On 11/10/2021 9:34 PM, antispam@math.uni.wroc.pl wrote:
> Don Y <blockedofcourse@foo.invalid> wrote:

>>> Classic trick it to parametrize.  However even if you
>>> parametrize there are hundreds of design decisions going
>>> into relatively small piece of code.  If you expose all
>>> design decisions then user as well may write his/her own
>>> code because complexity will be similar.  So normaly
>>> parametrization is limited and there will be users who
>>> find hardcoded desion choices inadequate.
>>>
>>> Another things is that current tools are rather weak
>>> at supporting parametrization.
>>
>> Look at a fleshy UART driver and think about how you would decompose
>> it into N different variants that could be "compile time configurable".
>> You'll be surprised as to how easy it is.  Even if the actual UART
>> hardware differs from instance to instance.
> 
> UART-s are simple.  And yet some things are tricky: in C to have
> "compile time configurable" buffer size you need to use macros.
> Works, but in a sense UART implementation "leaks" to user code.

You can configure using manifest constants, conditional compilation,
or even run-time switches.  Or, by linking against different
"support" routines.  How and where the configuration "leaks"
into user code is a function of the configuration mechanisms that
you decide to employ.

E.g., You'd likely NOT design your network stack to be tightly integrated
with your choice of NIC (all else being equal) -- simply because you'd
want to be able to reuse the stack with some *other* NIC without having
to rewrite it.

OTOH, it's not unexpected to want to isolate the caching of ARP results
in an "application specific" manner as you'll likely know the sorts (and
number!) of clients/services with which the device in question will be
connecting.  So, that (sub)module can be replaced with something most
appropriate to the application yet with a "standardized" interface to
the stack itself (*YOU* define that standard)

All of these require decisions up-front; you can't expect to be able to
retrofit an existing piece of code (cheaply) to support a more
modular/configurable implementation in the future.

But, personal experience teaches you what you are likely to need
by way of flexibility/configurability.  Most folks tend to eork
in a very narrow set of application domains.  Chances are, the
network stack you design for an embedded product will be considerably
different than one for a desktop OS.  If you plan to straddle
both domains, then the configurability challenge is greater!

>> There are costs to both approaches.  If I dedicate resource to
>> ensuring I don't miss anything, then some other aspect of the
>> design will bear that cost.  If I rely on detecting missed
>> messages, then I have to put a figure on their relative
>> likelihood so my device doesn't fail to provide its desired
>> functionality (because it is always missing one or two characters
>> out of EVERY message -- and, thus, sees NO messages).
> 
> My thinking goes toward using relatively short messages and
> buffer big enough for two messages.

You can also design with the intent of parsing messages before they are
complete and "reducing" them along the way.  This is particularly
important if messages can have varying length *or* there is a possibility
for the ass end of a message to get dropped (how do you know when the
message is complete?  Imagine THE USER misconfiguring your device
to expect CRLFs and the traffic only contains newlines; the terminating
CRLF never arrives!)

[At the limit case, a message reduces to a concept -- that is represented
in some application specific manner:  "Start the motor", "Clear the screen",
etc.]

Barcodes are messages (character sequences) of a sort.  I typically
process a barcode at several *concurrent* levels:
- an ISR that captures the times of transitions (black->white->black)
- a task that reduces the data captured by the ISR into "bar widths"
- a task that aggregates bar widths to form characters
- a task that parses character sequences to determine valid messages
- an application layer interpretation (or discard) of that message
This allows each layer to decide when the data on which it relies
does not represent a valid barcode and discard some (or all) of it...
without waiting for a complete message to be present.  So, the
resources that were consumed by that (partial?) message are
freed earlier.

As such, there is never a "start time" nor "end time" for a barcode
message -- because you don't want the user to have to "do something"
to tell you that he is now going to scan a barcode (otherwise, the
efficiency of using barcodes is subverted).

[Think about the sorts of applications that use barcodes; how many
require the user to tell the device "here comes a barcode, please start
your decoder algorithm NOW!"]

As users can abuse the barcode reader (there is nothing preventing them
from continuously scanning barcodes, in violation of any "protocol"
that the product may *intend*), you have to tolerate the case where
the data arrives faster than it can be consumed.  *Knowing* where
(in the event stream) you may have "lost" some data (transitions,
widths, characters or messages) lets you resync to a less pathological
event stream later (when the user starts "behaving properly")

> If there is need for
> high speed I would go for continous messages and DMA
> transfers (using break interrupt to discover end of message
> in case of variable length messages).  So device should
> be able to get all messages and in case of excess message
> trafic whole message could be dropped (possibly looking
> first for some high priority messages).  Of course, there
> may be some externaly mandated message format and/or
> communitation protocal making DMA inappropriate.
> Still, assuming interrupts, all characters should reach
> interrupt handler, causing possibly some extra CPU
> load.  The only possiblity of unnoticed loss of characters
> would be blocking interrupts too long.  If interrupts can
> be blocked for too long, then I would expect loss of whole
> messages.  In such case protocol should have something like
> "dont talk to me for next 100 miliseconds, I will be busy"
> to warn other nodes and request silence.  Now, if you
> need to faithfully support sillyness like Modbus RTU timeouts,
> then I hope that you are adequatly paid...

>> IMO, the advantages of writing in a multitasking environment so
>> far outweigh the "costs" of an MTOS that it behooves one to consider
>> how to shoehorn that functionality into EVERY design.
>>
>> When writing in a HLL, there are complications that impose
>> constraints on how the MTOS provides its services.  But, for small
>> projects written in ASM, you can gain the benefits of an MTOS
>> for very few bytes of code (and effectively zero RAM).
> 
> Well, looking at books and articles I did not find convincing
> argument/example showing that one really need multitasking for
> small systems.

The advantages of multitasking lie in problem decomposition.
Smaller problems are easier to "get right", in isolation.
The *challenge* of multitasking is coordinating the interactions
between these semi-concurrent actors.  Experience teaches you how
to partition a "job".

I want to blink a light at 1 Hz and check for a button to be
pressed which will start some action that may be lengthy.  I
can move the light blink into an ISR (which GENERALLY is a ridiculous
use of that "resource") to ensure the 1Hz timeliness is maintained
regardless of what the "lengthy" task may be doing, at the time.

Or, I can break the lengthy task into smaller chunks that
are executed sequentially with "peeks" at the "light timer"
between each of those segments.

    sequence1 := sequence2 := sequence3 := sequence4 := 0;
    while (FOREVER) {
task1:
       case sequence1++ {
       0 => do_task1_step0;
       1 => do_task1_step1;
       2 => do_task1_step2;
       ...
       }

       do_light;

task2:
       case sequence2++ {
       0 => do_task2_step0;
       1 => do_task2_step1;
       2 => do_task2_step2;
       ...
       }

       do_light;

task3:
       switch sequence3++ {
       0 => do_task3_step0;
       1 => do_task3_step1;
       2 => do_task3_step2;
       ...
       }

       do_light;

       ...
     }

When you need to do seven (or fifty) other "lengthy actions"
concurrently (each of which may introduce other "blinking
lights" or timeliness constraints), its easier (less brittle)
to put a structure in place that lets those competing actions
share the processor without requiring the developer to
micromanage at this level.

[50 tasks isn't an unusual load in a small system; video arcade
games from the early 80's -- 8 bit processors, kilobytes of
ROM+RAM -- would typically treat each object on the screen
(including bullets!) as a separate process]

The above example has low overhead for the apparent concurrency.
But, pushes all of the work onto the developer's lap.  He has
to carefully size each "step" of each "task" to ensure the
overall system is responsive.

A nicer approach is to just let an MTOS handle the switching
between tasks.  But, this comes at a cost of additional run-time
overhead (e.g., arbitrary context switches).

> I tend to think rather in terms of collection
> of coupled finite state machines (or if you prefer Petri net).
> State machines transition in response to events and may generate
> events.  Each finite state machine could be a task.  But it is
> not clear if it should.  Some transitions are simple and should
> be fast and that I would do in interrupt handlers.  Some
> other are triggered in regular way from other machines and
> are naturally handled by function calls.  Some need queues.
> The whole thing fits resonably well in "super loop" paradigm.

I use FSMs for UIs and message parsing.  They let the structure
of the code "rise to the top" where it is more visible (to another
developer) instead of burying it in subroutines and function calls.

"Event sources" create events which are consumed by FSMs, as
needed.  So, a "power monitor" could generate POWER_FAIL, LOW_BATTERY,
POWER_RESTORED, etc. events while a "keypad decoder" could put out
ENTER, CLEAR, ALPHA_M, NUMERIC_5, etc. events.

Because there is nothing *special* about an "event", *ANY* piece of
code can generate them.  Their significance assigns based on where
they are "placed" (in memory) and who/what can "see" them.  So,
you can use an FSM to parse a message (using "received characters"
as an ordered stream of events) and "signal" MESSAGE_COMPLETE to
another FSM that is awaiting "messages" (along with a pointer to the
completed message)

>>>>   From "KLEE: Unassisted and Automatic Generation of High-Coverage
>>>> Tests for Complex Systems Programs":
>>>>
>>>>       KLEE finds important errors in heavily-tested code. It
>>>>       found ten fatal errors in COREUTILS (including three
>>>>       that had escaped detection for 15 years), which account
>>>>       for more crashing bugs than were reported in 2006, 2007
>>>>       and 2008 combined. It further found 24 bugs in BUSYBOX, 21
>>>>       bugs in MINIX, and a security vulnerability in HISTAR? a
>>>>       total of 56 serious bugs.
>>>>
>>>> Ooops!  I wonder how many FOSS *eyes* missed those errors?
>>>
>>> Open source folks tend to be more willing to talk about bugs.
>>> And the above nicely shows that there is a lot of bugs, most
>>> waiting to by discovered.
>>
>> Part of the problem is ownership of the codebase.  You are
>> more likely to know where your own bugs lie -- and, more
>> willing to fix them ("pride of ownership").  When a piece
>> of code is shared, over time, there seems to be less incentive
>> for folks to tackle big -- often dubious -- issues as the
>> "reward" is minimal (i.e., you may not own the code when the bug
>> eventually becomes a problem)
> 
> Ownership may cause problems: there is tendency to "solve"
> problems locally, that is in code that given person "owns".
> This is good if there is easy local solution.  However, this
> may also lead to ugly workarounds that really do not work
> well, while problem is easily solvable in different part
> ("owned" by different programmer).  I have seen such thing
> several times, looking at whole codebase after some effort
> it was possible to do simple fix, while there were workarounds
> in different ("wrong") places.   I had no contact with
> original authors, but it seems that workarounds were due to
> "ownership".

You are *always* at the mercy of the code's owner.  Just as folks
are at YOUR mercy for the code that you (currently) exert ownership
over.  The best compliments you'll receive are from folks who
inherit your codebase and can appreciate its structure and
consistency.  Conversely, your worst nightmares will be inheriting
a codebase that was "hacked together", willy-nilly, by some number
of predecessors with no real concern over their "product" (code).

E.g., For FOSS projects, ownership isn't just a matter of who takes
"responsibility" for coordinating/merging diffs into the
codebase but, also, who has a compatible "vision" for the
codebase, going forward.  You'd not want a radically different
vision from one owner to the next as this leads to gyrations in
the codebase that will be seen as instability by its users
(i.e., other developers).

I use PostgreSQL in my current design.  I have no desire to
*develop* the RDBMS software -- let folks who understand that
sort of thing work their own magic on the codebase.  I can add
value *elsewhere* in my designs.

But, I eventually have to take ownership of *a* version of the
software as I can't expect the "real owners" to maintain some
version that *I* find useful, possibly years from now.  Once
I assume ownership of that chosen release, it will be my
priorities and skillset that drive how it evolves.  I can
choose to cherry pick "fixes" from the main branch and back-port
them into the version I've adopted.  Or, decide to live with
some particular set of problems/bugs/shortcomings.

If I am prudent, I will attempt to adopt the "style" of the
original developers in fitting any changes that I make to
that codebase.  I'd want my changes to "blend in" and seem
consistent with that which preceded them.

Folks following the main distribution would likely NOT be interested
in the changes that I choose to embrace as they'll likely have
different goals than I.  But that doesn't say my ownership is
"toxic", just that it doesn't suit the needs of (most) others.

---

I've got to bow out of this conversation.  I made a commitment to
release 6 designs to manufacturing before year end.  As it stands,
now, it looks like I'll only have time enough for four of them as
I got "distracted", spending the past few weeks gallavanting (but
it was wicked fun!).

OTOH, It won't be fun starting the new year two weeks "behind"...  :<

[Damn holidays eat into my work time.  And, no excuse on my part;
it's not like I didn't KNOW they were coming!!  :< ]

Don Y <blockedofcourse@foo.invalid> wrote:
> On 10/31/2021 3:54 PM, antispam@math.uni.wroc.pl wrote:
> >> Aren't THEY components?
> > 
> > Well, some folks expect more from components than from
> > traditional libraries.  Some evan claim to deliver.
> > However, libraries have limitations and ATM I see nothing
> > that fundamentally change situation.
> 
> A component is something that you can use as a black box,
> without having to reinvent it.  It is the epitome of reuse.
> 
> >>   How often do you reimplement
> >> ?printf() to avoid all of the bloat that typically accompanies it?
> > 
> > I did that once (for OS kernel where standard library would not
> > work).  If needed I can reuse it.  On PC-s I am not worried by
> > bloat due to printf.  OTOH, on MCU-s I am not sure if I ever used
> > printf.  Rather, printing was done by specialized routines
> > either library provided or my own.
> 
> You can also create a ?printf() that you can configure at build time to
> support the modifiers and specifiers that you know you will need.
> 
> Just like you can configure a UART driver to support a FIFO size defined
> at configuration, hardware handshaking, software flowcontrol, the
> high and low water marks for each of those (as they can be different),
> the character to send to request the remote to stop transmitting,
> the character you send to request resumption of transmission, which
> character YOU will recognize as requesting your Tx channel to pause,
> the character (or condition) you will recognize to resume your Tx,
> whether or not you will sample the condition codes in the UART, how
> you read/write the data register, how you read/write the status register,
> etc.
> 
> While these sound like lots of options, they are all relatively
> trivial additions to the code.
> 
> >> (when was the last time you needed ALL of those format specifiers
> >> in an application?  And modifiers?
> >>
> >>> There are some things which are reusable
> >>> if you accept modest to severe overhead.
> >>
> >> What you need is components with varying characteristics.
> >> You can buy diodes with all sorts of current carrying capacities,
> >> PIVs, package styles, etc.  But, they all still perform the
> >> same function.  Why so many different part numbers?  Why not
> >> just use the biggest, baddest diode in ALL your circuits?
<snip>
> > I am stuck with what is available on the market.  And diode
> > is logically pretty simple component, yet we need many kinds.
> > 
> >> I.e., we readily accept differences in "standard components"
> >> in other disciplines; why not when it comes to software
> >> modules?
> > 
> > Well, software is _much_ more compilcated than physical
> > engineering artifacts.  Physical thing may have 10000 joints,
> > but if joints are identical, then this is moral equivalent of
> > simple loop that just iterates fixed number of times.
> 
> This is the argument in favor of components.  You'd much rather
> read a comprehensive specification ("datasheet") for a software
> component than have to read through all of the code that implements
> it.

Well, if there is simple to use component that performs what
you need, then using it is fine.  However, for many tasks
once component is flexible enough to cover both your and
my needs its specification may be longer and more tricky
than code doing task at hand.

>  What if it was implemented in some programming language in
> which you aren't expert?  What if it was a binary "BLOB" and
> couldn't be inspected?

There are many reasons when existing code can not be reused.
Concerning BLOB-s, I am trying to avoid them and in first
order approximation I am not using them.  One (serious IMO)
problem with BLOB-s is that sooner or later they will be
incompatible with other things (OS/other libraries/my code).
Very old source code usually can be run on modern systems
with modest effort.  BLOB-s normally would require much
more effort.

> > At software level number of possible pre-composed blocks
> > is so large that it is infeasible to deliver all of them.
> 
> You don't have to deliver all of them.  When you wire a circuit,
> you still have to *solder* connections, don't you?  The
> components don't magically glue themselves together...

Yes, one needs to make connections.  In fact, in programming
most work is "making connections".  So you want something
which is simple to connect.  In other words, you can all
parts of your design to play nicely together.  With code
deliverd by other folks that is not always the case.

> > Classic trick it to parametrize.  However even if you
> > parametrize there are hundreds of design decisions going
> > into relatively small piece of code.  If you expose all
> > design decisions then user as well may write his/her own
> > code because complexity will be similar.  So normaly
> > parametrization is limited and there will be users who
> > find hardcoded desion choices inadequate.
> > 
> > Another things is that current tools are rather weak
> > at supporting parametrization.
> 
> Look at a fleshy UART driver and think about how you would decompose
> it into N different variants that could be "compile time configurable".
> You'll be surprised as to how easy it is.  Even if the actual UART
> hardware differs from instance to instance.

UART-s are simple.  And yet some things are tricky: in C to have
"compile time configurable" buffer size you need to use macros.
Works, but in a sense UART implementation "leaks" to user code.

> >>> For example things tends
> >>> to compose nicely if you dynamically allocate everything and use
> >>> garbage collection.  But performace cost may be substantial.
> >>> And in embedded setting garbage collection may be unacceptable.
> >>> In some cases I have found out that I can get much better
> >>> speed joing things that could be done as composition of library
> >>> operations into single big routine.
> >>
> >> Sure, but now you're tuning a solution to a specific problem.
> >> I've designed custom chips to solve particular problems.
> >> But, they ONLY solve those particular problems!  OTOH,
> >> I use lots of OTC components in my designs because those have
> >> been designed (for the most part) with an eye towards
> >> meeting a variety of market needs.
> > 
> > Maybe I made wrong impression, I think some explanation is in
> > place here.  I am trying to make my code reusable.  For my
> > problems performance is important part of reusablity: our
> > capability to solve problem is limited by performance and with
> > better perfomance users can solve bigger problems.  I am
> > re-using code that I can and I would re-use more if I could
> > but there there are technical obstacles.  Also, while I am
> > trying to make my code reusable, there are intrusive
> > design decision which may interfere with your possiobility
> > and willingness to re-use.
> 
> If you don't know where the design is headed, then you can't
> pick the components that it will need.

Well, there are routine tasks, for them it is natural to
re-use existing code.  There are new tasks that are "almost"
routine, than one can come with good design at the start.
But in a sense "interesting" tasks are when at start you
have only limited understanding.  In such case it is hard
to know "where the design is headed", except that it is
likely to change.  Of course, customer may be dissatisfied
if you tell "I will look at the problem and maybe I will
find solution".  But lack of understanding is normal
in research (at starting point), and I think that software
houses also do risky projects hoping that big win on succesful
ones will cover losses on failures.

> I approach a design from the top (down) and bottom (up).  This
> lets me gauge the types of information that I *may* have
> available from the hardware -- so I can sort out how to
> approach those limitations from above.  E.g., if I can't
> control the data rate of a comm channel, then I either have
> to ensure I can catch every (complete) message *or* design a
> protocol that lets me detect when I've missed something.

Well, with UART there will be some fixed transmission rate
(with wrong clock frequency UART would be unable to receive
anything).  I would expect MCU to be able to receive all
incoming characters (OK, assuming hardware UART with drivier
using high priority interrupt).  So, detecting that you got too
much should not be too hard.  OTOH, sensibly handling
excess input is different issue: if characters are coming
faster than you can process them, then either your CPU is
underpowered or there is some failure causing excess transmission.
In either case specific application will dictate what
should be avoided.

> There are costs to both approaches.  If I dedicate resource to
> ensuring I don't miss anything, then some other aspect of the
> design will bear that cost.  If I rely on detecting missed
> messages, then I have to put a figure on their relative
> likelihood so my device doesn't fail to provide its desired
> functionality (because it is always missing one or two characters
> out of EVERY message -- and, thus, sees NO messages).

My thinking goes toward using relatively short messages and
buffer big enough for two messages.  If there is need for
high speed I would go for continous messages and DMA
transfers (using break interrupt to discover end of message
in case of variable length messages).  So device should
be able to get all messages and in case of excess message
trafic whole message could be dropped (possibly looking
first for some high priority messages).  Of course, there
may be some externaly mandated message format and/or
communitation protocal making DMA inappropriate.
Still, assuming interrupts, all characters should reach
interrupt handler, causing possibly some extra CPU
load.  The only possiblity of unnoticed loss of characters
would be blocking interrupts too long.  If interrupts can
be blocked for too long, then I would expect loss of whole
messages.  In such case protocol should have something like
"dont talk to me for next 100 miliseconds, I will be busy"
to warn other nodes and request silence.  Now, if you
need to faithfully support sillyness like Modbus RTU timeouts,
then I hope that you are adequatly paid...

> > In slightly different spirit: in another thread you wrote
> > about accessing disc without OS file cache.  Here I
> > normaly depend on OS and OS file caching is big thing.
> > It is not perfect, but OS (OK, at least Linux) is doing
> > this resonably well I have no temptation to avoid it.
> > And I appreciate that with OS cache performance is
> > usually much better that would be "without cache".
> > OTOH, I routinly avoid stdio for I/O critical things
> > (so no printf in I/O critical code).
> 
> My point about the cache was that it is of no value in my case;
> I'm not going to revisit a file once I've seen it the first
> time (so why hold onto that data?)

Well, OS "cache" has many functions.  One of them is read-ahead,
another is scheduling of requests to minimize seek time.
And beside data there is also meta-data.  OS functions need
access to meta-data and OS-es are designed under assumption
that there is decent cache hit rate on meta-data access.

> >>>   In other cases I fixed
> >>> bugs by replacing composition of library routines by a single
> >>> routine: there were interactions making simple composition
> >>> incorrect.  Correct alterantive was single routine.
> >>>
> >>> As I wrote my embedded programs are simple and small.  But I
> >>> use almost no external libraries.  Trying some existing libraries
> >>> I have found out that some produce rather large programs, linking
> >>> in a lot of unneeded stuff.
> >>
> >> Because they try to address a variety of solution spaces without
> >> trying to be "optimal" for any.  You trade flexibility/capability
> >> for speed/performance/etc.
> > 
> > I think that this is more subtle: libraries frequently force some
> > way of doing things.  Which may be good if you try to quickly roll
> > solution and are within capabilities of library.  But if you
> > need/want different design, then library may be too inflexible
> > to deliver it.
> 
> Use a different diode.

Well, when needed I use my own library.

> >>> Of course, writing for scratch
> >>> will not scale to bigger programs.  OTOH, I feel that with
> >>> proper tooling it would be possible to retain efficiency and
> >>> small code size at least for large class of microntroller
> >>> programs (but existing tools and libraries do not support this).
> >>
> >> Templates are an attempt in this direction.  Allowing a class of
> >> problems to be solved once and then tailored to the specific
> >> application.
> > 
> > Yes, templates could help.  But they also have problems.  One
> > of them is that (among others) I would like to target STM8
> > and I have no C++ compiler for STM8.  My idea is to create
> > custom "optimizer/generator" for (annotated) C code.
> > ATM it is vapourware, but I think it is feasible with
> > reasonable effort.
> > 
> >> But, personal experience is where you win the most.  You write
> >> your second or third UART driver and start realizing that you
> >> could leverage a previous design if you'd just thought it out
> >> more fully -- instead of tailoring it to the specific needs
> >> of the original application.
> >>
> >> And, as you EXPECT to be reusing it in other applications (as
> >> evidenced by the fact that it's your third time writing the same
> >> piece of code!), you anticipate what those *might* need and
> >> think about how to implement those features "economically".
> >>
> >> It's rare that an application is *so* constrained that it can't
> >> afford a couple of extra lines of code, here and there.  If
> >> you've considered efficiency in the design of your algorithms,
> >> then these little bits of inefficiency will be below the noise floor.
> > 
> > Well, I am not talking about "couple of extra lines".  Rather
> > about IMO substantial fixed overhead.  As I wrote, one of my
> > targets is STM8 with 8k flash, another is MSP430 with 16k flash,
> > another is STM32 with 16k flash (there are also bigger targets).
> > One of libraries/frameworks for STM32 after activating few featurs
> > pulled in about 16k code, this is substantial overhead given
> > how little features I needed.  Other folks reported that for
> > trivial programs vendor supplied frameworks pulled close to 30k
> 
> A "framework" is considerably more than a set of individually
> selectable components.  I've designed products with 2KB of code and
> 128 bytes of RAM.  The "components" were ASM modules instead of
> HLL modules.  Each told me how big it was, how much RAM it required,
> how deep the stack penetration when invoked, how many T-states
> (worst case) to execute, etc.

Nice, but I am not sure how practical this would be in modern
times.  I have C code and can resonably estimate resource use.
But there are changable parameters which may enable/disable
some parts.  And size/speed/stack use depends on compiler
optimizations.  So there is variation.  And there are traps.
Linker transitively pulls dependencies, it there are "false"
dependencies, they can pull much more than strictly needed.
One example of "false" dependence are (or maybe were) C++
VMT-s.  Namely, any use of object/class pulled VMT which in
turn pulled all ancestors and methods.  If unused methods
referenced other classes that could easily cascade.  In both
cases authors of libraries probably thought that provided
"goodies" justified size (intended targets were larger).

> So, before I designed the hardware, I knew what I would need
> by way of ROM/RAM (before the days of FLASH) and could commit
> the hardware to foil without fear of running out of "space" or
> "time".
>
> > code.  That may be fine if you have bigger device and need features,
> > but for smaller MCU-s it may be difference between not fitting into
> > device or (without library) having plenty of free space.
> 
> Sure.  But a component will have a datasheet that tells you what
> it provides and at what *cost*.

My 16x2 text LCD routine may pull I2C driver.  If I2C is not needed
anyway, this is additional cost, otherwise cost is shared.
LCD routine depends also on timer.  Both timer and I2C affect
MCU initialization.  So even in very simple situations total
cost is rather complex.  And libraries that I tried presumably
were not "components" in your sense, you had to link the program
to learn total size.  Documentation mentioned dependencies,
when they affected correctness but otherwise not.  To say
the truth, when library supports hundreds or thousends of different
targets (combinations of CPU core, RAM/ROM sizes, peripherial
configurations) with different compilers, then there is hard
to make exact statements.

IMO, in ideal world for "standard" MCU functionality we would
have configuration tool where user can specify needed
functionality and tool would generate semi-custom code
and estimate its resource use.  MCU vendor tools attempt to
offer something like this, but reports I heard were rather
unfavourable, in particular it seems that vendors simply
deliver thick library that supports "everything", and
linking to this library causes code bloat.

> > When I tried it Free RTOS for STM32 needed about 8k flash.  Which
> > is fine if you need RTOS.  But ATM my designs run without RTOS.
> 
> RTOS is a commonly misused term.  Many are more properly called
> MTOSs (they provide no real timeliness guarantees, just multitasking
> primitives).

Well, Free RTOS comes with "no warranty", but AFAICS they make
honest effort to have good real time behaviour.  In particular,
code paths trough Free RTOS from events to user code are of
bounded and rather short length.  User code still may be
delayed by interrupts/process priorities, but they give resonable
explanation.  So it is up to user to code things in way that gives
needed real-time behaviour, but Free RTOS normally will not spoil it
and may help.

> IMO, the advantages of writing in a multitasking environment so
> far outweigh the "costs" of an MTOS that it behooves one to consider
> how to shoehorn that functionality into EVERY design.
> 
> When writing in a HLL, there are complications that impose
> constraints on how the MTOS provides its services.  But, for small
> projects written in ASM, you can gain the benefits of an MTOS
> for very few bytes of code (and effectively zero RAM).

Well, looking at books and articles I did not find convincing
argument/example showing that one really need multitasking for
small systems.  I tend to think rather in terms of collection
of coupled finite state machines (or if you prefer Petri net).
State machines transition in response to events and may generate
events.  Each finite state machine could be a task.  But it is
not clear if it should.  Some transitions are simple and should
be fast and that I would do in interrupt handlers.  Some
other are triggered in regular way from other machines and
are naturally handled by function calls.  Some need queues.
The whole thing fits resonably well in "super loop" paradigm.

I have found one issue that at first glance "requires"
multitasking.  Namely, when one wants to put system in
sleep mode when there is no work natural "super loop"
approach looks like

        if (work_to_do) {
           do_work();
        } else {
           wait_for_interrupt();
        }

where 'work_to_do' is flag which may be set by interrupt handlers.
But there is nasty race condition, if interrupt comes between
test for 'work_to_do' and 'wait_for_interrupt': despite
having work to do system will go to sleep and only wake on
next interrupt (which depending on specific requirements may
be harmless or disaster).  I was unable to find simple code
that avoids this race.  With multitasking kernel race vanishes:
there is idle task which is only doing 'wait_for_interrupt'
and OS scheduler passes control to worker tasks when there is
work to do.  But when one looks how multitasker avoids race,
then it is clear that crucial point is doing control transfer
via return from interrupt.  More precisely, variables are
tested with interrupts disabled and after decision is made
return from interrupt transfers control.  Important point is
that if interrupt comes after control transfer interrupt handler
will re-do test before returning to user code.  So what is needed
is piece of low-level code that uses return from interrupt for
control transfer and all interrupt handlers need to jump to
this code when finished.  The rest (usually majority) of
multitasker is not needed...

> >>>> Unlikely that this code will describe itself as "works well enough
> >>>> SOME of the time..."
> >>>>
> >>>> And, when/if you stumble on such faults, good luck explaining to
> >>>> your customer why it's going to take longer to fix and retest the
> >>>> *existing* codebase before you can get on with your modifications...
> >>>
> >>> Commercial vendors like to say how good their progam are.  But
> >>> market reality is that program my be quite bad and still sell.
> >>
> >> The same is true of FOSS -- despite the claim that many eyes (may)
> >> have looked at it (suggesting that bugs would have been caught!)
> >>
> >>  From "KLEE: Unassisted and Automatic Generation of High-Coverage
> >> Tests for Complex Systems Programs":
> >>
> >>      KLEE finds important errors in heavily-tested code. It
> >>      found ten fatal errors in COREUTILS (including three
> >>      that had escaped detection for 15 years), which account
> >>      for more crashing bugs than were reported in 2006, 2007
> >>      and 2008 combined. It further found 24 bugs in BUSYBOX, 21
> >>      bugs in MINIX, and a security vulnerability in HISTAR? a
> >>      total of 56 serious bugs.
> >>
> >> Ooops!  I wonder how many FOSS *eyes* missed those errors?
> > 
> > Open source folks tend to be more willing to talk about bugs.
> > And the above nicely shows that there is a lot of bugs, most
> > waiting to by discovered.
> 
> Part of the problem is ownership of the codebase.  You are
> more likely to know where your own bugs lie -- and, more
> willing to fix them ("pride of ownership").  When a piece
> of code is shared, over time, there seems to be less incentive
> for folks to tackle big -- often dubious -- issues as the
> "reward" is minimal (i.e., you may not own the code when the bug
> eventually becomes a problem)

Ownership may cause problems: there is tendency to "solve"
problems locally, that is in code that given person "owns".
This is good if there is easy local solution.  However, this
may also lead to ugly workarounds that really do not work
well, while problem is easily solvable in different part
("owned" by different programmer).  I have seen such thing
several times, looking at whole codebase after some effort
it was possible to do simple fix, while there were workarounds
in different ("wrong") places.   I had no contact with
original authors, but it seems that workarounds were due to
"ownership".

-- 
                              Waldek Hebisch

On 10/31/2021 3:54 PM, antispam@math.uni.wroc.pl wrote:
>> Aren't THEY components?
> 
> Well, some folks expect more from components than from
> traditional libraries.  Some evan claim to deliver.
> However, libraries have limitations and ATM I see nothing
> that fundamentally change situation.

A component is something that you can use as a black box,
without having to reinvent it.  It is the epitome of reuse.

>>   How often do you reimplement
>> ?printf() to avoid all of the bloat that typically accompanies it?
> 
> I did that once (for OS kernel where standard library would not
> work).  If needed I can reuse it.  On PC-s I am not worried by
> bloat due to printf.  OTOH, on MCU-s I am not sure if I ever used
> printf.  Rather, printing was done by specialized routines
> either library provided or my own.

You can also create a ?printf() that you can configure at build time to
support the modifiers and specifiers that you know you will need.

Just like you can configure a UART driver to support a FIFO size defined
at configuration, hardware handshaking, software flowcontrol, the
high and low water marks for each of those (as they can be different),
the character to send to request the remote to stop transmitting,
the character you send to request resumption of transmission, which
character YOU will recognize as requesting your Tx channel to pause,
the character (or condition) you will recognize to resume your Tx,
whether or not you will sample the condition codes in the UART, how
you read/write the data register, how you read/write the status register,
etc.

While these sound like lots of options, they are all relatively
trivial additions to the code.

>> (when was the last time you needed ALL of those format specifiers
>> in an application?  And modifiers?
>>
>>> There are some things which are reusable
>>> if you accept modest to severe overhead.
>>
>> What you need is components with varying characteristics.
>> You can buy diodes with all sorts of current carrying capacities,
>> PIVs, package styles, etc.  But, they all still perform the
>> same function.  Why so many different part numbers?  Why not
>> just use the biggest, baddest diode in ALL your circuits?
> 
> I heard such electronic analogies many times.  But they miss
> important point: there is no way for me to make my own diode,

Sure there is!  It is just not an efficient way of spending your
resources when you have so many OTS offerings available.

You can design your own processor.  Why do you "settle" for an
OTS device (ANS:  because there is so little extra added value
you will typically gain from rolling your own vs. the "inefficiency"
of using a COTS offering)

> I am stuck with what is available on the market.  And diode
> is logically pretty simple component, yet we need many kinds.
> 
>> I.e., we readily accept differences in "standard components"
>> in other disciplines; why not when it comes to software
>> modules?
> 
> Well, software is _much_ more compilcated than physical
> engineering artifacts.  Physical thing may have 10000 joints,
> but if joints are identical, then this is moral equivalent of
> simple loop that just iterates fixed number of times.

This is the argument in favor of components.  You'd much rather
read a comprehensive specification ("datasheet") for a software
component than have to read through all of the code that implements
it.  What if it was implemented in some programming language in
which you aren't expert?  What if it was a binary "BLOB" and
couldn't be inspected?

> At software level number of possible pre-composed blocks
> is so large that it is infeasible to deliver all of them.

You don't have to deliver all of them.  When you wire a circuit,
you still have to *solder* connections, don't you?  The
components don't magically glue themselves together...

> Classic trick it to parametrize.  However even if you
> parametrize there are hundreds of design decisions going
> into relatively small piece of code.  If you expose all
> design decisions then user as well may write his/her own
> code because complexity will be similar.  So normaly
> parametrization is limited and there will be users who
> find hardcoded desion choices inadequate.
> 
> Another things is that current tools are rather weak
> at supporting parametrization.

Look at a fleshy UART driver and think about how you would decompose
it into N different variants that could be "compile time configurable".
You'll be surprised as to how easy it is.  Even if the actual UART
hardware differs from instance to instance.

>>> For example things tends
>>> to compose nicely if you dynamically allocate everything and use
>>> garbage collection.  But performace cost may be substantial.
>>> And in embedded setting garbage collection may be unacceptable.
>>> In some cases I have found out that I can get much better
>>> speed joing things that could be done as composition of library
>>> operations into single big routine.
>>
>> Sure, but now you're tuning a solution to a specific problem.
>> I've designed custom chips to solve particular problems.
>> But, they ONLY solve those particular problems!  OTOH,
>> I use lots of OTC components in my designs because those have
>> been designed (for the most part) with an eye towards
>> meeting a variety of market needs.
> 
> Maybe I made wrong impression, I think some explanation is in
> place here.  I am trying to make my code reusable.  For my
> problems performance is important part of reusablity: our
> capability to solve problem is limited by performance and with
> better perfomance users can solve bigger problems.  I am
> re-using code that I can and I would re-use more if I could
> but there there are technical obstacles.  Also, while I am
> trying to make my code reusable, there are intrusive
> design decision which may interfere with your possiobility
> and willingness to re-use.

If you don't know where the design is headed, then you can't
pick the components that it will need.

I approach a design from the top (down) and bottom (up).  This
lets me gauge the types of information that I *may* have
available from the hardware -- so I can sort out how to
approach those limitations from above.  E.g., if I can't
control the data rate of a comm channel, then I either have
to ensure I can catch every (complete) message *or* design a
protocol that lets me detect when I've missed something.

There are costs to both approaches.  If I dedicate resource to
ensuring I don't miss anything, then some other aspect of the
design will bear that cost.  If I rely on detecting missed
messages, then I have to put a figure on their relative
likelihood so my device doesn't fail to provide its desired
functionality (because it is always missing one or two characters
out of EVERY message -- and, thus, sees NO messages).

> In slightly different spirit: in another thread you wrote
> about accessing disc without OS file cache.  Here I
> normaly depend on OS and OS file caching is big thing.
> It is not perfect, but OS (OK, at least Linux) is doing
> this resonably well I have no temptation to avoid it.
> And I appreciate that with OS cache performance is
> usually much better that would be "without cache".
> OTOH, I routinly avoid stdio for I/O critical things
> (so no printf in I/O critical code).

My point about the cache was that it is of no value in my case;
I'm not going to revisit a file once I've seen it the first
time (so why hold onto that data?)

>>>   In other cases I fixed
>>> bugs by replacing composition of library routines by a single
>>> routine: there were interactions making simple composition
>>> incorrect.  Correct alterantive was single routine.
>>>
>>> As I wrote my embedded programs are simple and small.  But I
>>> use almost no external libraries.  Trying some existing libraries
>>> I have found out that some produce rather large programs, linking
>>> in a lot of unneeded stuff.
>>
>> Because they try to address a variety of solution spaces without
>> trying to be "optimal" for any.  You trade flexibility/capability
>> for speed/performance/etc.
> 
> I think that this is more subtle: libraries frequently force some
> way of doing things.  Which may be good if you try to quickly roll
> solution and are within capabilities of library.  But if you
> need/want different design, then library may be too inflexible
> to deliver it.

Use a different diode.

>>> Of course, writing for scratch
>>> will not scale to bigger programs.  OTOH, I feel that with
>>> proper tooling it would be possible to retain efficiency and
>>> small code size at least for large class of microntroller
>>> programs (but existing tools and libraries do not support this).
>>
>> Templates are an attempt in this direction.  Allowing a class of
>> problems to be solved once and then tailored to the specific
>> application.
> 
> Yes, templates could help.  But they also have problems.  One
> of them is that (among others) I would like to target STM8
> and I have no C++ compiler for STM8.  My idea is to create
> custom "optimizer/generator" for (annotated) C code.
> ATM it is vapourware, but I think it is feasible with
> reasonable effort.
> 
>> But, personal experience is where you win the most.  You write
>> your second or third UART driver and start realizing that you
>> could leverage a previous design if you'd just thought it out
>> more fully -- instead of tailoring it to the specific needs
>> of the original application.
>>
>> And, as you EXPECT to be reusing it in other applications (as
>> evidenced by the fact that it's your third time writing the same
>> piece of code!), you anticipate what those *might* need and
>> think about how to implement those features "economically".
>>
>> It's rare that an application is *so* constrained that it can't
>> afford a couple of extra lines of code, here and there.  If
>> you've considered efficiency in the design of your algorithms,
>> then these little bits of inefficiency will be below the noise floor.
> 
> Well, I am not talking about "couple of extra lines".  Rather
> about IMO substantial fixed overhead.  As I wrote, one of my
> targets is STM8 with 8k flash, another is MSP430 with 16k flash,
> another is STM32 with 16k flash (there are also bigger targets).
> One of libraries/frameworks for STM32 after activating few featurs
> pulled in about 16k code, this is substantial overhead given
> how little features I needed.  Other folks reported that for
> trivial programs vendor supplied frameworks pulled close to 30k

A "framework" is considerably more than a set of individually
selectable components.  I've designed products with 2KB of code and
128 bytes of RAM.  The "components" were ASM modules instead of
HLL modules.  Each told me how big it was, how much RAM it required,
how deep the stack penetration when invoked, how many T-states
(worst case) to execute, etc.

So, before I designed the hardware, I knew what I would need
by way of ROM/RAM (before the days of FLASH) and could commit
the hardware to foil without fear of running out of "space" or
"time".

> code.  That may be fine if you have bigger device and need features,
> but for smaller MCU-s it may be difference between not fitting into
> device or (without library) having plenty of free space.

Sure.  But a component will have a datasheet that tells you what
it provides and at what *cost*.

> When I tried it Free RTOS for STM32 needed about 8k flash.  Which
> is fine if you need RTOS.  But ATM my designs run without RTOS.

RTOS is a commonly misused term.  Many are more properly called
MTOSs (they provide no real timeliness guarantees, just multitasking
primitives).

IMO, the advantages of writing in a multitasking environment so
far outweigh the "costs" of an MTOS that it behooves one to consider
how to shoehorn that functionality into EVERY design.

When writing in a HLL, there are complications that impose
constraints on how the MTOS provides its services.  But, for small
projects written in ASM, you can gain the benefits of an MTOS
for very few bytes of code (and effectively zero RAM).

> I have found libopencm3 to have small overhead.  But is routines
> are doing so little that direct register access may give simpler
> code.
> 
>>>> Unlikely that this code will describe itself as "works well enough
>>>> SOME of the time..."
>>>>
>>>> And, when/if you stumble on such faults, good luck explaining to
>>>> your customer why it's going to take longer to fix and retest the
>>>> *existing* codebase before you can get on with your modifications...
>>>
>>> Commercial vendors like to say how good their progam are.  But
>>> market reality is that program my be quite bad and still sell.
>>
>> The same is true of FOSS -- despite the claim that many eyes (may)
>> have looked at it (suggesting that bugs would have been caught!)
>>
>>  From "KLEE: Unassisted and Automatic Generation of High-Coverage
>> Tests for Complex Systems Programs":
>>
>>      KLEE finds important errors in heavily-tested code. It
>>      found ten fatal errors in COREUTILS (including three
>>      that had escaped detection for 15 years), which account
>>      for more crashing bugs than were reported in 2006, 2007
>>      and 2008 combined. It further found 24 bugs in BUSYBOX, 21
>>      bugs in MINIX, and a security vulnerability in HISTAR? a
>>      total of 56 serious bugs.
>>
>> Ooops!  I wonder how many FOSS *eyes* missed those errors?
> 
> Open source folks tend to be more willing to talk about bugs.
> And the above nicely shows that there is a lot of bugs, most
> waiting to by discovered.

Part of the problem is ownership of the codebase.  You are
more likely to know where your own bugs lie -- and, more
willing to fix them ("pride of ownership").  When a piece
of code is shared, over time, there seems to be less incentive
for folks to tackle big -- often dubious -- issues as the
"reward" is minimal (i.e., you may not own the code when the bug
eventually becomes a problem)

>> Every time you reinvent a solution, you lose much of the benefit
>> of the previous TESTED solution.
> 
> TESTED part works for simple repeatable tasks.  But if you have
> complex task it is quite likely that you will be the first
> person with given use case.  gcc is borderline case: if you
> throw really new code at it you can expect to see bugs.
> gcc user community it large and there is resonable chance that
> sombody wrote earlier code which is sufficiently similar to
> yours to catch troubles.  But there are domains that are at
> least as complicated as compilation and have much smaller
> user community.  You may find out that there are _no_ code
> that could be reasonably re-used.  Were you ever in situation
> when you looked how some "standard library" solves a tricky
> problem and realized that in fact library does not solve
> the problem?

As I said, your *personal* experience tells you where YOU will
likely benefit.  I did a stint with a company that manufactured
telecommunications kit.  We had all sorts of bizarre interface
protocols with which we had to contend (e.g., using RLSD as
a hardware "pacing" signal).  So, it was worthwhile to spend
time developing a robust UART driver (and handler, above it)
as you *knew* the next project would likely have need of it,
in some form or other.

If you're working free-lance and client A needs a BITBLTer
for his design, you have to decide how likely client B
(that you haven't yet met) will be to need the same sort
of module/component.

For example, I've never (until recently) needed to interface
to a disk controller in a product.  So, I don't have a
ready-made "component" in my bag-of-tricks.  When I look
at a new project, I "take inventory" of what I am likely to
need... and compare that to what I know I have "in stock".
If there's a lot of overlap, then my confidence in my bid
goes up.  If there'a a lot of new ground that I'll have to
cover, then it goes down (and the price goes up!).

Reuse helps you better estimate new projects, especially as
projects grow in complexity.

[There's nothing worse than having to upgrade someone else's
design that didn't plan for the future.  It's as if you
have to redesign the entire product from scratch --- despite
the fact that it *seems* to work, "as is" (but, not "as desired"!]

Don Y <blockedofcourse@foo.invalid> wrote:
> On 10/26/2021 10:22 PM, antispam@math.uni.wroc.pl wrote:
> >> One wants to be able to move towards the goal of software *components*.
> >> You don't want to have to inspect the design of every *diode* that
> >> you use; you want to look at it's overall specifications and decide
> >> if those fit your needs.
> > 
> > Sure, I would love to see really reusable components.  But IMHO we
> > are quite far from that.
> 
> Do you use the standard libraries?

Yes, I uses libraries when appropriate.

> Aren't THEY components?

Well, some folks expect more from components than from
traditional libraries.  Some evan claim to deliver.
However, libraries have limitations and ATM I see nothing
that fundamentally change situation.

> You rely on the compiler to decide how to divide X by Y -- instead
> of writing your own division routine.

Well, normally in C code I relay on compiler provied division.
To say the truth, my MCU code uses division sparingly, only
when I can not avoid it.  OTOH I also use languages with
multiprecision integers.  In one case I use complier provided
routines, but I am provider of modifed compiler and modification
includes replacement of division routine.  In other case I
override compiler supplied division routine by my own (which
in turn sends real work to external library).

>  How often do you reimplement
> ?printf() to avoid all of the bloat that typically accompanies it?

I did that once (for OS kernel where standard library would not
work).  If needed I can reuse it.  On PC-s I am not worried by
bloat due to printf.  OTOH, on MCU-s I am not sure if I ever used
printf.  Rather, printing was done by specialized routines
either library provided or my own.

> (when was the last time you needed ALL of those format specifiers
> in an application?  And modifiers?
> 
> > There are some things which are reusable
> > if you accept modest to severe overhead.
> 
> What you need is components with varying characteristics.
> You can buy diodes with all sorts of current carrying capacities,
> PIVs, package styles, etc.  But, they all still perform the
> same function.  Why so many different part numbers?  Why not
> just use the biggest, baddest diode in ALL your circuits?

I heard such electronic analogies many times.  But they miss
important point: there is no way for me to make my own diode,
I am stuck with what is available on the market.  And diode
is logically pretty simple component, yet we need many kinds.

> I.e., we readily accept differences in "standard components"
> in other disciplines; why not when it comes to software
> modules?

Well, software is _much_ more compilcated than physical
engineering artifacts.  Physical thing may have 10000 joints,
but if joints are identical, then this is moral equivalent of
simple loop that just iterates fixed number of times.
At software level number of possible pre-composed blocks
is so large that it is infeasible to deliver all of them.
Classic trick it to parametrize.  However even if you
parametrize there are hundreds of design decisions going
into relatively small piece of code.  If you expose all
design decisions then user as well may write his/her own
code because complexity will be similar.  So normaly
parametrization is limited and there will be users who
find hardcoded desion choices inadequate.

Another things is that current tools are rather weak
at supporting parametrization.

> > For example things tends
> > to compose nicely if you dynamically allocate everything and use
> > garbage collection.  But performace cost may be substantial.
> > And in embedded setting garbage collection may be unacceptable.
> > In some cases I have found out that I can get much better
> > speed joing things that could be done as composition of library
> > operations into single big routine.
> 
> Sure, but now you're tuning a solution to a specific problem.
> I've designed custom chips to solve particular problems.
> But, they ONLY solve those particular problems!  OTOH,
> I use lots of OTC components in my designs because those have
> been designed (for the most part) with an eye towards
> meeting a variety of market needs.

Maybe I made wrong impression, I think some explanation is in
place here.  I am trying to make my code reusable.  For my
problems performance is important part of reusablity: our
capability to solve problem is limited by performance and with
better perfomance users can solve bigger problems.  I am
re-using code that I can and I would re-use more if I could
but there there are technical obstacles.  Also, while I am
trying to make my code reusable, there are intrusive
design decision which may interfere with your possiobility
and willingness to re-use.

In slightly different spirit: in another thread you wrote
about accessing disc without OS file cache.  Here I
normaly depend on OS and OS file caching is big thing.
It is not perfect, but OS (OK, at least Linux) is doing
this resonably well I have no temptation to avoid it.
And I appreciate that with OS cache performance is
usually much better that would be "without cache".
OTOH, I routinly avoid stdio for I/O critical things
(so no printf in I/O critical code).

> >  In other cases I fixed
> > bugs by replacing composition of library routines by a single
> > routine: there were interactions making simple composition
> > incorrect.  Correct alterantive was single routine.
> > 
> > As I wrote my embedded programs are simple and small.  But I
> > use almost no external libraries.  Trying some existing libraries
> > I have found out that some produce rather large programs, linking
> > in a lot of unneeded stuff.
> 
> Because they try to address a variety of solution spaces without
> trying to be "optimal" for any.  You trade flexibility/capability
> for speed/performance/etc.

I think that this is more subtle: libraries frequently force some
way of doing things.  Which may be good if you try to quickly roll
solution and are within capabilities of library.  But if you
need/want different design, then library may be too inflexible
to deliver it.

> > Of course, writing for scratch
> > will not scale to bigger programs.  OTOH, I feel that with
> > proper tooling it would be possible to retain efficiency and
> > small code size at least for large class of microntroller
> > programs (but existing tools and libraries do not support this).
> 
> Templates are an attempt in this direction.  Allowing a class of
> problems to be solved once and then tailored to the specific
> application.

Yes, templates could help.  But they also have problems.  One
of them is that (among others) I would like to target STM8
and I have no C++ compiler for STM8.  My idea is to create
custom "optimizer/generator" for (annotated) C code.
ATM it is vapourware, but I think it is feasible with
reasonable effort.

> But, personal experience is where you win the most.  You write
> your second or third UART driver and start realizing that you
> could leverage a previous design if you'd just thought it out
> more fully -- instead of tailoring it to the specific needs
> of the original application.
> 
> And, as you EXPECT to be reusing it in other applications (as
> evidenced by the fact that it's your third time writing the same
> piece of code!), you anticipate what those *might* need and
> think about how to implement those features "economically".
> 
> It's rare that an application is *so* constrained that it can't
> afford a couple of extra lines of code, here and there.  If
> you've considered efficiency in the design of your algorithms,
> then these little bits of inefficiency will be below the noise floor.

Well, I am not talking about "couple of extra lines".  Rather
about IMO substantial fixed overhead.  As I wrote, one of my
targets is STM8 with 8k flash, another is MSP430 with 16k flash,
another is STM32 with 16k flash (there are also bigger targets).
One of libraries/frameworks for STM32 after activating few featurs
pulled in about 16k code, this is substantial overhead given
how little features I needed.  Other folks reported that for
trivial programs vendor supplied frameworks pulled close to 30k
code.  That may be fine if you have bigger device and need features,
but for smaller MCU-s it may be difference between not fitting into
device or (without library) having plenty of free space.

When I tried it Free RTOS for STM32 needed about 8k flash.  Which
is fine if you need RTOS.  But ATM my designs run without RTOS.

I have found libopencm3 to have small overhead.  But is routines
are doing so little that direct register access may give simpler
code.

> >> Unlikely that this code will describe itself as "works well enough
> >> SOME of the time..."
> >>
> >> And, when/if you stumble on such faults, good luck explaining to
> >> your customer why it's going to take longer to fix and retest the
> >> *existing* codebase before you can get on with your modifications...
> > 
> > Commercial vendors like to say how good their progam are.  But
> > market reality is that program my be quite bad and still sell.
> 
> The same is true of FOSS -- despite the claim that many eyes (may)
> have looked at it (suggesting that bugs would have been caught!)
> 
> From "KLEE: Unassisted and Automatic Generation of High-Coverage
> Tests for Complex Systems Programs":
> 
>     KLEE finds important errors in heavily-tested code. It
>     found ten fatal errors in COREUTILS (including three
>     that had escaped detection for 15 years), which account
>     for more crashing bugs than were reported in 2006, 2007
>     and 2008 combined. It further found 24 bugs in BUSYBOX, 21
>     bugs in MINIX, and a security vulnerability in HISTAR? a
>     total of 56 serious bugs.
> 
> Ooops!  I wonder how many FOSS *eyes* missed those errors?

Open source folks tend to be more willing to talk about bugs.
And the above nicely shows that there is a lot of bugs, most
waiting to by discovered.

> Every time you reinvent a solution, you lose much of the benefit
> of the previous TESTED solution.

TESTED part works for simple repeatable tasks.  But if you have
complex task it is quite likely that you will be the first
person with given use case.  gcc is borderline case: if you
throw really new code at it you can expect to see bugs.
gcc user community it large and there is resonable chance that
sombody wrote earlier code which is sufficiently similar to
yours to catch troubles.  But there are domains that are at
least as complicated as compilation and have much smaller
user community.  You may find out that there are _no_ code
that could be reasonably re-used.  Were you ever in situation
when you looked how some "standard library" solves a tricky
problem and realized that in fact library does not solve
the problem?

-- 
                              Waldek Hebisch

On 10/26/2021 10:22 PM, antispam@math.uni.wroc.pl wrote:
>> One wants to be able to move towards the goal of software *components*.
>> You don't want to have to inspect the design of every *diode* that
>> you use; you want to look at it's overall specifications and decide
>> if those fit your needs.
> 
> Sure, I would love to see really reusable components.  But IMHO we
> are quite far from that.

Do you use the standard libraries?  Aren't THEY components?
You rely on the compiler to decide how to divide X by Y -- instead
of writing your own division routine.  How often do you reimplement
?printf() to avoid all of the bloat that typically accompanies it?
(when was the last time you needed ALL of those format specifiers
in an application?  And modifiers?

> There are some things which are reusable
> if you accept modest to severe overhead.

What you need is components with varying characteristics.
You can buy diodes with all sorts of current carrying capacities,
PIVs, package styles, etc.  But, they all still perform the
same function.  Why so many different part numbers?  Why not
just use the biggest, baddest diode in ALL your circuits?

I.e., we readily accept differences in "standard components"
in other disciplines; why not when it comes to software
modules?

> For example things tends
> to compose nicely if you dynamically allocate everything and use
> garbage collection.  But performace cost may be substantial.
> And in embedded setting garbage collection may be unacceptable.
> In some cases I have found out that I can get much better
> speed joing things that could be done as composition of library
> operations into single big routine.

Sure, but now you're tuning a solution to a specific problem.
I've designed custom chips to solve particular problems.
But, they ONLY solve those particular problems!  OTOH,
I use lots of OTC components in my designs because those have
been designed (for the most part) with an eye towards
meeting a variety of market needs.

>  In other cases I fixed
> bugs by replacing composition of library routines by a single
> routine: there were interactions making simple composition
> incorrect.  Correct alterantive was single routine.
> 
> As I wrote my embedded programs are simple and small.  But I
> use almost no external libraries.  Trying some existing libraries
> I have found out that some produce rather large programs, linking
> in a lot of unneeded stuff.

Because they try to address a variety of solution spaces without
trying to be "optimal" for any.  You trade flexibility/capability
for speed/performance/etc.

> Of course, writing for scratch
> will not scale to bigger programs.  OTOH, I feel that with
> proper tooling it would be possible to retain efficiency and
> small code size at least for large class of microntroller
> programs (but existing tools and libraries do not support this).

Templates are an attempt in this direction.  Allowing a class of
problems to be solved once and then tailored to the specific
application.

But, personal experience is where you win the most.  You write
your second or third UART driver and start realizing that you
could leverage a previous design if you'd just thought it out
more fully -- instead of tailoring it to the specific needs
of the original application.

And, as you EXPECT to be reusing it in other applications (as
evidenced by the fact that it's your third time writing the same
piece of code!), you anticipate what those *might* need and
think about how to implement those features "economically".

It's rare that an application is *so* constrained that it can't
afford a couple of extra lines of code, here and there.  If
you've considered efficiency in the design of your algorithms,
then these little bits of inefficiency will be below the noise floor.

>> Unlikely that this code will describe itself as "works well enough
>> SOME of the time..."
>>
>> And, when/if you stumble on such faults, good luck explaining to
>> your customer why it's going to take longer to fix and retest the
>> *existing* codebase before you can get on with your modifications...
> 
> Commercial vendors like to say how good their progam are.  But
> market reality is that program my be quite bad and still sell.

The same is true of FOSS -- despite the claim that many eyes (may)
have looked at it (suggesting that bugs would have been caught!)

 From "KLEE: Unassisted and Automatic Generation of High-Coverage
Tests for Complex Systems Programs":

     KLEE finds important errors in heavily-tested code. It
     found ten fatal errors in COREUTILS (including three
     that had escaped detection for 15 years), which account
     for more crashing bugs than were reported in 2006, 2007
     and 2008 combined. It further found 24 bugs in BUSYBOX, 21
     bugs in MINIX, and a security vulnerability in HISTAR– a
     total of 56 serious bugs.

Ooops!  I wonder how many FOSS *eyes* missed those errors?

Every time you reinvent a solution, you lose much of the benefit
of the previous TESTED solution.

Don Y <blockedofcourse@foo.invalid> wrote:
> On 10/26/2021 5:20 PM, antispam@math.uni.wroc.pl wrote:
> > Don Y <blockedofcourse@foo.invalid> wrote:
> >> On 10/25/2021 2:32 PM, antispam@math.uni.wroc.pl wrote:
> >>> Don Y <blockedofcourse@foo.invalid> wrote:
> >>>> On 10/24/2021 4:14 AM, Dimiter_Popoff wrote:
> >>>>>> Disable interrupts while accessing the fifo. you really have to.
> >>>>>> alternatively you'll often get away not using a fifo at all,
> >>>>>> unless you're blocking for a long while in some part of the code.
> >>>>>
> >>>>> Why would you do that. The fifo write pointer is only modified by
> >>>>> the interrupt handler, the read pointer is only modified by the
> >>>>> interrupted code. Has been done so for times immemorial.
> >>>>
> >>>> The OPs code doesn't differentiate between FIFO full and empty.
> >>>
> >>> If you read carefuly what he wrote you would know that he does.
> >>> The trick he uses is that his indices may point outside buffer:
> >>> empty is equal indices, full is difference equal to buffer
> >>
> >> Doesn't matter as any index can increase by any amount and
> >> invalidate the "reality" of the buffer's contents (i.e.
> >> actual number of characters that have been tranfered to
> >> that region of memory).
> > 
> > AFAIK OP considers this not a problem in his application.
> 
> And I don't think I have to test for division by zero -- as
> *my* code is the code that is passing numerator and denominator
> to that operator, right?

Well, I do not test for zero if I know that divisor must be
nonzero.  To put it differently, having zero in such place
is a bug and there is already enough machinery so that
such bug will not remain undetected.  Having extra test
adds no value.

OTOH is zero is possible, then handling it is part of program
logic and test is needed to take correct action.

> Can you remember all of the little assumptions you've made in
> any non-trivial piece of code -- a week later?  a month later?
> 6 months later (when a bug manifests or a feature upgrade
> is requested)?

Well, my normal practice is that there are no "little assumptions".
To put it differently, code is structured to make things clear,
even if this requires more code than some "clever" solution.
There may be "big assumptions", that is highly nontrivial facts
used by the code.  Some of them are considered "well known",
with proper naming in code it is easy to recall them years later.
Some deserve comments/referece.  In most of may coding I have
pretty comfortable situation: for human there is quite clear
what is valid and what is invalid.  So code makes a lot of
effort to handle valid (but possibly quite unusual) cases

> Do not check the inputs of routines for validity -- assume everything is
> correct (cuz YOU wrote it to be so, right?).

Well, correct inputs are part of contract.  Some things (like
array indices inside bounds) are checked, but in general you can
expect garbage if you pass incorrect input.  Most of my code is
of sort that called routine can not really check validity of input
(there are complex invariants).  Note: here I am talking mostly
about my non-embedded code (which is majority of my coding).
In most of may coding I have pretty comfortable situation: for
human there is quite clear what is valid and what is invalid.
So code makes a lot of effort to handle valid (but possibly quite
unusual) cases.  User input is normally checked to give sensible
error message, but some things are deemed to tricky/expensive
to check.  Other routines are deemed "system level", and here
there us up to user/caller to respect the contract.

My embedded code consists of rather small systems, and normally
there are no explicit validity checks.  To clarify: when system
receives commands it recognizes and handles valid commands.
So there is implicit check: anything not recognized as valid
is invalid.  OTOH frequently there is nothing to do in case
of errors: if there are no display to print error message,
no persistent store to log erreor and shuting down is not helpful,
then what else potential error handler would do?

I do not check if 12-bit ADC really returns numbers in range.
My 'print_byte' routine takes integer argument and blindly
truncates it to 8-bit without worring about possible
spurious upper bits.  "Safety critical" folks my be worried
by such practice, but my embedded code is fairly non-critical.

> Do not handle error conditions -- because they can't exist (because
> you wrote the code and feel confident that you've anticipated
> every contingency -- including those for future upgrades).
> 
> Ignore compiler warnings -- surely you know better than a silly
> "generic" program!
> 
> Would you hire someone who viewed your product's quality (and
> your reputation) in this regard?

Well, you do not know what OP code is doing.  I would prefer
my code to be robust and I feel that I am doing resonably
well here.  OTOH, coming back to serial comunication, it
is not hard to design communication protocal such that in
normal operation there is no possibility for buffer
overflow.  It would still make sense to add a single line
to say drop excess characters.  But it does not make
sense to make big story of lack of this line.  In particular
issue that OP wanted to discuss is still valid.

> > Of course, if such changes were a problem he would need to
> > add test preventing writing to full buffer (he already have
> > test preventing reading from empty buffer).
> > 
> >> Buffer size is 128, for example.  in is 127, out is 127.
> >> What's that mean?
> > 
> > Empty buffer.
> 
> No, it means you can't sort out *if* there have been any characters
> received, based solely on this fact (and, what other facts are there
> to observe?)

Of course you can connect to system and change values of variables
in debugger, so specific values mean nothing.  I am telling
you what to protocal is.  If all part of system (including parts
that OP skipped) obey the protocal, then you have meaning above.
If something misbehaves (say cosmic ray flipped a bit), it does
not mean that protocal is incorrect.  Simply _if_ probability
of misbehaviour is too high you need to fix the system (add
radiation shielding, appropiate seal to avoid tampering with
internals, extra checks inside, etc).  But what/if to fix
something is for OP to decide.

> >>   Can you tell me what has happened prior
> >> to this point in time?  Have 127 characters been received?
> >> Or, 383?  Or, 1151?
> > 
> > Does not matter.
> 
> Of course it does!  Something has happened that the code MIGHT have
> detected in other circumstances (e.g., if uart_task had been invoked
> more frequently).  The world has changed and the code doesn't know it.
> Why write code that only *sometimes* works?

All code works only sometimes.  Parafrazing famous answer to
Napoleon: fisrt you need a processor.  There are a lot of
conditons so that code works as intended.  Granted, I would
not skip needed check in real code.  But this is obvious
thing to add.  You are somewhat making OP code as "broken
beyond repair".  Well, as discussion showed, OP had problem
using "volatile" and that IMHO is much more important to
fix.

> >> How many characters have been removed from the buffer?
> >> (same numeric examples).
> > 
> > The same as has been stored.  Point is that received is
> > always bigger or equal to removed and does not exceed
> > removed by more than 128.  So you can exactly recover
> > difference between received and removed.
> 
> If it can wrap, then "some data" can look like "no data".
> If "no data", then NOTHING has been received -- from the
> viewpoint of the code.
> 
> Tell me what prevents 256 characters from being received
> after .in (and .out) are initially 0 -- without any
> indication of their presence.  What "limits" the difference
> to "128"?  Do you see any conditionals in the code that
> do so?  Is there some magic in the hardware that enforces
> this?

That is the protocol.  How to avoid violation is different
matter: dropping characters _may_ be solution.  But dropping
characters means that some data is lost, and how to deal
with lost data is different issue.  As is OP code will loose
some old data.  It is OP problem to decide which failure
mode is more problematic and how much extra checks are
needed.

> This is how you end up with bugs in your code.  The sorts
> of bugs that you can witness -- with your own eyes -- and
> never reproduce (until the code has been released and
> lots of customers' eyes witness it as well).

IME it is issues that you can not prodict that catch you.
The above is obvious issue, and should not be a problem
(unless designer is seriously incompenent and misjudged
what can happen).

> >> The biggest practical limitation is that of expectations of
> >> other developers who may inherit (or copy) his code expecting
> >> the FIFO to be "well behaved".
> > 
> > Well, personally I would avoid storing to full buffer.  And
> > even on small MCU it is not clear for me if his "savings"
> > are worth it.  But his core design is sound.
> > 
> > Concerning other developers, I always working on assumption
> > that code is "as is" and any claim what it is doing are of
> > limited value unless there is convincing argument (proof
> > or outline of proof) what it is doing.
> 
> Ever worked on 100KLoC projects?  500KLoC?  Do you personally examine
> the entire codebase before you get started?

Of course I do not read all code before start.  But I accept
risc that code may turn out to be faulty and I may be forced
to fix or abandon it.  My main project has 450K wc lines.
I know that parts are wrong and I am working on fixing that
(which will probably involve substantial rewrite).  I worked
a little on gcc and I can tell you that only sure thing in
such projects is that there are bugs.  Of course, despite
bugs gcc is quite useful.  But I also met Modula 2 compiler
that carefuly checked programs for violation of language
rules, but miscompiled nested function calls.

>  Do you purchase source
> licenses for every library that you rely upon in your design?
> (or, do you just assume software vendors are infallible?)

Well, for several years I work exclusively with open source code.
I see a lot of defects.  While my experience with comercial codes
is limited I do not think that commercial codes have less defects
than open source ones.  In fact, there are reasons to suspect
that there are more defects in commercial codes.

> How would you feel if a fellow worker told you "yeah, the previous
> guy had a habit of cutting corners in his FIFO management code"?
> Or, "the previous guy always assumed malloc would succeed and
> didn't even build an infrastructure to address the possibility
> of it failing"

Well, there is a lot of bad code.  Sometimes best solution is simply
to throw it out.  In other cases (likely in your malloc scenario above)
there may be simple workaround (replace malloc by checking version).

> You could, perhaps, grep(1) for "malloc" or "FIFO" and manually
> examine those code fragments.

Yes, that one of possible appraches.

>  What about division operators?

I have a C parser.  In desperation I could try to search parse
tree or transform program.  Or, more likely decide that program
is broken beyond repair.

> Or, verifying that data types never overflow their limits?  Or...

Well, one thing is to look at structure of program.  Code may
look complicated, but some programs are reasonably testable:
few random inputs can give some confidence that "main"
execution path computes correct values.  Then you look if
you can hit limits.  Actually, much of my coding is in
arbitrary precision, so overflow is impossible.  Instead
program may run out of memory.  But there parts for speed
use fixed precision.  If I correctly computed limits
overflow is impossible.  But this is big if.

> > Fact that code
> > worked well in past system(s) is rather unconvincing.
> > I have seen small (few lines) pieces of code that contained
> > multiple bugs.  And that code was in "production" use
> > for several years and passed its tests.
> > 
> > Certainly code like FIFO-s where there are multiple tradeofs
> > and actual code tends to be relatively small deserves
> > examination before re-use.
> 
> It's not "FIFO code".  It's a UART driver.   Do you examine every piece
> of code that might *contain* a FIFO?  How do you know that there *is* a FIFO
> in a piece of code -- without manually inspecting it?  What if it is a
> FIFO mechanism but not explicitly named as a FIFO?
> 
> One wants to be able to move towards the goal of software *components*.
> You don't want to have to inspect the design of every *diode* that
> you use; you want to look at it's overall specifications and decide
> if those fit your needs.

Sure, I would love to see really reusable components.  But IMHO we
are quite far from that.  There are some things which are reusable
if you accept modest to severe overhead.  For example things tends
to compose nicely if you dynamically allocate everything and use
garbage collection.  But performace cost may be substantial.
And in embedded setting garbage collection may be unacceptable.
In some cases I have found out that I can get much better
speed joing things that could be done as composition of library
operations into single big routine.  In other cases I fixed
bugs by replacing composition of library routines by a single
routine: there were interactions making simple composition
incorrect.  Correct alterantive was single routine.

As I wrote my embedded programs are simple and small.  But I
use almost no external libraries.  Trying some existing libraries
I have found out that some produce rather large programs, linking
in a lot of unneeded stuff.  Of course, writing for scratch
will not scale to bigger programs.  OTOH, I feel that with
proper tooling it would be possible to retain efficiency and
small code size at least for large class of microntroller
programs (but existing tools and libraries do not support this).

> Unlikely that this code will describe itself as "works well enough
> SOME of the time..."
> 
> And, when/if you stumble on such faults, good luck explaining to
> your customer why it's going to take longer to fix and retest the
> *existing* codebase before you can get on with your modifications...

Commercial vendors like to say how good their progam are.  But
market reality is that program my be quite bad and still sell.

-- 
                              Waldek Hebisch

On 10/26/2021 5:20 PM, antispam@math.uni.wroc.pl wrote:
> Don Y <blockedofcourse@foo.invalid> wrote:
>> On 10/25/2021 2:32 PM, antispam@math.uni.wroc.pl wrote:
>>> Don Y <blockedofcourse@foo.invalid> wrote:
>>>> On 10/24/2021 4:14 AM, Dimiter_Popoff wrote:
>>>>>> Disable interrupts while accessing the fifo. you really have to.
>>>>>> alternatively you'll often get away not using a fifo at all,
>>>>>> unless you're blocking for a long while in some part of the code.
>>>>>
>>>>> Why would you do that. The fifo write pointer is only modified by
>>>>> the interrupt handler, the read pointer is only modified by the
>>>>> interrupted code. Has been done so for times immemorial.
>>>>
>>>> The OPs code doesn't differentiate between FIFO full and empty.
>>>
>>> If you read carefuly what he wrote you would know that he does.
>>> The trick he uses is that his indices may point outside buffer:
>>> empty is equal indices, full is difference equal to buffer
>>
>> Doesn't matter as any index can increase by any amount and
>> invalidate the "reality" of the buffer's contents (i.e.
>> actual number of characters that have been tranfered to
>> that region of memory).
> 
> AFAIK OP considers this not a problem in his application.

And I don't think I have to test for division by zero -- as
*my* code is the code that is passing numerator and denominator
to that operator, right?

Can you remember all of the little assumptions you've made in
any non-trivial piece of code -- a week later?  a month later?
6 months later (when a bug manifests or a feature upgrade
is requested)?

Do not check the inputs of routines for validity -- assume everything is
correct (cuz YOU wrote it to be so, right?).

Do not handle error conditions -- because they can't exist (because
you wrote the code and feel confident that you've anticipated
every contingency -- including those for future upgrades).

Ignore compiler warnings -- surely you know better than a silly
"generic" program!

Would you hire someone who viewed your product's quality (and
your reputation) in this regard?

> Of course, if such changes were a problem he would need to
> add test preventing writing to full buffer (he already have
> test preventing reading from empty buffer).
> 
>> Buffer size is 128, for example.  in is 127, out is 127.
>> What's that mean?
> 
> Empty buffer.

No, it means you can't sort out *if* there have been any characters
received, based solely on this fact (and, what other facts are there
to observe?)

>>   Can you tell me what has happened prior
>> to this point in time?  Have 127 characters been received?
>> Or, 383?  Or, 1151?
> 
> Does not matter.

Of course it does!  Something has happened that the code MIGHT have
detected in other circumstances (e.g., if uart_task had been invoked
more frequently).  The world has changed and the code doesn't know it.
Why write code that only *sometimes* works?

>> How many characters have been removed from the buffer?
>> (same numeric examples).
> 
> The same as has been stored.  Point is that received is
> always bigger or equal to removed and does not exceed
> removed by more than 128.  So you can exactly recover
> difference between received and removed.

If it can wrap, then "some data" can look like "no data".
If "no data", then NOTHING has been received -- from the
viewpoint of the code.

Tell me what prevents 256 characters from being received
after .in (and .out) are initially 0 -- without any
indication of their presence.  What "limits" the difference
to "128"?  Do you see any conditionals in the code that
do so?  Is there some magic in the hardware that enforces
this?

This is how you end up with bugs in your code.  The sorts
of bugs that you can witness -- with your own eyes -- and
never reproduce (until the code has been released and
lots of customers' eyes witness it as well).

>> The biggest practical limitation is that of expectations of
>> other developers who may inherit (or copy) his code expecting
>> the FIFO to be "well behaved".
> 
> Well, personally I would avoid storing to full buffer.  And
> even on small MCU it is not clear for me if his "savings"
> are worth it.  But his core design is sound.
> 
> Concerning other developers, I always working on assumption
> that code is "as is" and any claim what it is doing are of
> limited value unless there is convincing argument (proof
> or outline of proof) what it is doing.

Ever worked on 100KLoC projects?  500KLoC?  Do you personally examine
the entire codebase before you get started?  Do you purchase source
licenses for every library that you rely upon in your design?
(or, do you just assume software vendors are infallible?)

How would you feel if a fellow worker told you "yeah, the previous
guy had a habit of cutting corners in his FIFO management code"?
Or, "the previous guy always assumed malloc would succeed and
didn't even build an infrastructure to address the possibility
of it failing"

You could, perhaps, grep(1) for "malloc" or "FIFO" and manually
examine those code fragments.  What about division operators?
Or, verifying that data types never overflow their limits?  Or...

> Fact that code
> worked well in past system(s) is rather unconvincing.
> I have seen small (few lines) pieces of code that contained
> multiple bugs.  And that code was in "production" use
> for several years and passed its tests.
> 
> Certainly code like FIFO-s where there are multiple tradeofs
> and actual code tends to be relatively small deserves
> examination before re-use.

It's not "FIFO code".  It's a UART driver.   Do you examine every piece
of code that might *contain* a FIFO?  How do you know that there *is* a FIFO
in a piece of code -- without manually inspecting it?  What if it is a
FIFO mechanism but not explicitly named as a FIFO?

One wants to be able to move towards the goal of software *components*.
You don't want to have to inspect the design of every *diode* that
you use; you want to look at it's overall specifications and decide
if those fit your needs.

Unlikely that this code will describe itself as "works well enough
SOME of the time..."

And, when/if you stumble on such faults, good luck explaining to
your customer why it's going to take longer to fix and retest the
*existing* codebase before you can get on with your modifications...

Don Y <blockedofcourse@foo.invalid> wrote:
> On 10/25/2021 2:32 PM, antispam@math.uni.wroc.pl wrote:
> > Don Y <blockedofcourse@foo.invalid> wrote:
> >> On 10/24/2021 4:14 AM, Dimiter_Popoff wrote:
> >>>> Disable interrupts while accessing the fifo. you really have to.
> >>>> alternatively you'll often get away not using a fifo at all,
> >>>> unless you're blocking for a long while in some part of the code.
> >>>
> >>> Why would you do that. The fifo write pointer is only modified by
> >>> the interrupt handler, the read pointer is only modified by the
> >>> interrupted code. Has been done so for times immemorial.
> >>
> >> The OPs code doesn't differentiate between FIFO full and empty.
> > 
> > If you read carefuly what he wrote you would know that he does.
> > The trick he uses is that his indices may point outside buffer:
> > empty is equal indices, full is difference equal to buffer
> 
> Doesn't matter as any index can increase by any amount and
> invalidate the "reality" of the buffer's contents (i.e.
> actual number of characters that have been tranfered to
> that region of memory).

AFAIK OP considers this not a problem in his application.
Of course, if such changes were a problem he would need to
add test preventing writing to full buffer (he already have
test preventing reading from empty buffer).

> Buffer size is 128, for example.  in is 127, out is 127.
> What's that mean?

Empty buffer.

>  Can you tell me what has happened prior
> to this point in time?  Have 127 characters been received?
> Or, 383?  Or, 1151?

Does not matter.

> How many characters have been removed from the buffer?
> (same numeric examples).

The same as has been stored.  Point is that received is
always bigger or equal to removed and does not exceed
removed by more than 128.  So you can exactly recover
difference between received and removed.

> The biggest practical limitation is that of expectations of
> other developers who may inherit (or copy) his code expecting
> the FIFO to be "well behaved".

Well, personally I would avoid storing to full buffer.  And
even on small MCU it is not clear for me if his "savings"
are worth it.  But his core design is sound.

Concerning other developers, I always working on assumption
that code is "as is" and any claim what it is doing are of
limited value unless there is convincing argument (proof
or outline of proof) what it is doing.  Fact that code
worked well in past system(s) is rather unconvincing.
I have seen small (few lines) pieces of code that contained
multiple bugs.  And that code was in "production" use
for several years and passed its tests.

Certainly code like FIFO-s where there are multiple tradeofs
and actual code tends to be relatively small deserves
examination before re-use.

-- 
                              Waldek Hebisch

On 10/25/21 2:15 PM, pozz wrote:
> Il 23/10/2021 18:09, David Brown ha scritto:
> [...]
>> Marking "in" and "buf" as volatile is /far/ better than using a critical
>> section, and likely to be more efficient than a memory barrier.  You can
>> also use volatileAccess rather than making buf volatile, and it is often
>> slightly more efficient to cache volatile variables in a local variable
>> while working with them.
> 
> I think I got your point, but I'm wondering why there are plenty of 
> examples of ring-buffer implementations that don't use volatile at all, 
> even if the author explicitly refers to interrupts and multithreading.
> 
> Just an example[1] by Quantum Leaps. It promises to be a *lock-free* (I 
> think thread-safe) ring-buffer implementation in the scenario of single 
> producer/single consumer (that is my scenario too).
> 
> In the source code there's no use of volatile. I could call 
> RingBuf_put() in my rx uart ISR and call RingBuf_get() in my mainloop code.
> 
>  From what I learned from you, this code usually works, but the standard 
> doesn't guarantee it will work with every old, current and future 
> compilers.
> 
> 
> 
> [1] https://github.com/QuantumLeaps/lock-free-ring-buffer

The issue with not using 'volatile' (or some similar memory barrier) is 
that without it, the implementation is allowed to delay the actual write 
of the results into the variable.

If optimization is limited to just within a single translation unit, you 
can force it to work by having the execution path leave the translation 
unit, but with whole program optimization, it is theoretically possible 
that the implementation sees that the thread of execution NEVER needs it 
to be spilled out of the registers to memory, so the ISR will never see 
the change.

Il 25/10/2021 20:33, Niklas Holsti ha scritto:
> On 2021-10-25 20:52, pozz wrote:
>> Il 25/10/2021 17:34, Niklas Holsti ha scritto:
>>> On 2021-10-25 16:04, Dimiter_Popoff wrote:
>>>> On 10/25/2021 11:09, Niklas Holsti wrote:
>>>>> On 2021-10-24 23:27, Dimiter_Popoff wrote:
>>>>>> On 10/24/2021 22:54, Don Y wrote:
>>>>>>> On 10/24/2021 4:14 AM, Dimiter_Popoff wrote:
>>>>>>>>> Disable interrupts while accessing the fifo. you really have to.
>>>>>>>>> alternatively you'll often get away not using a fifo at all,
>>>>>>>>> unless you're blocking for a long while in some part of the code.
>>>>>>>>
>>>>>>>> Why would you do that. The fifo write pointer is only modified by
>>>>>>>> the interrupt handler, the read pointer is only modified by the
>>>>>>>> interrupted code. Has been done so for times immemorial.
>>>>>>>
>>>>>>> The OPs code doesn't differentiate between FIFO full and empty.
> 
> 
> (I suspect something is not quite right with the attributions of the 
> quotations above -- Dimiter probably did not suggest disabling 
> interrupts -- but no matter.)
> 
>  �� [snip]
> 
> 
>> When I have a small (<256) power-of-two (16, 32, 64, 128) buffer (and 
>> this is the case for a UART receiving ring-buffer), I like to use this 
>> implementation that works and doesn't waste any element.
>>
>> However I know this isn't the best implementation ever and it's a pity 
>> the thread emphasis has been against this implementation (that was 
>> used as *one* implementation just to have an example to discuss on).
>>
>> The main point was the use of volatile (and other techniques) to 
>> guarantee a correct compiler output, whatever legal (respect the C 
>> standard) optimizations the compiler thinks to do.
>>
>> It seems to me the arguments againts or for volatile are completely 
>> indipendent from the implementation of ring-buffer.
> 
> 
> Of course "volatile" is needed, in general, whenever anything is written 
> in one thread and read in another. The issue, I think, is when 
> "volatile" is _enough_.
> 
> I feel that detection of a full buffer (FIFO overflow) is required for a 
> proper ring buffer implementation, and that has implications for the 
> data structure needed, and that has implications for whether critical 
> sections are needed.
> 
> If the FIFO implementation is based on just two pointers (read and 
> write), and each pointer is modified by just one of the two threads 
> (main thread = reader, and interrupt handler = writer), and those 
> modifications are both "volatile" AND atomic (which has not been 
> discussed so far, IIRC...), then one can do without a critical region. 
> But then detection of a full buffer needs one "wasted" element in the 
> buffer.

Yeah, this is exactly the topic of my original post. Anyway it seems 
what you say isn't always correct. As per C standard, the compiler could 
reorder instructions that involve non-volatile data. So, even in your 
simplified scenario (atomic access for indexes), volatile for the head 
only (that ISR changes) is not sufficient.

The function called in the mainloop and that get data from the buffer 
access three variables: head (changed in ISR), tail (not changed in ISR) 
and buf[] (written in ISR ad read in mainloop).

The get function firstly check if some data is available in the FIFO and 
*next* read from buf[]. However compiler could rearrange instructions so 
reading from buf[] at first and then checking FIFO empty condition.
If the compiler goes this way, errors could occur during execution.

My original question was exactly if this could happen (without breaking 
C specifications) and, if yes, how to avoid this: volatile? critical 
section? memory barrier?

David Brown said this is possible and suggested to access both head and 
buf[] as volatile in get() function, forcing the compiler to respect the 
order of instructions.


> To avoid the wasted element, one could add a "full"/"not full" Boolean 
> flag. But that flag would be modified by both threads, and should be 
> modified atomically together with the pointer modifications, which (I 
> think) means that a critical section is needed.