Thanks everyone for the answers. Very useful info.

-- Pete

Brian Silverman wrote:
> psommerfeld@gmail.com wrote:
> > Another question is how small can a functional rootfs be? I am hoping I
> > can get it under 20 MB.
>
> I worked on linux project last year, which may give you some
> guidelines...  We had a very functional rootfs that fit snuggly into
> 4MB (with JFFS2).  We kept our app and additional packages (the most
> notable large ones were Python and Qt) in a separate partition in flash
> - that took up a little over 20MB (also JFFS2, and we got better than
> 2:1 compression).
>
> Of course, it all depends on what you want...
>
> I'm now again working on a linux project, and I've just the other day
> used Busybox to get a minimal rootfs (NOTHING, except init and sh).
> It's only 75K.  Of course, the caveat is you need glibc, too, and
> that's 2MB.
> 
> -bri

psommerfeld@gmail.com wrote:
> Another question is how small can a functional rootfs be? I am hoping I
> can get it under 20 MB.

I worked on linux project last year, which may give you some
guidelines...  We had a very functional rootfs that fit snuggly into
4MB (with JFFS2).  We kept our app and additional packages (the most
notable large ones were Python and Qt) in a separate partition in flash
- that took up a little over 20MB (also JFFS2, and we got better than
2:1 compression).

Of course, it all depends on what you want...

I'm now again working on a linux project, and I've just the other day
used Busybox to get a minimal rootfs (NOTHING, except init and sh).
It's only 75K.  Of course, the caveat is you need glibc, too, and
that's 2MB.

-bri

larwe wrote:
> psommerfeld@gmail.com wrote:
>
> > I have some questions about u-boot for a project I'm working on. The uP
> > is an ARM9.
>
> In answer to most of your questions, please refer to
> <http://www-128.ibm.com/developerworks/library/pa-migrate2/> which is
> an article I wrote last year on related topics.
>
> It is irksome to run Linux off a write-protected root filesystem.
> Possible, yes - with tweaking  - but it's much easier just to load the
> root filesystem into a RAMdisk and run entirely out of RAM. Faster,
> too.
>
> As far as memory footprint goes, are you asking about flash storage
> requirements or RAM requirements? You can fit a fully functional Linux
> distribution and applications into 8Mb without difficulty. (The root
> filesystem can be compressed, remember. I've implemented turnkey
> x86-based Linux devices on a 4Mb CompactFlash). I would suggest 64Mb
> RAM, more if you intend to use X. It can be squeezed into less, but
> again it's irritating.

There are no set rules for flash vs. ram.  Very often, it's a
trade off between costs and limitations.  Nowadays, flash
is sometimes cheaper than ram.  There are still many systems
with limited ram space, but flash is virtually unlimited.

You can compress everything on flash, but need more ram to
decompress and run them anyway.

Clifford Heath wrote:

> > Semantic gripe: "VFAT" is the name of a Microsoft filesystem driver.
>
> Ok, it's wrong that linux "mount" uses "-t vfat" then.

The Linux kernel vfat driver is so named because it is compatible with
the LFN support in current versions of the Microsoft VFAT driver.

The Microsoft driver was originally introduced in Windows for
Workgroups 3.11, at which time it was a VxD. When Win95 was released,
this VxD was rolled into the master VMM32 VxD under the name "vfat",
"Virtual FAT Driver" and LFN support was added.

This is a semantic issue but I am old and curmudgeonly, and expected to
be colorful.

larwe wrote:
> Semantic gripe: "VFAT" is the name of a Microsoft filesystem driver.

Ok, it's wrong that linux "mount" uses "-t vfat" then.

> ...inexperienced end-users can put the card into a PC

Yes, I thought that was the most sensible way to do it also.
Thanks for the confirmation.

Clifford Heath wrote:

> > I've implemented turnkey x86-based Linux devices on a 4Mb CompactFlash).
>
> Did you use VFAT on the CF or a Linux FS? With which bootloader?

Semantic gripe: "VFAT" is the name of a Microsoft filesystem driver.
The media format is FAT12, FAT16, or FAT32.

I used FAT (12, on such a small card) and SYSLINUX as the loader. A
strong side benefit of this approach is that the kernel and rootfs are
simply DOS-readable files. This means that inexperienced end-users can
put the card into a PC, copy across two new files, and the upgrade is
complete.

larwe wrote:
> I've implemented turnkey x86-based Linux devices on a 4Mb CompactFlash).

Did you use VFAT on the CF or a Linux FS? With which bootloader?

psommerfeld@gmail.com wrote:

> I have some questions about u-boot for a project I'm working on. The uP
> is an ARM9.

In answer to most of your questions, please refer to
<http://www-128.ibm.com/developerworks/library/pa-migrate2/> which is
an article I wrote last year on related topics.

It is irksome to run Linux off a write-protected root filesystem.
Possible, yes - with tweaking  - but it's much easier just to load the
root filesystem into a RAMdisk and run entirely out of RAM. Faster,
too.

As far as memory footprint goes, are you asking about flash storage
requirements or RAM requirements? You can fit a fully functional Linux
distribution and applications into 8Mb without difficulty. (The root
filesystem can be compressed, remember. I've implemented turnkey
x86-based Linux devices on a 4Mb CompactFlash). I would suggest 64Mb
RAM, more if you intend to use X. It can be squeezed into less, but
again it's irritating.

linnix wrote:
> psommerfeld@gmail.com wrote:
>> Hi,
>>
>> I have some questions about u-boot for a project I'm working on. The
>> uP is an ARM9.
>>
>> I am wondering if it's possible to use u-boot to two things in one 
>> boot-up:
>> - to create a root filesystem on an SD card (I'm not interested in the 
>> secure digital functionality)

You can use U-boot to store a file system, not to create it.
Whether you can use your specific ARM9 or not is depending on
if u-boot supports the SD-interface on your chip.
If it doesn't then you have to write a driver.

>> - to boot Linux using a Linux kernel image from inside this newly created 
>> root filesystem

May beed modifications to U-boot, but I know of people that have done such 
modifications.


>> Another question is how small can a functional rootfs be? I am
>> hoping I can get it under 20 MB.
>
> You can have a bare bone system in 8M or so.  But there is not much
> you can do other than starting a custom app.
>

You can have a complete system with Linux kernel and small file system in 4 
MB
but you would have very little room for your own application

>> And a final question is: Is it possible to have a read-only
>> filesystem? Or do some things (ie. /dev devices) have to be
>> read/write?
>
> No, /dev can be read-only, but /tmp amd /var must be writable.

Yes, in a way,
if you use a RAM disk, then the underlying flash image is not writable.

>
>>
>> Thanks, Pete

-- 
Best Regards,
Ulf Samuelsson
ulf@a-t-m-e-l.com
This message is intended to be my own personal view and it
may or may not be shared by my employer Atmel Nordic AB 

linnix wrote:
> No, /dev can be read-only, but /tmp amd /var must be writable.

But can be resident in a mounted ramdisk. I'm about to go
down this path also, after painstakingly wiring an IDE cable
to a CF socket so I can boot Linux from CF on a PC/104 P133.