An overview of Linux Boot Process for Embedded Systems

Kunal SinghDecember 25, 200811 comments

This Text provides an insight in to the Embedded Linux Boot Process. Reader should have a basic Knowledge of Boot Process in general and should be familiar with Embedded Linux Boot Process.


(1) Software components Involved in Embedded Linux Boot Process
    (a) Bootloader
    (b) kernel Image
    (c) root file system - either an initrd image or a NFS location

(2) Steps during Booting process of a conventional PC
    (a) System Startup - PC-BIOS/BootMonitor
    (b) Stage1 bootloader - MBR
    (c) stage2 bootloader - LILO,GRUB etc
    (d) kernel - Linux
    (e) init - The User Space

(3) Booting process for an Emebedded Systems
    (a) Instead of BIOS you will run program from a fixed location in Flash
    (b) The components involved in the first steps of PC boot process are combined in to a single "boot strap firmware", called "boot loader".
    (c) Bootloader also provides additional features useful for development & debugging.

(4) What is System Startup?
    [ Exact process depends on the Target Hardware ]
    (a) CPU starts exectuing BIOS at address 0xFFFF0   
    (b) POST (Power On Self Test) - is the first step of BIOS.
    (c) run time services - involve local device enumeration and
    (d) After the POST is complete, POST related code is flushed out of
memory. But BIOS runtime services remain in memory and are available to
the target OS.
    (e) The runtime searches for devices that are both active and
bootable in order of preference defined in CMOS settings
    (f) The primary boot loader is loaded and BIOS returns control to it

(5) The Primary boot loader
    (a) Performs  few optional initializations
    (b) Its main job is to Load the secondary boot loader

(6) Secondary boot loader
    (a) The Second Stage boot loader loads the Linux & an optional
initial RAM disk in to the memory
    (b) on PC, the initrd is used as a temporary root files
system, before final root file system gets mounted. However, on embedded
systems, the initrd is generally the final root file system.
    (c) The secondary loader passes control to the kernel image - kernel is
decompressed & initialized
    (d) So, the secondary boot loaderis the kernel Loader, can also load optional initial RAM disk (initrd), and then invokes the kernel image

(7) Kernel Invocation
    (a) As the kernel is invoked, it performs the checks on system hardware, enumerates the attached hardware devices, mounts the root device
    (b) Next it loads the necessary kernel modules
    (c) First user-space program (init) now starts and high-level system initialization is performed
    (d) The Kernel Invocation Process is similar on Embedded Linux Systems as well as on PC. We will discuss this in detail in following text.

(8) Kernel Image
    (a) Is typically a compressed image [zlib compression]
    (b) Typical named a zImage (<512 KB) or bzImage (> 512 KB)
    (c) At the head of this image (in file head.S) is a routine that does some minimal amount of hardware set up and then decompresses the kernel contained in the kernel image and places in to high memory (high memory & low memory)


(1) Kernel Invocation Process - A Summary

    (a) zImage Entry Point   
    (d) CLEAR BSS

    [Now We have set up the run time environment for the code to be
executed next]

    (f) Execute the decompressed Kernel Image
        - ENABLE MMU
        - DETECT CPU (& optoinal FPU) TYPE & SAVE THIS INFO

    [With above set up, we are now ready to execute a general C
Code. Till now we only executed asm routines.]

    (g) The First Kernel C function
        - LOAD INITRD

    [ The above code is being executed by swapper process, the one
with pid 0]

    (h) The Init Process
        - Init process is with pid 1
        - Invoke Scheduler

(2) zImage Entry point
    (a) This is a call to the absolute physical address by boot loader

    - Refer to file arch/***/boot/compressed/head.S: start() in kernel source.
    - For the ARM process this is "arch/arm/boot/compressed/head.S: start()"

    (b) start() performs
        - basic hardware set up
        - basic environment set up
        - clears bss
        - calls the decompress_kernel()

(3) Decompressing Kernel Image
    (a) This is a call to arch/***/boot/compressed/misc.c: decompress_kernel()
        - This function decompresses the kernel image, stores it in to the RAM & returns the address of decompressed image in RAM.
    (b) on ARM processor this maps to "arch/arm/boot/compressed/misc.c: decompress_kernel()" routine.

(4) Execute the decompressed Kernel Image
    (a) After we have got the (uncompressed) kernel image in RAM, we execute it.
    (b) Execution starts with call_kernel() function call [from start()].
    (c) call_kernel() will start executing the kernel code, from Kernel
entry point.
    (d) arch/***/kernel/head.S contains the kernel entry point.
        - separate entry points for Master CPU and Secondary CPUs (for SMP
        - This code is in asm
        - Page Tables are Initialized & MMU is enabled.
        - type of CPU alongwith optional FPU is detected and stored
        - For Master CPU; start_kernel(), which is the first C function to
be executed in kernel, is called.
        - For secondary CPUs (on an SMP system); secondary_start_kernel()
is the first C function to be called.
    (e) On ARM process it maps to "arch/arm/kernel/head.S
        - Contains kernel ENTRY points for master and secondary CPU.
        - For Master CPU "mmap_switched()" is called as soon as mmu gets enabled. The mmap_switched() saves the CPU info makes a call to start_kernel()
        - For Secondary CPU "secondary_start_kernel()" is called as soon as MMU gets enabled.

(5) The first kernel C function
    (a) The start_kernel() function is being executed by the swapper process.
    (b) Refer to init/main.c: start_kernel() in the kernel source.
    (c) start_kernel():
        - a long list of initialization functions are called: this sets up interrupts, performs further memory configuration & loads the initrd.
        - calls rest_init() in the End.
    (d) econdary_start_kernel() for secondary CPUs (on SMP systems).
        - arch/***/kernel/smp.c: secondary_start_kernel()
        - for ARM, arch/arm/kernel/smp.c
        - there is not rest_init() call for secondary CPUs.

(6) Init process
    (a) Refer to init/main.c: rest_init() in kernel source.
    (b) Executed only on the Master CPU
    (c) rest_init() forks new process by calling kernel_thread() function
    (d) kernel_thread(kern_init,*,*); kern_init has PID-1
    (e) kern_init() will call the initialization scripts.
    (f) kernel_thread() defined in "arch/***/kernel/process.c: kernel_thread()".
    (g) on ARM, arch/arm/kernel/process.c

(7) Invoke scheduler
    (a) The rest_init() calls cpu_idle() in end [after it is done creating the init process]
    (b) For the Secondary CPUs (on SMP systems), cpu_idle is directly
called from secondary_start_kernel [no step-5 & hence no init process].
    (c) cpu_idle() defined in "arch/***/kernel/process.c: cpu_idle()".
    (d) on ARM, arch/arm/kernel/process.c

(8) initrd image
    (a) The initrd serves as a temporary root file system in RAM & allows the kernel to fully boot without having to mount and physical disks. Since the necessary modules needed to interface with peripherals
can be part of initrd the kernel can be very small.
    (b) pivot_root() routine: the root files system is pivoted where the initrd root file system is unmounted & the real root file system is mounted.
    (c) In any embedded system, the initrd could be the root file system.



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Previous post by Kunal Singh:
   Building Linux Kernel for Desktops


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Comment by ShikhaJune 4, 2009
Your information is really very helpful.......... Actually i am new to uclinux and i am using ax4510 board. When i connect my board to pc;s com port .in hyperterminal nothing is displaying .i think it indicates device can't be opened ......... perhaps reason for this is that bios is not present in board........if it is like that so how can i load new flash in board......plz help me out
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Comment by breadDecember 15, 2009
Hi, Thank you for this precious summary for us. One question is that what does the kernel do in the step of the BASIC HARDWARE SETUP at the beginng?
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Comment by 8mohan8March 17, 2010
I am going to enter into embedded field throgh my higher situdies .... i heared as C ,Linux ,Electronics knowledge are enough to this field .... is it true or not?????
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Comment by emBGeekMarch 22, 2010
really good info , i will use for next interview
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Comment by rxsApril 28, 2010
Very nice good information............... thanks.......
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Comment by rxsApril 28, 2010
I am new to this area . do u know what are all steps performs in Uboot during start time .. I am going to work on at91rm9200 platform so I need to port uboot pls help me ,,
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Comment by Sue JangMay 6, 2010
Hi, I am working on at91sam9g20 platform, I found two boards, one of which i unable to load kernel after uboot. -> there i can load the kernel to nand flash. but while loading it is showing CRC error. second board is failing to fail to load rfs-> some segmentation fault is coming but not consistently. It is failing to load RFS. But when i disable cache in kernel, i can load both kernel and RFS. My image is correct and it is working on other board. I wanna foud the root cause of this problem. Is it a bad at91sam9g20 part no? is so then how can i test it on uboot?? please tell the concept of cache? is it on controller side (SRAM)?? please reply fast ASAP?? send one copy of ur reply to my mail also
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Comment by bvasurajJuly 14, 2010 Dear Vikas, As Since you are loading in to the NAND Flash, these are the common errors occurs most of the time. Please make sure to erase the NAND Flash before you load the new kernel / rootfs on it. You can reach me at (9535504414)
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Comment by sriharsha050484November 1, 2010
can u sent me the steps after power is on for embedded linux up to uboot
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Comment by sriharsha050484November 10, 2010
as iam using comcerto 1000 chipset. now can u tell me the boot sequence.
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Comment by sriharsha050484November 10, 2010
how can we find unwanted library files during booting and how can we block and allow that files after booting is finished

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