Booting Linux on ARM Processor in Depth

• ∙Kernel entry point is in arch/arm/kernel/head.S
  ∙This file starts with a check (TEXT_ADDR & 0xffff) != 0x8000, this is because page tables are placed 16k below KERNEL_RAM_VADDR
  ∙arch/arm/kernel/head.S: stext
  ∙enable supervisor mode
  ∙disable IRQ
  ∙call __lookup_processor_type
  ∙defined in arch/arm/kernel/head-common.S
  ∙search list of supported processor types __proc_info_begin
  ∙kernel may be built to support more than one processor type
  ∙list of proc_info_list structs
  ∙defined in arch/arm/mm/proc-arm926.S and other corresponding proc-*.S files
  ∙linked into list by section declaration: .section ".proc.info.init"
  ∙return pointer to proc_info_list struct corresponding to processor if found, or loop in
  error if not
  ∙call __lookup_machine_type
  ∙defined in arch/arm/kernel/head-common.S
  ∙search list of supported machines (boards)
  ∙kernel may be built to support more than one board
  ∙list of machine_desc structs
  ∙machine_desc struct for boards defined in board-specific file vx115_vep.c
  ∙linked into list by section declaration that's part of MACHINE_DESC macro
  ∙return pointer to machine_desc struct corresponding to machine (board)
  ∙call __vet_atags
  
  

  ∙defined in arch/arm/kernel/head-common.S

  ∙validate the R2 reg, which locates to the structure that holds the parameters passed
  by the bootloader.

  ∙Return either valid atags pointer or zero

  ∙call __create_page_tables to set up initial MMU tables

  ∙Return – R4 containing the physical page table address

  ∙set lr to __enable_mmu, r13 to address of __switch_data

  ∙lr and r13 used for jumps after the following calls

  ∙__switch_data defined in arch/arm/kernel/head-common.S

  ∙call the __cpu_flush function pointer in the previously returned proc_info_list struct

  ∙offset is #PROCINFO_INITFUNC into struct

  ∙this function is __arm926_setup for the ARM 926EJ-S, defined in arch/arm/mm/proc-arm926.S

  ∙initialize caches, writebuffer jump to lr, previously set to address of __enable_mmu

  ∙__enable_mmu

  ∙set page table pointer (TTB) in MMU hardware so it knows where to start page-table walks

  ∙enable MMU so running with virtual addresses

  ∙jump to r13, previously set to address of __switch_data, whose first field is address of __mmap_switched

  ∙__switch_data defined in arch/arm/kernel/head-common.S

  ∙arch/arm/kernel/head-common.S: __mmap_switched

  . copy data segment to RAM

  . zero BSS

  . branch to start_kernel

  
  

  
  

  Start_kernel – (init/main.c)

  ∙tick_init – (kernel/time/tick-common.c)

  ∙setup_arch (arch/arm/kernel/setup.c)

  ∙Locate processor in the list of supported processors and print CPU version and name

  ∙Setup stack for exception handlers

  ∙Setp page tables

  ∙setup_command_line (init/main.c)

  ∙Store the unmodified and modified command line parameters for future ref

  ∙build_all_zonelists (mm/page_alloc)

  ∙Construct zonelist for each memory zone(DMA, DMA32, NORMAL, HIGH_MEMORY, MOVABLE). Zonelist specifies the zones/nodes to visit when allocation request for a selected zone/node is not met. This is called overflow or fallback.

  ∙setup_log_buf (kernel/printk.c)

  ∙Allocate memory for log_buf, free early log_buf and copy early log buf to newly allocated log_buf

  ∙mm_init (init/main.c)

  ∙Initialize memory allocators. Calculate available, reserved and highmem.

  ∙sched_init (kernel/sched.c)

  ∙Setup the scheduler, note we still have not enabled preemption and interrupts.

  ∙preempt_disable(include/linux/preempt.h)

  ∙Preemption is enabled, and in the previous step scheduler is also initialized.

  ∙init_IRQ (arch/arm/kernel/irq.c)

  ∙init_timers (kernel/timer.c)

  ∙hrtimers_init (kernel/hrtimer.c)

  ∙softirq_init (kernel/softirq.c)

  ∙local_irq_enable(include/linux/irqflags.h)

  ∙console_init (drivers/char/tty_io.c)

  ∙vfs_caches_init (fs/dcache.c)

  ∙mnt_init (fs/namespace.c)

  ∙init_rootfs (fs/ramfs/inode.c)

  ∙init_mount_tree (fs/namespace.c)

  ∙bdev_cache_init (fs/block_dev.c)

  ∙chrdev_init (fs/char_dev.c:)

  ∙signals_init (kernel/signal.c)

  ∙rest_init (init/main.c)

  ∙kernel_thread ( arch/arm/kernel/process.c)

  ∙kernel_thread() creates a kernel init thread and control is given to kernel_init().

  ∙

  ∙Kthreadd

  ∙Is a kernel thread deamon, all kthreads are forked off this thread

  ∙kernel_init (init/main.c)

  ∙smp_prepare_cpus (arch/arm/smp.c)

  ∙smp_init(kernel/smp.c)

  ∙do_basic_setup (init/main.c)

  ∙driver_init (drivers/base/init.c)

  ∙do_initcalls (init/main.c)

  ∙Calls all subsytems init functions, __initcall_start doesn’t appear anywhere in *.c or *.h file, it is present in linker script (*.lds)

  ∙prepare_namespace (init/do_mounts.c)

  ∙Decides where to mount and mounts the ramdisk

  ∙init_post (init/main.c)

  ∙Initial boot is done clear all initmem segments