ARM linux核心啟動時幾個關鍵地址

 

1.       核心啟動地址
1.1.   名詞解釋
ZTEXTADDR

解壓代碼啟動並執行開始地址。沒有物理地址和虛擬位址之分，因為此時MMU處於關閉狀態。這個地址不一定時RAM的地址，可以是支援讀寫定址的flash等儲存中介。

Start address of decompressor. here's no point in talking about virtual or physical addresses here, since the MMU will be off at the time when you call the decompressor code. You normally call   the kernel at this address to
start it booting. This doesn't have to be located in RAM, it can be in flash or other read-only or      read-write addressable medium.

      

ZRELADDR

       核心啟動在RAM中的地址。壓縮的核心映像被解壓到這個地址，然後執行。

This is the address where the decompressed kernel will be written, and eventually executed. The following constraint must be valid:

              __virt_to_phys(TEXTADDR) == ZRELADDR

       The initial part of the kernel is carefully coded to be position independent.

TEXTADDR

       核心啟動的虛擬位址，與ZRELADDR相對應。一般核心啟動的虛擬位址為RAM的第一個bank地址加上0x8000。

       TEXTADDR = PAGE_OFFSET + TEXTOFFST

       Virtual start address of kernel, normally PAGE_OFFSET + 0x8000.This is where the kernel image ends up. With the latest kernels, it must be located at 32768 bytes into a 128MB region. Previous kernels placed a restriction
of 256MB here.

TEXTOFFSET

       核心位移地址。在arch/arm/makefile中設定。

PHYS_OFFSET

       RAM第一個bank的物理起始地址。

Physical start address of the first bank of RAM.

PAGE_OFFSET

RAM第一個bank的虛擬起始地址。

       Virtual start address of the first bank of RAM. During the kernel

       boot phase, virtual address PAGE_OFFSET will be mapped to physical

       address PHYS_OFFSET, along with any other mappings you supply.

       This should be the same value as TASK_SIZE.

1.2.   核心啟動地址確定
核心啟動引導地址由bootp.lds決定。 Bootp.lds : arch/arm/bootp

OUTPUT_ARCH(arm)

ENTRY(_start)

SECTIONS

{

. = 0;

.text : {

   _stext = .;

   *(.start)

   *(.text)

   initrd_size = initrd_end - initrd_start;

   _etext = .;

}

}

由上 .= 0可以確定解壓代碼啟動並執行開始地址在0x0的位置。ZTEXTADDR的值決定了這個值得選取。

Makefile : arch/arm/boot/compressed

如果設定核心從ROM中啟動的話，可以在make menuconfig 的配置介面中設定解壓代碼的起始地址，否則解壓代碼的起始地址為0x0。實際上，預設從ROM啟動時，解壓代碼的起始地址也是0x0。

feq ($(CONFIG_ZBOOT_ROM),y)

ZTEXTADDR := $(CONFIG_ZBOOT_ROM_TEXT)

ZBSSADDR    := $(CONFIG_ZBOOT_ROM_BSS)

else

ZTEXTADDR :=0                                                       ZBSSADDR := ALIGN(4)

endif

SEDFLAGS    = s/TEXT_START/$(ZTEXTADDR)/;s/BSS_START/$(ZBSSADDR)/                                                                                                                                             

……

$(obj)/vmlinux.lds: $(obj)/vmlinux.lds.in arch/arm/mach-s3c2410/Makefile .config

       @sed "$(SEDFLAGS)" < $< > $@

@sed "$(SEDFLAGS)" < $< > $@ 規則將TEXT_START設定為ZTEXTADDR。TEXT_START在arch/arm/boot/compressed/vmlinux.lds.in 中被用來設定解壓代碼的起始地址。

OUTPUT_ARCH(arm)

ENTRY(_start)

SECTIONS

{

. = TEXT_START;  

_text = .;

.text : {

    _start = .;

    *(.start)

    *(.text)

*(.text.*)

……

}

}

核心的編譯依靠vmlinux.lds，vmlinux.lds由vmlinux.lds.s 產生。從下面代碼可以看出核心啟動的虛擬位址被設定為PAGE_OFFSET + TEXT_OFFSET，而核心啟動的物理地址ZRELADDR在arch/arm/boot/Makefile中設定。

OUTPUT_ARCH(arm)

ENTRY(stext)

SECTIONS

{

#ifdef CONFIG_XIP_KERNEL

       . = XIP_VIRT_ADDR(CONFIG_XIP_PHYS_ADDR);

#else

       . = PAGE_OFFSET + TEXT_OFFSET;   

#endif

       .init : {                  /* Init code and data             */

              _stext = .;

                     _sinittext = .;

                     *(.init.text)

                     _einittext = .;

……

       }

}

# arch/arm/boot/Makefile

# Note: the following conditions must always be true:

#   ZRELADDR == virt_to_phys(PAGE_OFFSET + TEXT_OFFSET)

#   PARAMS_PHYS must be within 4MB of ZRELADDR

#   INITRD_PHYS must be in RAM

ZRELADDR    := $(zreladdr-y)                                      

#---> zrealaddr-y is specified with 0x30008000 in arch/arm/boot/makefile.boot

PARAMS_PHYS := $(params_phys-y)

INITRD_PHYS := $(initrd_phys-y)

export ZRELADDR INITRD_PHYS PARAMS_PHYS

通過下面的命令編譯核心映像，由參數-a, -e設定其入口地址為ZRELADDR，此值在上面ZRELADDR    := $(zreladdr-y)指定。

quiet_cmd_uimage= UIMAGE $@                                                                     

      cmd_uimage = $(CONFIG_SHELL) $(MKIMAGE) -A arm -O linux -T kernel \

                 -C none -a $(ZRELADDR) -e $(ZRELADDR) \

                 -n 'Linux-$(KERNELRELEASE)' -d $< $@

1.3.   小結
從上面分析可知道，linux核心被bootloader拷貝到RAM後，解壓代碼從ZTEXTADDR開始運行（這段代碼是與位置無關的PIC）。核心被解壓縮到ZREALADDR處，也就是核心啟動的物理地址處。相應地，核心啟動的虛擬位址被設定為TEXTADDR，滿足如下條件：

TEXTADDR = PAGE_OFFSET + TEXT_OFFSET

核心啟動的物理地址和虛擬位址滿足入下條件：

ZRELADDR == virt_to_phys(PAGE_OFFSET + TEXT_OFFSET)= virt_to_phys(TEXTADDR)

假定開發板為smdk2410，則有：

核心啟動的虛擬位址

TEXTADDR     = 0xC0008000

核心啟動的物理地址  

ZRELADDR     = 0x30008000

如果直接從flash中啟動還需要設定ZTEXTADDR地址。

2.       核心啟動過程分析
核心啟動過程經過大體可以分為兩個階段：核心映像的自引導；linux核心子模組的初始化。

start

Decompress_kernel()

Call_kernel

Stext:

Prepare_namespace

Do_basic_setup

init

Rest_init

Setup_arch ……

Start_kernel

_enable_mmu

Execve(“/sbin/init”))

核心啟動流程圖

2.1.   核心映像的自引導
這階段的主要工作是實現壓縮核心的解壓和進入核心代碼的入口。

Bootloader完成系統引導後，核心映像被調入記憶體指定的物理地址ZTEXTADDR。典型的核心映像由自引導程式和壓縮的VMlinux組成。因此在啟動核心之前需要先把核心解壓縮。核心映像的入口的第一條代碼就是自引導程式。它在arch/arm/boot/compressed/head.S檔案中。

Head.S檔案主要功能是實現壓縮核心的解壓和跳轉到核心vmlinux核心的入口。Decompress_kernel(): arch/arm/boot/compressed/misc.c 和call_kernel這兩個函數實現了上述功能。在調用decompress_kernel()解壓核心之前，需要確保解壓後的核心代碼不會覆蓋掉原來的核心映像。以及設定核心代碼的入口地址ZREALADDR。

              .text

              adr   r0, LC0

              ldmia       r0, {r1, r2, r3, r4, r5, r6, ip, sp}

              .type       LC0, #object

LC0:              .word      LC0               @ r1

              .word      __bss_start            @ r2

              .word      _end                     @ r3

              .word      zreladdr          @ r4

              .word      _start                    @ r5

              .word      _got_start              @ r6

              .word      _got_end        @ ip

              .word      user_stack+4096           @ sp

上面這段代碼得到核心代碼的入口地址，儲存在r4中。

/*

* Check to see if we will overwrite ourselves.

*   r4 = final kernel address

*   r5 = start of this image

*   r2 = end of malloc space (and therefore this image)

* We basically want:

*   r4 >= r2 -> OK

*   r4 + image length <= r5 -> OK

*/

              cmp r4, r2

              bhs wont_overwrite

              add r0, r4, #4096*1024 @ 4MB largest kernel size

              cmp r0, r5

              bls   wont_overwrite

              mov r5, r2                    @ decompress after malloc space

              mov r0, r5

              mov r3, r7

              bl     decompress_kernel

              b     call_kernel

上面代碼判斷解壓後的核心代碼會不會覆蓋原來的核心映像，然後調用核心解壓縮函數decompress_kernel()。

ulg

decompress_kernel(ulg output_start, ulg free_mem_ptr_p, ulg free_mem_ptr_end_p,

                int arch_id)

{

       output_data            = (uch *)output_start;    /* 指定核心執行地址，儲存在r4中*/

       free_mem_ptr        = free_mem_ptr_p;

       free_mem_ptr_end = free_mem_ptr_end_p;

       __machine_arch_type    = arch_id;

       arch_decomp_setup(); /*解壓縮前的初始化和設定，包括串口傳輸速率設定等*/

       makecrc();           /*CRC校正*/

       putstr("Uncompressing Linux...");

       gunzip();            /*調用解壓縮函數*/

       putstr(" done, booting the kernel.\n");

       return output_ptr;

}