1.?????? 內(nèi)核啟動(dòng)地址

ZTEXTADDR

解壓代碼運(yùn)行的開(kāi)始地址。沒(méi)有物理地址和虛擬地址之分，因?yàn)榇藭r(shí)MMU處于關(guān)閉狀態(tài)。這個(gè)地址不一定時(shí)RAM的地址，可以是支持讀寫(xiě)尋址的flash等存儲(chǔ)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.

在arch/arm/boot/compressed/Makefile中說(shuō)的很明確

#

# We now have a PIC decompressor implementation.? Decompressors running

# from RAM should not define ZTEXTADDR.? Decompressors running directly

# from ROM or Flash must define ZTEXTADDR (preferably via the config)

# FIXME: Previous assignment to ztextaddr-y is lost here. See SHARK

ifeq ($(CONFIG_ZBOOT_ROM),y)

ZTEXTADDR?? := $(CONFIG_ZBOOT_ROM_TEXT)

ZBSSADDR??? := $(CONFIG_ZBOOT_ROM_BSS)

else

ZTEXTADDR?? := 0

ZBSSADDR??? := ALIGN(8)

endif

ZRELADDR

內(nèi)核啟動(dòng)在RAM中的地址。壓縮的內(nèi)核映像被解壓到這個(gè)地址，然后執(zhí)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.

一般定義在項(xiàng)目目錄下，比如：

arch/arm/mach-at91/Makefile.boot: zreladdr-y += 0x70008000

arch/arm/mach-at91/Makefile.boot: zreladdr-y += 0x20008000

arch/arm/mach-cns3xxx/Makefile.boot: zreladdr-y += 0x00008000

arch/arm/mach-davinci/Makefile.boot: zreladdr-y += 0xc0008000

arch/arm/mach-davinci/Makefile.boot: zreladdr-y += 0x80008000

arch/arm/mach-dove/Makefile.boot: zreladdr-y += 0x00008000

arch/arm/mach-ebsa110/Makefile.boot: zreladdr-y += 0x00008000

arch/arm/mach-exynos/Makefile.boot: zreladdr-y += 0x40008000

arch/arm/mach-footbridge/Makefile.boot: zreladdr-y += 0x00008000

arch/arm/mach-gemini/Makefile.boot: zreladdr-y += 0x00008000

arch/arm/mach-gemini/Makefile.boot: zreladdr-y += 0x10008000

arch/arm/mach-integrator/Makefile.boot: zreladdr-y += 0x00008000

arch/arm/mach-iop13xx/Makefile.boot: zreladdr-y += 0x00008000

在arch/arm/boot/Makefile中被賦值：

ZRELADDR := $(zreladdr-y)

PARAMS_PHYS := $(params_phys-y)

INITRD_PHYS := $(initrd_phys-y)

... ...

ifneq ($(LOADADDR),)

UIMAGE_LOADADDR=$(LOADADDR)

else

ifeq ($(CONFIG_ZBOOT_ROM),y)

UIMAGE_LOADADDR=$(CONFIG_ZBOOT_ROM_TEXT)

else

UIMAGE_LOADADDR=$(ZRELADDR)

endif

endif

check_for_multiple_loadaddr = \

if [ $(words $(UIMAGE_LOADADDR)) -ne 1 ]; then \

echo 'multiple (or no) load addresses: $(UIMAGE_LOADADDR)'; \

echo 'This is incompatible with uImages'; \

echo 'Specify LOADADDR on the commandline to build an uImage'; \

false; \

fi

從最后一句紅色說(shuō)明可以看出，如果沒(méi)有在代碼中指定zreladdr-y，也可以在編譯的命令行中指定LOADADDR來(lái)確定內(nèi)核加載的實(shí)際物理地址。

TEXTADDR

內(nèi)核啟動(dòng)的虛擬地址，與ZRELADDR相對(duì)應(yīng)。一般內(nèi)核啟動(dòng)的虛擬地址為RAM的第一個(gè)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

內(nèi)核偏移地址。在arch/arm/makefile中設(shè)定。

PHYS_OFFSET

RAM第一個(gè)bank的物理起始地址。

Physical start address of the first bank of RAM.

PAGE_OFFSET

RAM第一個(gè)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.

這個(gè)值由make menuconfig進(jìn)行配置

1.2.?? 內(nèi)核啟動(dòng)地址確定

內(nèi)核啟動(dòng)引導(dǎo)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可以確定解壓代碼運(yùn)行的開(kāi)始地址在0x0的位置。ZTEXTADDR的值決定了這個(gè)值得選取。

Makefile : arch/arm/boot/compressed

如果設(shè)定內(nèi)核從ROM中啟動(dòng)的話，可以在make menuconfig 的配置界面中設(shè)置解壓代碼的起始地址，否則解壓代碼的起始地址為0x0。實(shí)際上，默認(rèn)從ROM啟動(dòng)時(shí)，解壓代碼的起始地址也是0x0。

ifeq ($(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(guī)則將TEXT_START設(shè)定為ZTEXTADDR。TEXT_START在arch/arm/boot/compressed/vmlinux.lds.in 中被用來(lái)設(shè)定解壓代碼的起始地址。

OUTPUT_ARCH(arm)

ENTRY(_start)

SECTIONS

{

. = TEXT_START;

_text = .;

.text : {

_start = .;

*(.start)

*(.text)

*(.text.*)

……

}

}

內(nèi)核的編譯依靠vmlinux.lds，vmlinux.lds由vmlinux.lds.s 生成。從下面代碼可以看出內(nèi)核啟動(dòng)的虛擬地址被設(shè)置為PAGE_OFFSET + TEXT_OFFSET，而內(nèi)核啟動(dòng)的物理地址ZRELADDR在arch/arm/boot/Makefile中設(shè)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(guò)下面的命令編譯內(nèi)核映像，由參數(shù)-a, -e設(shè)置其入口地址為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(jié)

從上面分析可知道，linux內(nèi)核被bootloader拷貝到RAM后，解壓代碼從ZTEXTADDR開(kāi)始運(yùn)行(這段代碼是與位置無(wú)關(guān)的PIC(Position independent code))。內(nèi)核被解壓縮到ZREALADDR處，也就是內(nèi)核啟動(dòng)的物理地址處。相應(yīng)地，內(nèi)核啟動(dòng)的虛擬地址被設(shè)定為T(mén)EXTADDR，滿足如下條件：

TEXTADDR = PAGE_OFFSET + TEXT_OFFSET

內(nèi)核啟動(dòng)的物理地址和虛擬地址滿足入下條件：

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

假定開(kāi)發(fā)板為smdk2410，則有：

內(nèi)核啟動(dòng)的虛擬地址

TEXTADDR???? = 0xC0008000

內(nèi)核啟動(dòng)的物理地址

ZRELADDR???? = 0x30008000

如果直接從flash中啟動(dòng)還需要設(shè)置ZTEXTADDR地址。

2.?????? 內(nèi)核啟動(dòng)過(guò)程分析

內(nèi)核啟動(dòng)過(guò)程經(jīng)過(guò)大體可以分為兩個(gè)階段：內(nèi)核映像的自引導(dǎo)；linux內(nèi)核子模塊的初始化。

start

Decompress_kernel()

Call_kernel

Stext:

Prepare_namespace

Do_basic_setup

init

Rest_init

Setup_arch ……

Start_kernel

_enable_mmu

Execve(“/sbin/init”))

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