linux内核启动解析（三）
发布时间：2012-4-1 09:57
发布者：李宽
关键词：
linux
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freshtree 1.2 __lookup_machine_type() 机器类型的查找代码如下： __lookup_machine_type: adr r3, 3b ldmia r3, {r4, r5, r6} sub r3, r3, r4 @ get offset between virt&phys add r5, r5, r3 @ convert virt addresses to add r6, r6, r3 @ physical address space 1: ldr r3, [r5, #MACHINFO_TYPE] @ get machine type teq r3, r1 @ matches loader number? beq 2f @ found add r5, r5, #SIZEOF_MACHINE_DESC @ next machine_desc cmp r5, r6 blo 1b mov r5, #0 @ unknown machine 2: mov pc, lr ENDPROC(__lookup_machine_type) 我们可以看到，这和处理器类型查找函数很类似，在这里只进行简单的解说。 .long __proc_info_begin .long __proc_info_end 3: .long . .long __arch_info_begin .long __arch_info_end __arch_info_begin和__arch_info_end在arch/arm/kernel/vlinux.lds.S中定义： __arch_info_begin = .; *(.arch.info.init) __arch_info_end = .; .arch.info.init段我们可以找到在arch/arm/include/asm/mach/arch.h中有引用： #define MACHINE_START(_type,_name) \ static const struct machine_desc __mach_desc_##_type \ __used \ __attribute__((__section__(".arch.info.init"))) = { \ .nr = MACH_TYPE_##_type, \ .name = _name, #define MACHINE_END \ }; 我们可以在arch/arm/mach-*.c文件中找到一系列关于MACHINE_START所定义的结构。 1.3 __vet_atags() 函数代码如下： __vet_atags: tst r2, #0x3 @ aligned? bne 1f ldr r5, [r2, #0] @ is first tag ATAG_CORE? subs r5, r5, #ATAG_CORE_SIZE bne 1f ldr r5, [r2, #4] ldr r6, =ATAG_CORE cmp r5, r6 bne 1f mov pc, lr @ atag pointer is ok 1: mov r2, #0 mov pc, lr ENDPROC(__vet_atags) atag是bootloader传递给linux内核的参数列表。这个参数列表是以tag的列表形式来表示的。这个列表起始位置的tag是ATAG_CORE，用来表示这是一个有效的tag列表。如果起始tag不是ATAG_CORE，就认为bootloader没有传递tag参数给内核。以下是tag值的定义和描述，以及tag结构的定义。 Tag name Value Size Description ATAG_NONE 0x00000000 2 Empty tag used to end list ATAG_CORE 0x54410001 5 (2 if empty) First tag used to start list ATAG_MEM 0x54410002 4 Describes a physical area of memory ATAG_VIDEOTEXT 0x54410003 5 Describes a VGA text display ATAG_RAMDISK 0x54410004 5 Describes how the ramdisk will be used in kernel ATAG_INITRD2 0x54420005 4 Describes where the compressed ramdisk image is placed in memory ATAG_SERIAL 0x54410006 4 64 bit board serial number ATAG_REVISION 0x54410007 3 32 bit board revision number ATAG_VIDEOLFB 0x54410008 8 Initial values for vesafb-type framebuffers ATAG_CMDLINE 0x54410009 2 + ((length_of_cmdline + 3) / 4) Command line to pass to kernel struct tag_header { __u32 size; __u32 tag; }; struct tag { struct tag_header hdr; union { struct tag_core core; struct tag_mem32 mem; struct tag_videotext videotext; struct tag_ramdisk ramdisk; struct tag_initrd initrd; struct tag_serialnr serialnr; struct tag_revision revision; struct tag_videolfb videolfb; struct tag_cmdline cmdline; struct tag_acorn acorn; struct tag_memclk memclk; } u; }; __vet_atags()函数实现的就是判断r2是否是有效的tag列表指针，如果不是，就将零指针赋值给r2。 |
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