1、 and irqs disabledmrcp15, 0, r9, c0, c0 get processor idbl_lookup_processor_type r5=procinfo r9=cpuidmovsr10, r5 invalid processor (r5=0)?beq_error_p yes, error pbl_lookup_machine_type r5=machinfomovsr8, r5 invalid machine (r5=0)?beq_error_a yes, error abl_create_page_tables/* * The following calls
2、CPU specific code in a position independent * manner. See arch/arm/mm/proc-*.S for details. r10 = base of * xxx_proc_info structure selected by _lookup_machine_type * above. On return, the CPU will be ready for the MMU to be * turned on, and r0 will hold the CPU control register value. */ldrr13, _sw
3、itch_data address to jump to after mmu has been enabledadrlr, _enable_mmu return (PIC) address第一步,执行的是_lookup_processor_type,这个函数是检查处理器型号,它读取你的电路板的CPU型号与内核支持的处理器进行比较看是否能够处理。这个我们不关心它的具体实现过程,因为现在主流处理器内核都提供了支持。第二步,执行的是_lookup_machine_type,这个函数是来检查机器型号的,它会读取你bootloader传进来的机器ID和他能够处理的机器ID进行比较看是否能够处理。内核的I
4、D号定义在arc/arm/tool/mach_types文件中MACH_TYPE_xxxx宏定义。内核究竟就如何检查是否是它支持的机器的呢?实际上每个机器都会在/arc/arm/mach-xxxx/smdk-xxxx.c文件中有个描述特定机器的数据结构,如下MACHINE_START(S3C2440, SMDK2440)/* Maintainer: Ben Dooks */.phys_io= S3C2410_PA_UART,.io_pg_offst= (u32)S3C24XX_VA_UART) 18) & 0xfffc,.boot_params= S3C2410_SDRAM_PA + 0x10
5、0,.init_irq= s3c24xx_init_irq,.map_io= smdk2440_map_io,.init_machine= smdk2440_machine_init,.timer= &s3c24xx_timer,MACHINE_ENDMACHINE_START 和 MACHINE_END实际上被展开成一个结构体#define MACHINE_START(_type,_name)static const struct machine_desc _mach_desc_#_type _used _attribute_(_section_(.arch.info.init) = .nr
6、= MACH_TYPE_#_type,.name= _name,#define MACHINE_END;于是上面的数据结构就被展开为static const struct machine_desc _mach_desc_S3C2440.nr= MACH_TYPE_S3C2440,.name=” SMDK2440”,;.phys_io= S3C2410_PA_UART,每个机器都会有一个machine_desc _mach_desc结构,内核通过检查每个machine_desc _mach_desc的nr号和bootloader传上来的ID进行比较,如果相同,内核就认为支持该机器,而且内核在后面
7、的工作中会调用该机器的machine_desc _mach_desc_结构中的方法进行一些初始化工作。第三步,创建一级页表。第四步,在R13中保存_switch_data这个函数的地址,在第四步使能mmu完成后会跳到该函数执行。第五步,执行的是_enable_mmu,它是使能MMU,这个函数调用了_turn_mmu_on函数,让后在_turn_mmu_on在最后将第三步赋给R13的值传给了PC指针(movpc, r13),于是内核开始跳到_switch_data这个函数开始执行。我们再来看arch/arm/kenel/head-common.S这个文件中的_switch_data函数_swit
8、ch_data:.long_mmap_switched.long_data_loc r4.long_data_start r5.long_bss_start r6.long_end r7.longprocessor_id r4.long_machine_arch_type r5.longcr_alignment r6.longinit_thread_union + THREAD_START_SP sp/* * The following fragment of code is executed with the MMU on in MMU mode, * and uses absolute a
9、ddresses; this is not position independent. * * r0 = cp#15 control register * r1 = machine ID * r9 = processor ID.type_mmap_switched, %function_mmap_switched:adrr3, _switch_data + 4ldmiar3!, r4, r5, r6, r7cmpr4, r5 Copy data segment if needed1:cmpner5, r6ldrnefp, r4, #4strnefp, r5, #4bne1bmovfp, #0
10、Clear BSS (and zero fp)cmpr6, r7strccfp, r6,#4bcc1bldmiar3, r4, r5, r6, spstrr9, r4 Save processor IDstrr1, r5 Save machine typebicr4, r0, #CR_A Clear A bitstmiar6, r0, r4 Save control register valuesbstart_kernel这个函数做的工作是,复制数据段清楚BBS段,设置堆在指针,然后保存处理器内核和机器内核等工作,最后跳到start_kernel函数。于是内核开始执行第二阶段。第二阶段:我们再
11、来看init/目录下的main.c的start_kernel函数,这里我只截图了部分。asmlinkage void _init start_kernel(void).printk(KERN_NOTICE);printk(linux_banner);setup_arch(&command_line);setup_command_line(command_line);parse_early_param();parse_args(Booting kernel, static_command_line, _start_param, _stop_param - _start_param, &unknown_bootoption);init_IRQ();pidhash_init();init_timers();hrtimers_init();softirq_init();
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