linux kernel中的cmdline的详细介绍

cmdline

1、向linux kernel添加cmdline的四种方式

(1)、 在dts中的bootargs中添加

(2)、在BoardConfig中添加

(3)、在uboot中添加

(4)、在android的Makefile中添加

2、在uboot中,将cmdline统一放置在FDT中

3、在kernel中,从FDT中解析处cmdline并使用

(1)、跳转linux kernel之前-准备cmdline

(2)、kernel启动-解析cmdline

1、向linux kernel添加cmdline的四种方式

在linux启动时候，串口log中会打印cmdline

[  0.000000] c0 0 (swapper) Kernel command line: earlycon androidboot.selinux=permissive uart_dma keep_dbgclk_on clk_ignore_unused initrd=0xd0000000,38711808 rw crash_page=0x8f040000 initrd=/recoveryrc boot_reason=0x2000 ota_status=0x1001

在linux启动完成后，通过 cat /proc/cmdline也是可以看到cmdline. 那么cmdline是如何添加的呢？

(1)、 在dts中的bootargs中添加

1. / {

2.   model = "yyyyyyy";

3.   compatible = "yyyyyyy", "xxxxxxxx";

5.   chosen {

6.     /*

7.      * initrd parameters not set in dts file since the ramdisk.img size

8.      * need to check in uboot, and the initrd load address and size will

9.      * set in uboot stage.

10.      */

11.     bootargs = "earlycon androidboot.selinux=permissive uart_dma keep_dbgclk_on clk_ignore_unused";

12.     stdout-path = "serial0:115200";

13.   };

14. ......

15. }

(2)、在BoardConfig中添加

vim device/xxx/xxx_evb/BoardConfigCommon.mk

BOARD_KERNEL_CMDLINE += androidboot.selinux=enforcing androidboot.hardware=xxxx_phone androidboot.dtbo_idx=0

(3)、在uboot中添加

vim u-boot/common/cmd_bootm.c

• append_bootargs("recovery=1");

• sprintf(dm_buf,"init=/init skip_initramfs rootwait root=/dev/dm-0 dm="system none ro,0 1 android-verity /dev/mmcblk0p%d"",ret);

• append_bootargs((const char *)dm_buf);

2、在uboot中,将cmdline统一放置在FDT中

以后再写，哈哈哈哈

3、在kernel中,从FDT中解析处cmdline并使用

(1)、跳转linux kernel之前-准备cmdline

在跳转linux kernel之前(如uboot中)，将cmdline数据放到了FDT中，然后将FDT的地址写入到了X0中。然后再跳转linux kernel.

别问我怎么知道的，请看kernel-4.14/Documentation/arm64/booting.txt

1. Before jumping into the kernel, the following conditions must be met:

3. • Quiesce all DMA capable devices so that memory does not get

4.  corrupted by bogus network packets or disk data. This will save

5.  you many hours of debug.

7. • Primary CPU general-purpose register settings

8.  x0 = physical address of device tree blob (dtb) in system RAM.

9.  x1 = 0 (reserved for future use)

10.  x2 = 0 (reserved for future use)

11.  x3 = 0 (reserved for future use)

(2)、kernel启动-解析cmdline

linux kernel从stext开始启动，整个流程大概就是读取X0(FDT地址)保存到X21中，又将X21保存到__fdt_pointer全局变量中

然后再将__fdt_pointer解析处cmdline数据到boot_command_line全局变量中

1. /*

2. • The following callee saved general purpose registers are used on the
3. • primary lowlevel boot path:

5. * Register  Scope           Purpose

6. * x21    stext() .. start_kernel() FDT pointer passed at boot in x0

7. * x23    stext() .. start_kernel() physical misalignment/KASLR offset

8. * x28    __create_page_tables()   callee preserved temp register

9. * x19/x20  __primary_switch()     callee preserved temp registers

10. */

11. ENTRY(stext)

12. bl preserve_boot_args

13. bl el2_setup // Drop to EL1, w0=cpu_boot_mode

14. adrp x23, __PHYS_OFFSET

15. and x23, x23, MIN_KIMG_ALIGN - 1 // KASLR offset, defaults to 0

16. bl set_cpu_boot_mode_flag

17. bl __create_page_tables

18. /*

19. • The following calls CPU setup code, see arch/arm64/mm/proc.S for
20. • details.
21. • On return, the CPU will be ready for the MMU to be turned on and
22. • the TCR will have been set.

23. */

24. bl __cpu_setup // initialise processor

25. b __primary_switch

26. ENDPROC(stext)

这里调用了：

preserve_boot_args

__primary_switch

在preserve_boot_args将X0(fdt地址)暂时先保存到了X21中

1. preserve_boot_args:

2. mov x21, x0 // x21=FDT

4. adr_l x0, boot_args // record the contents of

5. stp x21, x1, [x0] // x0 .. x3 at kernel entry

6. stp x2, x3, [x0, #16]

8. dmb sy // needed before dc ivac with

9. // MMU off

11. mov x1, #0x20 // 4 x 8 bytes

12. b __inval_dcache_area // tail call

13. ENDPROC(preserve_boot_args)

14. __primary_switch调用了__primary_switched

15. __primary_switch:

16. #ifdef CONFIG_RANDOMIZE_BASE

17. mov x19, x0 // preserve new SCTLR_EL1 value

18. mrs x20, sctlr_el1 // preserve old SCTLR_EL1 value

19. #endif

21. bl __enable_mmu

22. #ifdef CONFIG_RELOCATABLE

23. bl __relocate_kernel

24. #ifdef CONFIG_RANDOMIZE_BASE

25. ldr x8, =__primary_switched

26. adrp x0, __PHYS_OFFSET

27. blr x8

__primary_switched将X21(fdt地址)保存到了__fdt_pointer全局变量中

1. __primary_switched:

2. adrp x4, init_thread_union

3. add sp, x4, #THREAD_SIZE

4. adr_l x5, init_task

5. msr sp_el0, x5 // Save thread_info

7. adr_l x8, vectors // load VBAR_EL1 with virtual

8. msr vbar_el1, x8 // vector table address

9. isb

11. stp xzr, x30, [sp, #-16]!

12. mov x29, sp

14. str_l x21, __fdt_pointer, x5 // Save FDT pointer

16. ldr_l x4, kimage_vaddr // Save the offset between

17. sub x4, x4, x0 // the kernel virtual and

18. str_l x4, kimage_voffset, x5 // physical mappings

20. // Clear BSS

21. adr_l x0, __bss_start

22. mov x1, xzr

23. adr_l x2, __bss_stop

24. sub x2, x2, x0

25. bl __pi_memset

26. dsb ishst // Make zero page visible to PTW

在setup_arch()的时候，调用setup_machine_fdt将fdt解析到了boot_command_line全局变量中

1. void __init setup_arch(char **cmdline_p)

2. {

3. pr_info("Boot CPU: AArch64 Processor [%08x]\n", read_cpuid_id());

4. ......

5. *cmdline_p = boot_command_line;

6. ......

7. setup_machine_fdt(__fdt_pointer);

8. ......

9. }

setup_machine_fdt()—>early_init_dt_scan()—>early_init_dt_scan_nodes()

在中，将fdt解析到了boot_command_line中

1. of_scan_flat_dt(early_init_dt_scan_chosen, boot_command_line)

3. static void __init setup_machine_fdt(phys_addr_t dt_phys)

4. {

5. void *dt_virt = fixmap_remap_fdt(dt_phys);

6. const char *name;

8. if (!dt_virt || !early_init_dt_scan(dt_virt)) {

9. pr_crit("\n"

10. "Error: invalid device tree blob at physical address %pa (virtual address 0x%p)\n"

11. "The dtb must be 8-byte aligned and must not exceed 2 MB in size\n"

12. "\nPlease check your bootloader.",

13. &dt_phys, dt_virt);

15. while (true)

16. cpu_relax();

17. }

19. name = of_flat_dt_get_machine_name();

20. if (!name)

21. return;

22. /* backward-compatibility for third-party applications */

23. machine_desc_set(name);

25. pr_info("Machine model: %s\n", name);

26. dump_stack_set_arch_desc("%s (DT)", name);

27. }

28.  bool __init early_init_dt_scan(void *params)

29.  {

30.   bool status;

31.  

32.   status = early_init_dt_verify(params);

33.   if (!status)

34.   return false;

35.  

36.   early_init_dt_scan_nodes();

37.   return true;

38.  }

39.  void __init early_init_dt_scan_nodes(void)

40.  {

41.   /* Retrieve various information from the /chosen node */

42.   of_scan_flat_dt(early_init_dt_scan_chosen, boot_command_line);

43.  

44.   /* Initialize {size,address}-cells info */

45.   of_scan_flat_dt(early_init_dt_scan_root, NULL);

46.  

47.   /* Setup memory, calling early_init_dt_add_memory_arch */

48.   of_scan_flat_dt(early_init_dt_scan_memory, NULL);

49.  }

在start_kernel()打印了cmdline.

asmlinkage __visible void __init start_kernel(void)

{

…

pr_notice(“Kernel command line: %s\n”, boot_command_line);

…

}