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

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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

/ {model = "yyyyyyy";compatible = "yyyyyyy", "xxxxxxxx";chosen {/** initrd parameters not set in dts file since the ramdisk.img size* need to check in uboot, and the initrd load address and size will* set in uboot stage.*/bootargs = "earlycon androidboot.selinux=permissive uart_dma keep_dbgclk_on clk_ignore_unused";stdout-path = "serial0:115200";};
......
}

(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);

(4)、在android的Makefile中添加

vim build/core/Makefile

INTERNAL_KERNEL_CMDLINE := $(strip $(BOARD_KERNEL_CMDLINE) buildvariant=$(TARGET_BUILD_VARIANT) $(VERITY_KEYID))
ifdef INTERNAL_KERNEL_CMDLINE
INTERNAL_BOOTIMAGE_ARGS += --cmdline "$(INTERNAL_KERNEL_CMDLINE)"
endif

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

Before jumping into the kernel, the following conditions must be met:- Quiesce all DMA capable devices so that memory does not getcorrupted by bogus network packets or disk data.  This will saveyou many hours of debug.- Primary CPU general-purpose register settingsx0 = physical address of device tree blob (dtb) in system RAM.x1 = 0 (reserved for future use)x2 = 0 (reserved for future use)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

 /** The following callee saved general purpose registers are used on the* primary lowlevel boot path:**  Register   Scope                      Purpose*  x21        stext() .. start_kernel()  FDT pointer passed at boot in x0*  x23        stext() .. start_kernel()  physical misalignment/KASLR offset*  x28        __create_page_tables()     callee preserved temp register*  x19/x20    __primary_switch()         callee preserved temp registers*/
ENTRY(stext)bl  preserve_boot_argsbl    el2_setup           // Drop to EL1, w0=cpu_boot_modeadrp   x23, __PHYS_OFFSETand   x23, x23, MIN_KIMG_ALIGN - 1    // KASLR offset, defaults to 0bl    set_cpu_boot_mode_flagbl    __create_page_tables/** The following calls CPU setup code, see arch/arm64/mm/proc.S for* details.* On return, the CPU will be ready for the MMU to be turned on and* the TCR will have been set.*/bl   __cpu_setup         // initialise processorb    __primary_switch
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

preserve_boot_args:mov   x21, x0             // x21=FDTadr_l    x0, boot_args           // record the contents ofstp    x21, x1, [x0]           // x0 .. x3 at kernel entrystp  x2, x3, [x0, #16]dmb    sy              // needed before dc ivac with// MMU offmov  x1, #0x20           // 4 x 8 bytesb __inval_dcache_area     // tail call
ENDPROC(preserve_boot_args)

__primary_switch调用了__primary_switched
__primary_switch:
#ifdef CONFIG_RANDOMIZE_BASEmov x19, x0             // preserve new SCTLR_EL1 valuemrs  x20, sctlr_el1          // preserve old SCTLR_EL1 value
#endifbl    __enable_mmu
#ifdef CONFIG_RELOCATABLEbl __relocate_kernel
#ifdef CONFIG_RANDOMIZE_BASEldr x8, =__primary_switchedadrp    x0, __PHYS_OFFSETblr    x8

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

__primary_switched:adrp  x4, init_thread_unionadd    sp, x4, #THREAD_SIZEadr_l   x5, init_taskmsr    sp_el0, x5          // Save thread_infoadr_l    x8, vectors         // load VBAR_EL1 with virtualmsr    vbar_el1, x8            // vector table addressisbstp   xzr, x30, [sp, #-16]!mov    x29, spstr_l    x21, __fdt_pointer, x5      // Save FDT pointerldr_l    x4, kimage_vaddr        // Save the offset betweensub   x4, x4, x0          // the kernel virtual andstr_l  x4, kimage_voffset, x5      // physical mappings// Clear BSSadr_l   x0, __bss_startmov  x1, xzradr_l    x2, __bss_stopsub   x2, x2, x0bl    __pi_memsetdsb  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

void __init setup_arch(char **cmdline_p)
{pr_info("Boot CPU: AArch64 Processor [%08x]\n", read_cpuid_id());
......*cmdline_p = boot_command_line;
......setup_machine_fdt(__fdt_pointer);
......
}

setup_machine_fdt()—>early_init_dt_scan()—>early_init_dt_scan_nodes()
在中，将fdt解析到了boot_command_line中
of_scan_flat_dt(early_init_dt_scan_chosen, boot_command_line)

static void __init setup_machine_fdt(phys_addr_t dt_phys)
{void *dt_virt = fixmap_remap_fdt(dt_phys);const char *name;if (!dt_virt || !early_init_dt_scan(dt_virt)) {pr_crit("\n""Error: invalid device tree blob at physical address %pa (virtual address 0x%p)\n""The dtb must be 8-byte aligned and must not exceed 2 MB in size\n""\nPlease check your bootloader.",&dt_phys, dt_virt);while (true)cpu_relax();}name = of_flat_dt_get_machine_name();if (!name)return;/* backward-compatibility for third-party applications */machine_desc_set(name);pr_info("Machine model: %s\n", name);dump_stack_set_arch_desc("%s (DT)", name);
}

 bool __init early_init_dt_scan(void *params){bool status;status = early_init_dt_verify(params);if (!status)return false;early_init_dt_scan_nodes();return true;}

 void __init early_init_dt_scan_nodes(void){/* Retrieve various information from the /chosen node */of_scan_flat_dt(early_init_dt_scan_chosen, boot_command_line);/* Initialize {size,address}-cells info */of_scan_flat_dt(early_init_dt_scan_root, NULL);/* Setup memory, calling early_init_dt_add_memory_arch */of_scan_flat_dt(early_init_dt_scan_memory, NULL);}

在start_kernel()打印了cmdline.
asmlinkage __visible void __init start_kernel(void)
{
…
pr_notice(“Kernel command line: %s\n”, boot_command_line);
…
}