linux kernel中的cmdline的详细介绍

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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 get
  corrupted by bogus network packets or disk data.  This will save
  you many hours of debug.

- Primary CPU general-purpose register settings
  x0 = 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_args
	bl	el2_setup			// Drop to EL1, w0=cpu_boot_mode
	adrp	x23, __PHYS_OFFSET
	and	x23, x23, MIN_KIMG_ALIGN - 1	// KASLR offset, defaults to 0
	bl	set_cpu_boot_mode_flag
	bl	__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 processor
	b	__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=FDT

	adr_l	x0, boot_args			// record the contents of
	stp	x21, x1, [x0]			// x0 .. x3 at kernel entry
	stp	x2, x3, [x0, #16]

	dmb	sy				// needed before dc ivac with
						// MMU off

	mov	x1, #0x20			// 4 x 8 bytes
	b	__inval_dcache_area		// tail call
ENDPROC(preserve_boot_args)

__primary_switch调用了__primary_switched
__primary_switch:
#ifdef CONFIG_RANDOMIZE_BASE
	mov	x19, x0				// preserve new SCTLR_EL1 value
	mrs	x20, sctlr_el1			// preserve old SCTLR_EL1 value
#endif

	bl	__enable_mmu
#ifdef CONFIG_RELOCATABLE
	bl	__relocate_kernel
#ifdef CONFIG_RANDOMIZE_BASE
	ldr	x8, =__primary_switched
	adrp	x0, __PHYS_OFFSET
	blr	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_union
	add	sp, x4, #THREAD_SIZE
	adr_l	x5, init_task
	msr	sp_el0, x5			// Save thread_info

	adr_l	x8, vectors			// load VBAR_EL1 with virtual
	msr	vbar_el1, x8			// vector table address
	isb

	stp	xzr, x30, [sp, #-16]!
	mov	x29, sp

	str_l	x21, __fdt_pointer, x5		// Save FDT pointer

	ldr_l	x4, kimage_vaddr		// Save the offset between
	sub	x4, x4, x0			// the kernel virtual and
	str_l	x4, kimage_voffset, x5		// physical mappings

	// Clear BSS
	adr_l	x0, __bss_start
	mov	x1, xzr
	adr_l	x2, __bss_stop
	sub	x2, x2, x0
	bl	__pi_memset
	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

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);
…
}