Linux 内核调试篇1

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于 2025-01-04 11:57:22 发布

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分类专栏： Linux内核子系统文章标签： linux 运维服务器

本文链接：https://blog.youkuaiyun.com/zouhaicheng/article/details/144926065

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Linux 内核调试篇1（基于Linux6.6）---earlyprintk建立过程

一、earlyprintk

内核中使用__section设置了很多的段属性，使用段属性可以很方便的对函数调用时间分层。

比如之前常见的subsys_initcall，就要比module_init更早的执行，因为subsys_initcall在驱动中通常是bus和class，驱动程序调用执行需要class和bus已经创建才能执行驱动函数。

和subsys_initcall、arch_initcall、core_initcall、device_initcall等类似，内核在别的地方也是用了这样的模式。

include/linux/init.h

__setup这个宏里面又是调用了另一个宏

 #define __setup(str, fn)						\
	__setup_param(str, fn, fn, 0)

在看这个#define __setup_param(str, unique_id, fn, early) 宏之前，先看一个结构体，这个结构体也算是这个宏的基础。

 struct obs_kernel_param {
	const char *str;
	int (*setup_func)(char *);
	int early;
};

很简单，一个字符指针，主要是为了匹配。一个函数指针，主要是为了绑定函数。一个eraly主要是为了确定执行次序。

接下来就看__setup_param

include/linux/init.h

/*
 * Only for really core code.  See moduleparam.h for the normal way.
 *
 * Force the alignment so the compiler doesn't space elements of the
 * obs_kernel_param "array" too far apart in .init.setup.
 */
#define __setup_param(str, unique_id, fn, early)			\
	static const char __setup_str_##unique_id[] __initconst		\
		__aligned(1) = str; 					\
	static struct obs_kernel_param __setup_##unique_id		\
		__used __section(".init.setup")				\
		__aligned(__alignof__(struct obs_kernel_param))		\
		= { __setup_str_##unique_id, fn, early }

定义了一个字符串数组放置字符串。定义了一个obs_kernel_param结构体用来存放参数，这个结构体用__section(.init.setup)做了修饰，表示调用这个宏定义的obs_kernel_param结构体都会被编译进一个.init.setup段中。

结构体就占用3个long，一个是存放前面定义的这个字符串的地址，另一个就是函数指针，随后一个就是一个整数。

1.1、__setup的调用

include/linux/init.h

/*
 * NOTE: __setup functions return values:
 * @fn returns 1 (or non-zero) if the option argument is "handled"
 * and returns 0 if the option argument is "not handled".
 */
#define __setup(str, fn)						\
	__setup_param(str, fn, fn, 0)

/*
 * NOTE: @fn is as per module_param, not __setup!
 * I.e., @fn returns 0 for no error or non-zero for error
 * (possibly @fn returns a -errno value, but it does not matter).
 * Emits warning if @fn returns non-zero.
 */
#define early_param(str, fn)						\
	__setup_param(str, fn, fn, 1)

#define early_param_on_off(str_on, str_off, var, config)		\
									\
	int var = IS_ENABLED(config);					\
									\
	static int __init parse_##var##_on(char *arg)			\
	{								\
		var = 1;						\
		return 0;						\
	}								\
	early_param(str_on, parse_##var##_on);				\
									\
	static int __init parse_##var##_off(char *arg)			\
	{								\
		var = 0;						\
		return 0;						\
	}								\
	early_param(str_off, parse_##var##_off)

/* Relies on boot_command_line being set */
void __init parse_early_param(void);
void __init parse_early_options(char *cmdline);
#endif /* __ASSEMBLY__ */

#else /* MODULE */

#define __setup_param(str, unique_id, fn)	/* nothing */
#define __setup(str, func) 			/* nothing */
#endif

一个是__setup宏，一个是early_param宏，最后就是early_param_on_off宏

但是经过我搜索内核，实际上这个宏early_param_on_off是从未使用过的，可能是为了新功能预留的，

__setup和early_param宏只是最后一个整数参数的不同。这里就以__setup为例分析下去。

这里就以内核调试主体要分析的这个为例

kernel/printk/printk.c

__setup("console=", console_setup);

这个宏将来就被定义为在一个段属性里面了。

static const char __setup_str_console_setup[] __initconst	__aligned(1) = "console="; 				
static struct obs_kernel_param __setup_console_setup
		__used __section(.init.setup)			
		__attribute__((aligned((sizeof(long)))))
= { 
        __setup_str_console_setup 
        console_setup, 
        0
}

定义这个段属性，肯定就是为了在某个阶段里面有函数执行，那么什么时候执行呢？

再次回到链接脚本

 __setup_start = .; 
KEEP(*(.init.setup)    ) 
__setup_end = .;

可以看到有两个标号在这个段属性前后。

搜索一下这两个标号：

init/main.c

static int __init set_reset_devices(char *str)
{
	reset_devices = 1;
	return 1;
}

__setup("reset_devices", set_reset_devices);

static const char *argv_init[MAX_INIT_ARGS+2] = { "init", NULL, };
const char *envp_init[MAX_INIT_ENVS+2] = { "HOME=/", "TERM=linux", NULL, };
static const char *panic_later, *panic_param;

extern const struct obs_kernel_param __setup_start[], __setup_end[];

在init目录下的main.c函数导出了。

1.2、obsolete_checksetup

既然已经知道了这段在内存的位置，那么就可以直接使用里面的结构体，进而函数执行了。

而执行的函数就在extern导出的下面：

init/main.c

static bool __init obsolete_checksetup(char *line)
{
	const struct obs_kernel_param *p;
	bool had_early_param = false;

	p = __setup_start;
	do {
		int n = strlen(p->str);
		if (parameqn(line, p->str, n)) {
			if (p->early) {
				/* Already done in parse_early_param?
				 * (Needs exact match on param part).
				 * Keep iterating, as we can have early
				 * params and __setups of same names 8( */
				if (line[n] == '\0' || line[n] == '=')
					had_early_param = true;
			} else if (!p->setup_func) {
				pr_warn("Parameter %s is obsolete, ignored\n",
					p->str);
				return true;
			} else if (p->setup_func(line + n))
				return true;
		}
		p++;
	} while (p < __setup_end);

	return had_early_param;
}

可以看到，这是一个do_while的循环，就是调用parameqn比较，在obsolete_checksetup这个函数传入字符串参数和段属性中所有结构体比较，相等的情况下执行，找到的结构体obs_kernel_param对应的函数。

字符串比较函数很简单，唯一就是把'-'字符替换，按照‘_’进行比较。

kernel/params.c

  
static char dash2underscore(char c)
{
	if (c == '-')
		return '_';
	return c;
}
 
bool parameqn(const char *a, const char *b, size_t n)
{
	size_t i;
 
	for (i = 0; i < n; i++) {
		if (dash2underscore(a[i]) != dash2underscore(b[i]))
			return false;
	}
	return true;
}

看到这里，调用这个函数的上层函数给这个函数传入一个字符串，段属性中找到对应的字符串，然后执行这个字符串对应的结构体里面的函数。

init/main.c

这个函数只被下面这个函数调用了一次。

/*
 * Unknown boot options get handed to init, unless they look like
 * unused parameters (modprobe will find them in /proc/cmdline).
 */
static int __init unknown_bootoption(char *param, char *val,
				     const char *unused, void *arg)
{
	size_t len = strlen(param);

	repair_env_string(param, val);

	/* Handle obsolete-style parameters */
	if (obsolete_checksetup(param))
		return 0;

	/* Unused module parameter. */
	if (strnchr(param, len, '.'))
		return 0;

	if (panic_later)
		return 0;

	if (val) {
		/* Environment option */
		unsigned int i;
		for (i = 0; envp_init[i]; i++) {
			if (i == MAX_INIT_ENVS) {
				panic_later = "env";
				panic_param = param;
			}
			if (!strncmp(param, envp_init[i], len+1))
				break;
		}
		envp_init[i] = param;
	} else {
		/* Command line option */
		unsigned int i;
		for (i = 0; argv_init[i]; i++) {
			if (i == MAX_INIT_ARGS) {
				panic_later = "init";
				panic_param = param;
			}
		}
		argv_init[i] = param;
	}
	return 0;
}

看一下它的调用之处，可以看到这个函数是以函数指针的形式调用的。

init/main.c

 	after_dashes = parse_args("Booting kernel",
				  static_command_line, __start___param,
				  __stop___param - __start___param,
				  -1, -1, NULL, &unknown_bootoption);

在这里看到了很重要的一条就是需要comdline。

1.3、comdline

看一下comdline是设置的。

init/main.c

asmlinkage __visible void __init __no_sanitize_address start_kernel(void)
{
	char *command_line;
	char *after_dashes;

	set_task_stack_end_magic(&init_task);
	smp_setup_processor_id();
	debug_objects_early_init();
	init_vmlinux_build_id();

	cgroup_init_early();

	local_irq_disable();
	early_boot_irqs_disabled = true;

	/*
	 * Interrupts are still disabled. Do necessary setups, then
	 * enable them.
	 */
	boot_cpu_init();
	page_address_init();
	pr_notice("%s", linux_banner);
	early_security_init();
	setup_arch(&command_line);
	setup_boot_config();
	setup_command_line(command_line);
	setup_nr_cpu_ids();
	setup_per_cpu_areas();
	smp_prepare_boot_cpu();	/* arch-specific boot-cpu hooks */
	boot_cpu_hotplug_init();

	build_all_zonelists(NULL);
	page_alloc_init();

	pr_notice("Kernel command line: %s\n", saved_command_line);
	/* parameters may set static keys */
	jump_label_init();
	parse_early_param();
	after_dashes = parse_args("Booting kernel",
				  static_command_line, __start___param,
				  __stop___param - __start___param,
				  -1, -1, NULL, &unknown_bootoption);
	print_unknown_bootoptions();
	if (!IS_ERR_OR_NULL(after_dashes))
		parse_args("Setting init args", after_dashes, NULL, 0, -1, -1,
			   NULL, set_init_arg);
	if (extra_init_args)
		parse_args("Setting extra init args", extra_init_args,
			   NULL, 0, -1, -1, NULL, set_init_arg);

	/* Architectural and non-timekeeping rng init, before allocator init */
	random_init_early(command_line);

	/*
	 * These use large bootmem allocations and must precede
	 * kmem_cache_init()
	 */
	setup_log_buf(0);
	vfs_caches_init_early();
	sort_main_extable();
	trap_init();
	mm_init();

	ftrace_init();

	/* trace_printk can be enabled here */
	early_trace_init();

	/*
	 * Set up the scheduler prior starting any interrupts (such as the
	 * timer interrupt). Full topology setup happens at smp_init()
	 * time - but meanwhile we still have a functioning scheduler.
	 */
	sched_init();

	if (WARN(!irqs_disabled(),
		 "Interrupts were enabled *very* early, fixing it\n"))
		local_irq_disable();
	radix_tree_init();
	maple_tree_init();

	/*
	 * Set up housekeeping before setting up workqueues to allow the unbound
	 * workqueue to take non-housekeeping into account.
	 */
	housekeeping_init();

	/*
	 * Allow workqueue creation and work item queueing/cancelling
	 * early.  Work item execution depends on kthreads and starts after
	 * workqueue_init().
	 */
	workqueue_init_early();

	rcu_init();

	/* Trace events are available after this */
	trace_init();

	if (initcall_debug)
		initcall_debug_enable();

	context_tracking_init();
	/* init some links before init_ISA_irqs() */
	early_irq_init();
	init_IRQ();
	tick_init();
	rcu_init_nohz();
	init_timers();
	srcu_init();
	hrtimers_init();
	softirq_init();
	timekeeping_init();
	time_init();

	/* This must be after timekeeping is initialized */
	random_init();

	/* These make use of the fully initialized rng */
	kfence_init();
	boot_init_stack_canary();

	perf_event_init();
	profile_init();
	call_function_init();
	WARN(!irqs_disabled(), "Interrupts were enabled early\n");

	early_boot_irqs_disabled = false;
	local_irq_enable();

	kmem_cache_init_late();

	/*
	 * HACK ALERT! This is early. We're enabling the console before
	 * we've done PCI setups etc, and console_init() must be aware of
	 * this. But we do want output early, in case something goes wrong.
	 */
	console_init();
	if (panic_later)
		panic("Too many boot %s vars at `%s'", panic_later,
		      panic_param);

	lockdep_init();

	/*
	 * Need to run this when irqs are enabled, because it wants
	 * to self-test [hard/soft]-irqs on/off lock inversion bugs
	 * too:
	 */
	locking_selftest();

	/*
	 * This needs to be called before any devices perform DMA
	 * operations that might use the SWIOTLB bounce buffers. It will
	 * mark the bounce buffers as decrypted so that their usage will
	 * not cause "plain-text" data to be decrypted when accessed.
	 */
	mem_encrypt_init();

#ifdef CONFIG_BLK_DEV_INITRD
	if (initrd_start && !initrd_below_start_ok &&
	    page_to_pfn(virt_to_page((void *)initrd_start)) < min_low_pfn) {
		pr_crit("initrd overwritten (0x%08lx < 0x%08lx) - disabling it.\n",
		    page_to_pfn(virt_to_page((void *)initrd_start)),
		    min_low_pfn);
		initrd_start = 0;
	}
#endif
	setup_per_cpu_pageset();
	numa_policy_init();
	acpi_early_init();
	if (late_time_init)
		late_time_init();
	sched_clock_init();
	calibrate_delay();
	pid_idr_init();
	anon_vma_init();
#ifdef CONFIG_X86
	if (efi_enabled(EFI_RUNTIME_SERVICES))
		efi_enter_virtual_mode();
#endif
	thread_stack_cache_init();
	cred_init();
	fork_init();
	proc_caches_init();
	uts_ns_init();
	key_init();
	security_init();
	dbg_late_init();
	net_ns_init();
	vfs_caches_init();
	pagecache_init();
	signals_init();
	seq_file_init();
	proc_root_init();
	nsfs_init();
	cpuset_init();
	cgroup_init();
	taskstats_init_early();
	delayacct_init();

	poking_init();
	check_bugs();

	acpi_subsystem_init();
	arch_post_acpi_subsys_init();
	kcsan_init();

	/* Do the rest non-__init'ed, we're now alive */
	arch_call_rest_init();

	prevent_tail_call_optimization();
}

当然最初的comdline是通过uboot传过来的，可以是设备树，也可以是地址传，在下面这个函数解析的。

	setup_arch(&command_line);

内核中定义了好多的comdline，刚启动时，是完全一样使用uboot的传参，后面可以根据需要更改。

init/main.c

/*
 * We need to store the untouched command line for future reference.
 * We also need to store the touched command line since the parameter
 * parsing is performed in place, and we should allow a component to
 * store reference of name/value for future reference.
 */
static void __init setup_command_line(char *command_line)
{
	size_t len, xlen = 0, ilen = 0;

	if (extra_command_line)
		xlen = strlen(extra_command_line);
	if (extra_init_args)
		ilen = strlen(extra_init_args) + 4; /* for " -- " */

	len = xlen + strlen(boot_command_line) + 1;

	saved_command_line = memblock_alloc(len + ilen, SMP_CACHE_BYTES);
	if (!saved_command_line)
		panic("%s: Failed to allocate %zu bytes\n", __func__, len + ilen);

	static_command_line = memblock_alloc(len, SMP_CACHE_BYTES);
	if (!static_command_line)
		panic("%s: Failed to allocate %zu bytes\n", __func__, len);

	if (xlen) {
		/*
		 * We have to put extra_command_line before boot command
		 * lines because there could be dashes (separator of init
		 * command line) in the command lines.
		 */
		strcpy(saved_command_line, extra_command_line);
		strcpy(static_command_line, extra_command_line);
	}
	strcpy(saved_command_line + xlen, boot_command_line);
	strcpy(static_command_line + xlen, command_line);

	if (ilen) {
		/*
		 * Append supplemental init boot args to saved_command_line
		 * so that user can check what command line options passed
		 * to init.
		 * The order should always be
		 * " -- "[bootconfig init-param][cmdline init-param]
		 */
		if (initargs_offs) {
			len = xlen + initargs_offs;
			strcpy(saved_command_line + len, extra_init_args);
			len += ilen - 4;	/* strlen(extra_init_args) */
			strcpy(saved_command_line + len,
				boot_command_line + initargs_offs - 1);
		} else {
			len = strlen(saved_command_line);
			strcpy(saved_command_line + len, " -- ");
			len += 4;
			strcpy(saved_command_line + len, extra_init_args);
		}
	}
}

1.4、parse_early_param

	parse_early_param();
	after_dashes = parse_args("Booting kernel",
				  static_command_line, __start___param,
				  __stop___param - __start___param,
				  -1, -1, NULL, &unknown_bootoption);

先看一下早期的解析函数

init/main.c

void __init parse_early_options(char *cmdline)
{
	parse_args("early options", cmdline, NULL, 0, 0, 0, NULL,
		   do_early_param);
}

/* Arch code calls this early on, or if not, just before other parsing. */
void __init parse_early_param(void)
{
	static int done __initdata;
	static char tmp_cmdline[COMMAND_LINE_SIZE] __initdata;

	if (done)
		return;

	/* All fall through to do_early_param. */
	strscpy(tmp_cmdline, boot_command_line, COMMAND_LINE_SIZE);
	parse_early_options(tmp_cmdline);
	done = 1;
}

kernel/params.c

  
/* Args looks like "foo=bar,bar2 baz=fuz wiz". */
char *parse_args(const char *doing,
		 char *args,
		 const struct kernel_param *params,
		 unsigned num,
		 s16 min_level,
		 s16 max_level,
		 void *arg,
		 int (*unknown)(char *param, char *val,
				const char *doing, void *arg))
{
	char *param, *val, *err = NULL;
 
	/* Chew leading spaces */
    /* 忽略掉参数最前面的的空格 */
	args = skip_spaces(args);
 
	if (*args)
		pr_debug("doing %s, parsing ARGS: '%s'\n", doing, args);
 
	while (*args) {
		int ret;
		int irq_was_disabled;
 
		args = next_arg(args, &param, &val);
		/* Stop at -- */
		if (!val && strcmp(param, "--") == 0)
			return err ?: args;
		irq_was_disabled = irqs_disabled();
		ret = parse_one(param, val, doing, params, num,
				min_level, max_level, arg, unknown);
		if (irq_was_disabled && !irqs_disabled())
			pr_warn("%s: option '%s' enabled irq's!\n",
				doing, param);
 
		switch (ret) {
		case 0:
			continue;
		case -ENOENT:
			pr_err("%s: Unknown parameter `%s'\n", doing, param);
			break;
		case -ENOSPC:
			pr_err("%s: `%s' too large for parameter `%s'\n",
			       doing, val ?: "", param);
			break;
		default:
			pr_err("%s: `%s' invalid for parameter `%s'\n",
			       doing, val ?: "", param);
			break;
		}
 
		err = ERR_PTR(ret);
	}
 
	return err;
}

上函数很简单，因为cmdline里面可能有多个命令，每个命令之间应用空格间隔，依次拿出每一个命令执行parse_one函数。

 root=/dev/nfs nfsroot=192.168.0.101:/home/run/work/rootfs/rootfs_3.16.57
ip=192.168.0.20:192.168.0.101:192.168.0.1:255.255.255.0::eth0:off init=/linuxrc 
console=ttyS0,115200 earlyprintk

这个parse_one函数也比较简单，因为上面传入的params为NULL，num_params为0，min_level和max_level都是0，所以for里面的也肯定不会执行的，也就是只执行handle_unknown。

kernel/params.c

  
static int parse_one(char *param,
		     char *val,
		     const char *doing,
		     const struct kernel_param *params,
		     unsigned num_params,
		     s16 min_level,
		     s16 max_level,
		     void *arg,
		     int (*handle_unknown)(char *param, char *val,
				     const char *doing, void *arg))
{
	unsigned int i;
	int err;
 
	/* Find parameter */
	for (i = 0; i < num_params; i++) {
		if (parameq(param, params[i].name)) {
			if (params[i].level < min_level
			    || params[i].level > max_level)
				return 0;
			/* No one handled NULL, so do it here. */
			if (!val &&
			    !(params[i].ops->flags & KERNEL_PARAM_OPS_FL_NOARG))
				return -EINVAL;
			pr_debug("handling %s with %p\n", param,
				params[i].ops->set);
			kernel_param_lock(params[i].mod);
			param_check_unsafe(&params[i]);
			err = params[i].ops->set(val, &params[i]);
			kernel_param_unlock(params[i].mod);
			return err;
		}
	}
 
	if (handle_unknown) {
		pr_debug("doing %s: %s='%s'\n", doing, param, val);
		return handle_unknown(param, val, doing, arg);
	}
 
	pr_debug("Unknown argument '%s'\n", param);
	return -ENOENT;
}

这个handle_unknown就是上面传进去的早期的参数函数了。

init/main.c

  
/* Check for early params. */
static int __init do_early_param(char *param, char *val,
				 const char *unused, void *arg)
{
	const struct obs_kernel_param *p;
 
	for (p = __setup_start; p < __setup_end; p++) {
		if ((p->early && parameq(param, p->str)) ||
		    (strcmp(param, "console") == 0 &&
		     strcmp(p->str, "earlycon") == 0)
		) {
			if (p->setup_func(val) != 0)
				pr_warn("Malformed early option '%s'\n", param);
		}
	}
	/* We accept everything at this stage. */
	return 0;
}

可以看到这里也用到了段属性定义的__setup_start，__setup_end。在这个段属性中结构体中扫描，看是否那个结构体中rerlay这个整数不为0，且命令行参数中有传给内核，或者比较命令行参数有为consloe，且这个.init.setup段属性有earlycon这个名字的结构体。

1.5、命令行参数

看一下命令行参数，有定义earlayprintk和console=ttySAC2

 root=/dev/nfs nfsroot=192.168.0.101:/home/run/work/rootfs/rootfs_3.16.57
ip=192.168.0.20:192.168.0.101:192.168.0.1:255.255.255.0::eth0:off init=/linuxrc 
console=ttyS0,115200 earlyprintk

内核搜索"earlyprintk"，所以早期的rarlay_printk就是在这里初始化的。

arch/arm/kernel/early_printk.c

 static int __init setup_early_printk(char *buf)
{
	early_console = &early_console_dev;
	register_console(&early_console_dev);
	return 0;
}
 
early_param("earlyprintk", setup_early_printk);

这里把早期的打印原理也说一下，打印字符串。

arch/arm/kernel/early_printk.c

 
 
static void early_write(const char *s, unsigned n)
{
	char buf[128];
	while (n) {
		unsigned l = min(n, sizeof(buf)-1);
		memcpy(buf, s, l);
		buf[l] = 0;
		s += l;
		n -= l;
		printascii(buf);
	}
}
 
static void early_console_write(struct console *con, const char *s, unsigned n)
{
	early_write(s, n);
}
 
static struct console early_console_dev = {
	.name =		"earlycon",
	.write =	early_console_write,
	.flags =	CON_PRINTBUFFER | CON_BOOT,
	.index =	-1,
};

最终就调用一个串口的发送函数printascii，这个函数使用汇编定义的。

1.6、printascii

发送函数很简单

 ENTRY(printascii)
		addruart_current r3, r1, r2
1:		teq	r0, #0
		ldrneb	r1, [r0], #1
		teqne	r1, #0
		reteq	lr
2:		teq     r1, #'\n'
		bne	3f
		mov	r1, #'\r'
		waituart r2, r3
		senduart r1, r3
		busyuart r2, r3
		mov	r1, #'\n'
3:		waituart r2, r3
		senduart r1, r3
		busyuart r2, r3
		b	1b
ENDPROC(printascii)

可以看到，早期的earlay_printk只能在串口打印，而且实现方式很简单，

二、console

earlay_printk完了，还定义了console。

2.1、console调用

drivers/tty/serial/earlycon.c

  
early_param("earlycon", param_setup_earlycon);

这个搜索后也是定义了的，所以这函数也要执行。

关于这个也涉及到另一个段，__earlycon_table

include/linux/serial_core.h

#define OF_EARLYCON_DECLARE(_name, compat, fn)				\
	static const struct earlycon_id __UNIQUE_ID(__earlycon_##_name) \
		EARLYCON_USED_OR_UNUSED  __section("__earlycon_table")  \
		__aligned(__alignof__(struct earlycon_id))		\
		= { .name = __stringify(_name),				\
		    .compatible = compat,				\
		    .setup = fn }

#define EARLYCON_DECLARE(_name, fn)	OF_EARLYCON_DECLARE(_name, "", fn)

extern const struct earlycon_id *__earlycon_table[];
extern const struct earlycon_id *__earlycon_table_end[];

以sprd为例

不分析串口驱动，只是对内核调试的原理分析一下。

static int __init sprd_early_console_setup(struct earlycon_device *device,
					   const char *opt)
{
	if (!device->port.membase)
		return -ENODEV;

	device->con->write = sprd_early_write;
	return 0;
}
OF_EARLYCON_DECLARE(sprd_serial, "sprd,sc9836-uart",
		    sprd_early_console_setup);

搜索所有的earlaycon后，和值比较，找到匹配的。

init/main.c

  
/* Check for early params. */
static int __init do_early_param(char *param, char *val,
				 const char *unused, void *arg)
{
	const struct obs_kernel_param *p;
 
	for (p = __setup_start; p < __setup_end; p++) {
		if ((p->early && parameq(param, p->str)) ||
		    (strcmp(param, "console") == 0 &&
		     strcmp(p->str, "earlycon") == 0)
		) {
			if (p->setup_func(val) != 0)
				pr_warn("Malformed early option '%s'\n", param);
		}
	}
	/* We accept everything at this stage. */
	return 0;
}

2.2、解析console

可以看一下上面的earlay函数，这里传的参数是val，earlayprintk没val，但console有val，为tyyS0

 root=/dev/nfs nfsroot=192.168.0.101:/home/run/work/rootfs/rootfs_3.16.57
ip=192.168.0.20:192.168.0.101:192.168.0.1:255.255.255.0::eth0:off init=/linuxrc 
console=ttyS0,115200 earlyprintk

可以看一下面的解析函数：

drivers/tty/serial/earlycon.c

/* early_param wrapper for setup_earlycon() */
static int __init param_setup_earlycon(char *buf)
{
	int err;

	/* Just 'earlycon' is a valid param for devicetree and ACPI SPCR. */
	if (!buf || !buf[0]) {
		if (IS_ENABLED(CONFIG_ACPI_SPCR_TABLE)) {
			earlycon_acpi_spcr_enable = true;
			return 0;
		} else if (!buf) {
			return early_init_dt_scan_chosen_stdout();
		}
	}

	err = setup_earlycon(buf);
	if (err == -ENOENT || err == -EALREADY)
		return 0;
	return err;
}
early_param("earlycon", param_setup_earlycon);

drivers/tty/serial/earlycon.c

int __init setup_earlycon(char *buf)
{
	const struct earlycon_id *match;
	bool empty_compatible = true;

	if (!buf || !buf[0])
		return -EINVAL;

	if (early_con.flags & CON_ENABLED)
		return -EALREADY;

again:
	for (match = __earlycon_table; match < __earlycon_table_end; match++) {
		size_t len = strlen(match->name);

		if (strncmp(buf, match->name, len))
			continue;

		/* prefer entries with empty compatible */
		if (empty_compatible && *match->compatible)
			continue;

		if (buf[len]) {
			if (buf[len] != ',')
				continue;
			buf += len + 1;
		} else
			buf = NULL;

		return register_earlycon(buf, match);
	}

	if (empty_compatible) {
		empty_compatible = false;
		goto again;
	}

	return -ENOENT;
}

比较的是ttyS0和name，很明显，没有一个设备的name是ttyS0。

OF_EARLYCON_DECLARE(sprd_serial, "sprd,sc9836-uart",
		    sprd_early_console_setup);
 
#define _OF_EARLYCON_DECLARE(_name, compat, fn, unique_id)		\
	static const struct earlycon_id unique_id			\
	     EARLYCON_USED_OR_UNUSED __initconst			\
		= { .name = __stringify(_name),				\
		    .compatible = compat,				\
		    .setup = fn  };					\
	static const struct earlycon_id EARLYCON_USED_OR_UNUSED		\
		__section(__earlycon_table)				\
		* const __PASTE(__p, unique_id) = &unique_id
 
#define OF_EARLYCON_DECLARE(_name, compat, fn)				\
	_OF_EARLYCON_DECLARE(_name, compat, fn,				\
			     __UNIQUE_ID(__earlycon_##_name))

这里的name是sprd，不是ttyS0。.

2.3、ttyS0执行

那ttyS0什么时候执行呢，这个就不属于parse_early_param函数的功能了，而是下面的这个parse_args了。

 	parse_early_param();
	after_dashes = parse_args("Booting kernel",
				  static_command_line, __start___param,
				  __stop___param - __start___param,
				  -1, -1, NULL, &unknown_bootoption);