http://mcuos.com/thread-8169-1-1.html
（一）知識背景：
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[color=Red]Uncompressing Linux... done, booting the kernel.
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------------setup_arch------------
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------------setup_machine_fdt return 0------------
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Machine: MCUOS6410[/color]
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Linux version 3.0.30-g4e794c6-dirty (zswan@zswan-laptop-ubuntu) (gcc version
4.2.1) #17 Sun May 6 00:55:44 CST 2012
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bootconsole [earlycon0] enabled
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S3C24XX Clocks, Copyright 2004 Simtec Electronics
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camera: no parent clock specified
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S3C64XX: PLL settings, A=532000000, M=532000000, E=24000000
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S3C64XX: HCLK2=266000000, HCLK=133000000, PCLK=66500000
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mout_apll: source is fout_apll (1), rate is 532000000
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mout_epll: source is epll (1), rate is 24000000
複製代碼
我們知道line 5-12行代碼部分，是printk打印出來的。而如果你想要在打印linux的版本之前的函數line1-4行中也加打印信息，那麼printk這個時候其實串口，console什麼的還沒註冊呢，所以肯定沒有信息的。比如說，我想打印setup函數中的部分調試信息：
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void __init setup_arch(char **cmdline_p)
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{
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struct machine_desc *mdesc;
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unwind_init();
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setup_processor();
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early_printk("------------setup_arch------------\n");
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//return ;
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mdesc = setup_machine_fdt(__atags_pointer);
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early_printk("------------setup_machine_fdt return %d------------\n", mdesc);
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if (!mdesc)
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mdesc = setup_machine_tags(machine_arch_type);
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machine_desc = mdesc;
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machine_name = mdesc->name;
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early_printk("Machine: %s\n", machine_name);
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if (mdesc->soft_reboot)
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reboot_setup("s");
複製代碼
就可以使用early printk的功能啦，那麼如何打開這個功能呢，下面我做個介紹：（二）支持early printk對內核需要做的配置(1)Kernel hacking ---> Kernel low-level debugging functions --> Early printk (2)boot option中你需要添加 earlyprintk項。類似於：root=/dev/ram0 console=ttySAC0,115200n8 rdinit=/sbin/init earlyprintk（三）在需要加打印信息的地方使用early_printk函數代替printk函數。（四）對early printk的驅動實現的分析arch/arm/kernel/early_printk.c文件，上代碼：
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extern void printch(int);
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static void early_write(const char *s, unsigned n)
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{
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while (n-- > 0) {
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if (*s == '\n')
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printch('\r');
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printch(*s);
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s++;
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}
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}
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static void early_console_write(struct console *con, const char *s, unsigned n)
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{
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early_write(s, n);
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}
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static struct console early_console = {
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.name = "earlycon",
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.write = early_console_write,
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.flags = CON_PRINTBUFFER | CON_BOOT,
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.index = -1,
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};
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asmlinkage void early_printk(const char *fmt, ...)
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{
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char buf[512];
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int n;
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va_list ap;
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va_start(ap, fmt);
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n = vscnprintf(buf, sizeof(buf), fmt, ap);
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early_write(buf, n);
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va_end(ap);
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}
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static int __init setup_early_printk(char *buf)
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{
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register_console(&early_console);
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return 0;
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}
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early_param("earlyprintk", setup_early_printk);
複製代碼
其實這段code最終的實現都是靠：extern void printch(int);這個函數。這個函數實現是在：arch/arm/kernel/debug.S中：
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ENTRY(printch)
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addruart_current r3, r1, r2
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mov r1, r0
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mov r0, #0
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b 1b
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ENDPROC(printch)
複製代碼
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.macro addruart_current, rx, tmp1, tmp2
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addruart \tmp1, \tmp2
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mrc p15, 0, \rx, c1, c0
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tst \rx, #1
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moveq \rx, \tmp1
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movne \rx, \tmp2
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.endm
複製代碼
printch會調用到 addruart_current函數，而addruart_current函數用調用到：addruart函數，該函數實現是在：arch\arm\mach-s3c64xx\include\mach中的debug-macro.S彙編文件中：
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.macro addruart, rp, rv
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ldr \rp, = S3C_PA_UART
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ldr \rv, = (S3C_VA_UART + S3C_PA_UART & 0xfffff)
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#if CONFIG_DEBUG_S3C_UART != 0
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add \rp, \rp, #(0x400 * CONFIG_DEBUG_S3C_UART)
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add \rv, \rv, #(0x400 * CONFIG_DEBUG_S3C_UART)
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#endif
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.endm
複製代碼
我們從上面的代碼可以看到S3C_PA_UART， S3C_PA_UART都是實際的6410的串口寄存器物理和虛擬地址，從而進行真正的硬件底層操作。
