本文主要是介绍printk与 uart console关系分析(草稿)，希望对大家解决编程问题提供一定的参考价值，需要的开发者们随着小编来一起学习吧！
1，了解kernel启动过程
asmlinkage void __init start_kernel(void)
{...parse_args("Booting kernel", static_command_line, __start___param,__stop___param - __start___param,-1, -1, &unknown_bootoption);...console_init();...rest_init();
}了解下console_setup, console_init, uart_add_one_port.
console_setup如何调用而来：
在kernel/printk.c中console_setup代码：
static int __init console_setup(char *str)
{char buf[sizeof(console_cmdline[0].name) + 4]; /* 4 for index */char *s, *options, *brl_options = NULL;int idx;#ifdef CONFIG_A11Y_BRAILLE_CONSOLEif (!memcmp(str, "brl,", 4)) {brl_options = "";str += 4;} else if (!memcmp(str, "brl=", 4)) {brl_options = str + 4;str = strchr(brl_options, ',');if (!str) {printk(KERN_ERR "need port name after brl=\n");return 1;}*(str++) = 0;}
#endif/** Decode str into name, index, options.*/if (str[0] >= '0' && str[0] <= '9') {strcpy(buf, "ttyS");strncpy(buf + 4, str, sizeof(buf) - 5);} else {strncpy(buf, str, sizeof(buf) - 1);}buf[sizeof(buf) - 1] = 0;if ((options = strchr(str, ',')) != NULL)*(options++) = 0;//记录option，例如1152008n
#ifdef __sparc__if (!strcmp(str, "ttya"))strcpy(buf, "ttyS0");if (!strcmp(str, "ttyb"))strcpy(buf, "ttyS1");
#endiffor (s = buf; *s; s++)if ((*s >= '0' && *s <= '9') || *s == ',')break;idx = simple_strtoul(s, NULL, 10);//得到console=ttySxxx中串口号, 例如0，1，2...*s = 0;//使得buf内容不再包含串口号。__add_preferred_console(buf, idx, options, brl_options);//完成对console_cmdline数组的填充console_set_on_cmdline = 1;return 1;
}
__setup("console=", console_setup);__add_preferred_console函数：
static int __add_preferred_console(char *name, int idx, char *options,char *brl_options)
{struct console_cmdline *c;int i;/** See if this tty is not yet registered, and* if we have a slot free.*/for (i = 0; i < MAX_CMDLINECONSOLES && console_cmdline[i].name[0]; i++)if (strcmp(console_cmdline[i].name, name) == 0 &&console_cmdline[i].index == idx) {if (!brl_options)selected_console = i;return 0;}if (i == MAX_CMDLINECONSOLES)return -E2BIG;if (!brl_options)selected_console = i;//记录selected_console的值>=0c = &console_cmdline[i];strlcpy(c->name, name, sizeof(c->name));//记录串口名字c->options = options;//记录串口设置参数
#ifdef CONFIG_A11Y_BRAILLE_CONSOLEc->brl_options = brl_options;
#endif c->index = idx;//记录作为串口的index号，在console_setup中从console=ttySxxx中得到。return 0;
}
__setup跟踪， 在include/linux/init.h
#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) \__attribute__((aligned((sizeof(long))))) \= { __setup_str_##unique_id, fn, early }#define __setup(str, fn) \__setup_param(str, fn, fn, 0)console_init()函数定义在drivers/tty/tty_io.c，主要完成对console_initcall 声明函数的调用：
void __init console_init(void)
{initcall_t *call;/* Setup the default TTY line discipline. */tty_ldisc_begin();/** set up the console device so that later boot sequences can* inform about problems etc..*/call = __con_initcall_start;while (call < __con_initcall_end) {(*call)();call++;//依次调用console_initcall声明的函数,like：console_initcall(sirfsoc_uart_console_init);}
}
static int __init sirfsoc_uart_console_init(void)
{register_console(&sirfsoc_uart_console);return 0;
}
console_initcall(sirfsoc_uart_console_init);
void register_console(struct console *newcon)
{int i;unsigned long flags;struct console *bcon = NULL;/** before we register a new CON_BOOT console, make sure we don't* already have a valid console*/if (console_drivers && newcon->flags & CON_BOOT) {//boot console 如果开启了early_printk hacking,可以跟踪分析arch/arm/kernel/early_printk.c/* find the last or real console */for_each_console(bcon) {if (!(bcon->flags & CON_BOOT)) {printk(KERN_INFO "Too late to register bootconsole %s%d\n",newcon->name, newcon->index);return;}}}if (console_drivers && console_drivers->flags & CON_BOOT)//boot consolebcon = console_drivers;if (preferred_console < 0 || bcon || !console_drivers)preferred_console = selected_console;//对于real console preferred_console >=0if (newcon->early_setup)newcon->early_setup();/** See if we want to use this console driver. If we* didn't select a console we take the first one* that registers here.*/if (preferred_console < 0) {if (newcon->index < 0)newcon->index = 0;if (newcon->setup == NULL ||newcon->setup(newcon, NULL) == 0) {newcon->flags |= CON_ENABLED;if (newcon->device) {newcon->flags |= CON_CONSDEV;preferred_console = 0;}}}/** See if this console matches one we selected on* the command line.*/for (i = 0; i < MAX_CMDLINECONSOLES && console_cmdline[i].name[0];i++) {if (strcmp(console_cmdline[i].name, newcon->name) != 0)continue;if (newcon->index >= 0 &&newcon->index != console_cmdline[i].index)continue;if (newcon->index < 0)newcon->index = console_cmdline[i].index;//对于console_initcall类型函数调用的register_console, 对console的index
//附上console=ttySxxx传过来的串口号。
#ifdef CONFIG_A11Y_BRAILLE_CONSOLEif (console_cmdline[i].brl_options) {newcon->flags |= CON_BRL;braille_register_console(newcon,console_cmdline[i].index,console_cmdline[i].options,console_cmdline[i].brl_options);return;}
#endifif (newcon->setup &&newcon->setup(newcon, console_cmdline[i].options) != 0)break;//第一次register_cosole不能成功从这里退出，因为port->mapbase == 0, mapbase 没有在probe中被设置。newcon->flags |= CON_ENABLED;//如果setup成功设置CON_ENABLED标记。newcon->index = console_cmdline[i].index;if (i == selected_console) {newcon->flags |= CON_CONSDEV;preferred_console = selected_console;}break;}if (!(newcon->flags & CON_ENABLED))return;/** If we have a bootconsole, and are switching to a real console,* don't print everything out again, since when the boot console, and* the real console are the same physical device, it's annoying to* see the beginning boot messages twice*/if (bcon && ((newcon->flags & (CON_CONSDEV | CON_BOOT)) == CON_CONSDEV))newcon->flags &= ~CON_PRINTBUFFER;//如注释所示，避免了real-console再次打印log信息/** Put this console in the list - keep the* preferred driver at the head of the list.*/console_lock();if ((newcon->flags & CON_CONSDEV) || console_drivers == NULL) {newcon->next = console_drivers;console_drivers = newcon;if (newcon->next)newcon->next->flags &= ~CON_CONSDEV;} else {newcon->next = console_drivers->next;console_drivers->next = newcon;}if (newcon->flags & CON_PRINTBUFFER) {/** console_unlock(); will print out the buffered messages* for us.*/raw_spin_lock_irqsave(&logbuf_lock, flags);console_seq = syslog_seq;console_idx = syslog_idx;console_prev = syslog_prev;raw_spin_unlock_irqrestore(&logbuf_lock, flags);/** We're about to replay the log buffer. Only do this to the* just-registered console to avoid excessive message spam to* the already-registered consoles.*/exclusive_console = newcon;}console_unlock();console_sysfs_notify();/** By unregistering the bootconsoles after we enable the real console* we get the "console xxx enabled" message on all the consoles -* boot consoles, real consoles, etc - this is to ensure that end* users know there might be something in the kernel's log buffer that* went to the bootconsole (that they do not see on the real console)*/if (bcon &&((newcon->flags & (CON_CONSDEV | CON_BOOT)) == CON_CONSDEV) &&!keep_bootcon) {/* we need to iterate through twice, to make sure we print* everything out, before we unregister the console(s)*/printk(KERN_INFO "console [%s%d] enabled, bootconsole disabled\n",newcon->name, newcon->index);//开启了early_printkfor_each_console(bcon)if (bcon->flags & CON_BOOT)unregister_console(bcon);} else {printk(KERN_INFO "%sconsole [%s%d] enabled\n",(newcon->flags & CON_BOOT) ? "boot" : "" ,newcon->name, newcon->index);//未开启early_printk}
}
EXPORT_SYMBOL(register_console);
对probe函数中uart_add_one_port的分析：
1， 名词解析：
console_sem:
/** console_sem protects the console_drivers list, and also* provides serialisation for access to the entire console* driver system.*/
console_drivers:
static DEFINE_SEMAPHORE(console_sem);
struct console *console_drivers;
EXPORT_SYMBOL_GPL(console_drivers);
在这里将console_drivers设置成export_symbol的原因是在include/linux/console.h中定义了一个宏：
#define for_each_console(con) \for (con = console_drivers; con != NULL; con = con->next)
用来便利console_drivers的列表。
2，函数解析
register_console:
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); static struct console early_console_dev = {.name = "earlycon",.write = early_console_write,.flags = CON_PRINTBUFFER | CON_BOOT,.index = -1,
};
在boot/uboot/include/configs/atlas6cb.h中设置bootargs:
"devargs=" \"setenv bootargs no_console_suspend retain_initrd " \"earlyprintk resumewait " \"mem=${meminfo} " \"console=ttySiRF3 " \"lpj=7995392 " \"real_root=/dev/${bootinf}blk${bootdev}p${rootpart} " \"resume=/dev/${bootinf}blk${bootdev}p${resumepart} "\"scert_start=0x3800 " \"scert_size=0x1000\0" \
与pr_notice("Kernel command line: %s\n", boot_command_line);
3， 解压过程中log 打印“Uncompressing Linux... done, booting the kernel.”
make kmenuconfig->Kernel Hacking->Kernel low-level debugging functions->Platform ownself debug console.
由于选择了自己平台的console口，所以在选中CONFIG_DEBUG_LL的同时就没有选中Kernel low-level debug output via semihosting I/O，这就是CONFIG_DEBUG_SEMIHOSTING选项，开启DEBUG_LL只能选择一个debug口，因而CONFIG_DEBUG_SEMIHOSTING没有打开。代开arch/arm/Kconfig.debug文件：
choiceprompt "Kernel low-level debugging port"depends on DEBUG_LLconfig DEBUG_SEMIHOSTINGbool "Kernel low-level debug output via semihosting I/O"helpSemihosting enables code running on an ARM target to usethe I/O facilities on a host debugger/emulator through asimple SVC call. The host debugger or emulator must havesemihosting enabled for the special svc call to be trappedotherwise the kernel will crash.This is known to work with OpenOCD, as well asARM's Fast Models, or any other controlling environmentthat implements semihosting.For more details about semihosting, please seechapter 8 of DUI0203I_rvct_developer_guide.pdf from ARM Ltd.endchoice
config中的choice选项只能选择一种。
接着打开arch/arm/kernel/debug.S
#if !defined(CONFIG_DEBUG_SEMIHOSTING)
#include CONFIG_DEBUG_LL_INCLUDE
#endif第一句就是要包含DEBUG_LL_INCLUDE config选项：即arch/arm/Kconfig.debug文件中：
config DEBUG_LL_INCLUDEstringdefault "debug/bcm2835.S" if DEBUG_BCM2835default "debug/cns3xxx.S" if DEBUG_CNS3XXXdefault "debug/exynos.S" if DEBUG_EXYNOS_UARTdefault "debug/highbank.S" if DEBUG_HIGHBANK_UARTdefault "debug/icedcc.S" if DEBUG_ICEDCCdefault "debug/imx.S" if DEBUG_IMX1_UART || \DEBUG_IMX25_UART || \DEBUG_IMX21_IMX27_UART || \DEBUG_IMX31_UART || \DEBUG_IMX35_UART || \DEBUG_IMX51_UART || \DEBUG_IMX53_UART ||\DEBUG_IMX6Q_UARTdefault "debug/mvebu.S" if DEBUG_MVEBU_UARTdefault "debug/mxs.S" if DEBUG_IMX23_UART || DEBUG_IMX28_UARTdefault "debug/nomadik.S" if DEBUG_NOMADIK_UARTdefault "debug/omap2plus.S" if DEBUG_OMAP2PLUS_UARTdefault "debug/picoxcell.S" if DEBUG_PICOXCELL_UARTdefault "debug/pxa.S" if DEBUG_PXA_UART1 || DEBUG_MMP_UART2 || \DEBUG_MMP_UART3default "debug/sirf.S" if DEBUG_SIRFPRIMA2_UART1 || DEBUG_SIRFMARCO_UART1default "debug/socfpga.S" if DEBUG_SOCFPGA_UARTdefault "debug/sunxi.S" if DEBUG_SUNXI_UART0 || DEBUG_SUNXI_UART1default "debug/tegra.S" if DEBUG_TEGRA_UARTdefault "debug/ux500.S" if DEBUG_UX500_UARTdefault "debug/vexpress.S" if DEBUG_VEXPRESS_UART0_DETECT || \DEBUG_VEXPRESS_UART0_CA9 || DEBUG_VEXPRESS_UART0_RS1default "debug/vt8500.S" if DEBUG_VT8500_UART0default "debug/zynq.S" if DEBUG_ZYNQ_UART0 || DEBUG_ZYNQ_UART1default "mach/debug-macro.S"包含板子内部所带的debug文件。
同时在arch/arm/kernel/debug.S中：
#ifndef CONFIG_DEBUG_SEMIHOSTINGENTRY(printascii)addruart_current r3, r1, r2b 2f
1: waituart r2, r3senduart r1, r3busyuart r2, r3teq r1, #'\n'moveq r1, #'\r'beq 1b
2: teq r0, #0ldrneb r1, [r0], #1teqne r1, #0bne 1bmov pc, lr
ENDPROC(printascii)ENTRY(printch)addruart_current r3, r1, r2mov r1, r0mov r0, #0b 1b
ENDPROC(printch)
定义了printascii 和 printch函数，并调用了waituart,senduart,busyuart等函数。这些函数就是DEBUG_LL_INCLUDE中要包含的板子调试信息中所定义，如：
在arch/arm/include/debug/sirf.S
#if defined(CONFIG_DEBUG_SIRFPRIMA2_UART1)
#define SIRFSOC_UART1_PA_BASE 0xb0060000
#elif defined(CONFIG_DEBUG_SIRFMARCO_UART1)
#define SIRFSOC_UART1_PA_BASE 0xcc060000
#else
#define SIRFSOC_UART1_PA_BASE 0
#endif#define SIRFSOC_UART1_VA_BASE 0xFEC60000#define SIRFSOC_UART_TXFIFO_STATUS 0x0114
#define SIRFSOC_UART_TXFIFO_DATA 0x0118#define SIRFSOC_UART1_TXFIFO_FULL (1 << 5)
#define SIRFSOC_UART1_TXFIFO_EMPTY (1 << 6).macro addruart, rp, rv, tmpldr \rp, =SIRFSOC_UART1_PA_BASE @ physicalldr \rv, =SIRFSOC_UART1_VA_BASE @ virtual.endm.macro senduart,rd,rxstr \rd, [\rx, #SIRFSOC_UART_TXFIFO_DATA].endm.macro busyuart,rd,rx.endm.macro waituart,rd,rx
1001: ldr \rd, [\rx, #SIRFSOC_UART_TXFIFO_STATUS]tst \rd, #SIRFSOC_UART1_TXFIFO_EMPTYbeq 1001b.endm
定义了寄存器的地址，以及上述*uart的含义，直接想硬件寄存器放字符去打印出来。这就是lowlevel打印的函数形成的过程。
putc形成过程：
arch/arm/Kconfig.debug文件：
config DEBUG_UNCOMPRESSbooldefault y if ARCH_MULTIPLATFORM && DEBUG_LL && \!DEBUG_OMAP2PLUS_UART && \!DEBUG_TEGRA_UART
在打开DEBUG_LL后，默认应该打开了DEBUG_UNCOMPRESS config 选项。在arch/arm/boot/compressd/Makefile文件中：
ifeq ($(CONFIG_DEBUG_UNCOMPRESS),y)
OBJS += debug.o即编译arch/arm/boot/compressed/debug.S文件。该文件对putc函数进行了定义同样是使用arch/arm/include/debug/sirf.S中定义的*uart 函数。
在arch/arm/include/debug/uncompress.h文件中包含putc()函数的定义
#ifdef CONFIG_DEBUG_UNCOMPRESS
extern void putc(int c);
#else
static inline void putc(int c) {}
#endif
因而在真正的uart console启动起来之前，putc和printascii均调用的是直接想串口丢数据到txfifo，数据打印出来。
在sirf平台默认的就是uart1口。
4, console_setup:
__setup("console=", console_setup);/** Decode str into name, index, options.
*/
__add_preferred_console(buf, idx, options, brl_options);
console_set_on_cmdline = 1;
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