console驱动：一、基本概念终端是一种字符型设备，通常使用tty简称各种类型的终端。linux的终端类型：/dev/ttySn,串行口终端/dev/pty,伪终端/dev/tty,当前进程的控制终端，可以是介绍的其它任何一种终端/dev/ttyn,tty1~tty6是虚拟终端，tty0当前虚拟终端的别名。/dev/console，控制台终端（显示器）二、uboot传参数的处理linux启动时uboot传递进console=ttyS2,115200n8的参数内核中用__setup()宏声明参数处理的方法：__setup("console=", console_setup); 1.console_cmdline结构体struct console_cmdline { char name[8]; //驱动名 int index; //次设备号 char *options; //选项 #ifdef CONFIG_A11Y_BRAILLE_CONSOLE char *brl_options; #endif }; 2.内核调用console_setup()函数处理uboot传进的console参数static int __init console_setup(char *str) { char buf[sizeof(console_cmdline[0].name) + 4]; //分配驱动名+index的缓冲区，分配12个字节 char *s, *options, *brl_options = NULL; int idx; #ifdef CONFIG_A11Y_BRAILLE_CONSOLE if (!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 if (str[0] >= '0' && str[0] <= '9') { //第一个参数属于[0,9] strcpy(buf, "ttyS"); //则将其驱动名设为ttyS strncpy(buf + 4, str, sizeof(buf) - 5);//将次设备号放其后面 } else { strncpy(buf, str, sizeof(buf) - 1); //否则直接将驱动名+设备号拷贝到buf中 } buf[sizeof(buf) - 1] = 0; if ((options = strchr(str, ',')) != NULL) //获取options，即“115200n8” *(options++) = 0; #ifdef __sparc__ if (!strcmp(str, "ttya")) strcpy(buf, "ttyS0"); if (!strcmp(str, "ttyb")) strcpy(buf, "ttyS1"); #endif for (s = buf; *s; s++) if ((*s >= '0' && *s <= '9') || *s == ',')//移动指针s到次设备号处 break; idx = simple_strtoul(s, NULL, 10); //获取次设备号，字符串转换成unsigend long long型数据，s表示字符串的开始，NULL表示字符串的结束，10表示进制 //这里返回的是次设备号=2 *s = 0; __add_preferred_console(buf, idx, options, brl_options); console_set_on_cmdline = 1; return 1; } 3.__add_preferred_console()函数//整体的作用是根据uboot传递的参数设置全局console_cmdline数组//该数组及全局selected_console，在register_console中会使用到static int __add_preferred_console(char *name, int idx, char *options,char *brl_options) { struct console_cmdline *c; int i; for (i = 0; i < MAX_CMDLINECONSOLES && console_cmdline[i].name[0]; i++)//可以最多8个console if (strcmp(console_cmdline[i].name, name) == 0 && console_cmdline[i].index == idx) { //比较已注册的console_cmdline数组中的项的名字及次设备号,若console_cmdline已经存在 if (!brl_options) selected_console = i;//设置全局selected_console索引号 return 0;//则返回 } if (i == MAX_CMDLINECONSOLES)//判断console_cmdline数组是否满了 return -E2BIG; if (!brl_options) selected_console = i; //设置全局selected_console索引号 c = &console_cmdline[i];//获取全局console_cmdline数组的第i项地址 strlcpy(c->name, name, sizeof(c->name)); //填充全局console_cmdline的驱动名“ttyS2” c->options = options; //填充配置选项115200n8 #ifdef CONFIG_A11Y_BRAILLE_CONSOLE c->brl_options = brl_options; #endif c->index = idx; //填充索引号2，即次设备号 return 0; } 三、在console初始化之前能使用printk，使用内核提供的early printk支持。//在调用console_init之前调用printk也能打印出信息，这是為什麼呢？在start_kernel函数中很早就调用了 parse_early_param函数，//该函数会调用到链接脚本中.init.setup段的函数。其中就有 setup_early_serial8250_console函数。//该函数通过 register_console(&early_serial8250_console);//注册了一个比较简单的串口设备。可以用来打印内核启 动早期的信息。//对于early printk的console注册往往通过内核的early_param完成。early_param(“earlycon”,setup_early_serial8250_console);//定义一个earlycon的内核参数，内核解析这个参数时调用setup_early_serial8250_console()函数1.setup_early_serial8250_console()函数//earlycon = uart8250,mmio,0xff5e0000,115200n8int __init setup_early_serial8250_console(char *cmdline){ char *options; int err; options = strstr(cmdline, "uart8250,");//找到“uart8250,”字符串，返回此字符串的起始位置 if (!options) { options = strstr(cmdline, "uart,"); if (!options) return 0; } options = strchr(cmdline, ',') + 1;//options指针指向第一个逗号后边的字符串地址 err = early_serial8250_setup(options);//进行配置 if (err < 0) return err; /* static struct console early_serial8250_console __initdata = { .name = "uart", .write = early_serial8250_write, .flags = CON_PRINTBUFFER | CON_BOOT,//所用具有CON_BOOT属性的console都会在内核初始化到late initcall阶段被注销，相互消他们的函数是 .index = -1, }; */ //注册一个早期的console,到真正的console_init时，此console会被注销，因为设置了CON_BOOT标志 register_console(&early_serial8250_console); return 0;}static int __init early_serial8250_setup(char *options){ struct early_serial8250_device *device = &early_device; int err; if (device->port.membase || device->port.iobase)//early_device设备的端口地址若配置过则返回 return 0; err = parse_options(device, options);//解析参数并配置early_device设备对应的uart_port结构 if (err < 0) return err; init_port(device);//early_device设备对应的初始化uart_port结构 return 0;}static int __init parse_options(struct early_serial8250_device *device,char *options){ struct uart_port *port = &device->port;//找到early_device设备对应的uart_port结构 int mmio, mmio32, length; if (!options) return -ENODEV; port->uartclk = BASE_BAUD * 16;//串口时钟 mmio = !strncmp(options, "mmio,", 5);//查找"mmio,"字符串，找到mmio=1 mmio32 = !strncmp(options, "mmio32,", 7);//mmio32=0 if (mmio || mmio32) { port->iotype = (mmio ? UPIO_MEM : UPIO_MEM32);//串口类型设为UPIO_MEM=2 port->mapbase = simple_strtoul(options + (mmio ? 5 : 7),&options, 0);//获得串口的配置寄存器基础地址（物理地址），这里是得到0xff5e0000 if (mmio32) port->regshift = 2;#ifdef CONFIG_FIX_EARLYCON_MEM set_fixmap_nocache(FIX_EARLYCON_MEM_BASE,port->mapbase & PAGE_MASK); port->membase =(void __iomem *)__fix_to_virt(FIX_EARLYCON_MEM_BASE); port->membase += port->mapbase & ~PAGE_MASK;#else port->membase = ioremap_nocache(port->mapbase, 64);//映射到内存的配置寄存器基础地址 if (!port->membase) { printk(KERN_ERR "%s: Couldn't ioremap 0x%llx\n", __func__,(unsigned long long) port->mapbase); return -ENOMEM; }#endif } else if (!strncmp(options, "io,", 3)) { port->iotype = UPIO_PORT; port->iobase = simple_strtoul(options + 3, &options, 0); mmio = 0; } else return -EINVAL; options = strchr(options, ',');//指针移到“115200n8”字符串处 if (options) {//存在 options++; device->baud = simple_strtoul(options, NULL, 0);//取得波特率115200 length = min(strcspn(options, " "), sizeof(device->options)); strncpy(device->options, options, length);//将字符串115200n8拷贝到设备的device->options字段中 } else { device->baud = probe_baud(port); snprintf(device->options, sizeof(device->options), "%u",device->baud); } if (mmio || mmio32) printk(KERN_INFO "Early serial console at MMIO%s 0x%llx (options '%s')\n",mmio32 ? "32" : "",(unsigned long long)port->mapbase,device->options); else printk(KERN_INFO "Early serial console at I/O port 0x%lx (options '%s')\n",port->iobase,device->options); return 0;}static void __init init_port(struct early_serial8250_device *device){ struct uart_port *port = &device->port; unsigned int divisor; unsigned char c; serial_out(port, UART_LCR, 0x3); /* 8n1 */ serial_out(port, UART_IER, 0); /* no interrupt */ serial_out(port, UART_FCR, 0); /* no fifo */ serial_out(port, UART_MCR, 0x3); /* DTR + RTS */ divisor = port->uartclk / (16 * device->baud);//根据波特率设置分频 c = serial_in(port, UART_LCR); serial_out(port, UART_LCR, c | UART_LCR_DLAB); serial_out(port, UART_DLL, divisor & 0xff); serial_out(port, UART_DLM, (divisor >> 8) & 0xff); serial_out(port, UART_LCR, c & ~UART_LCR_DLAB);}void register_console(struct console *newcon){ int i; unsigned long flags; struct console *bcon = NULL; /* 现在是注册一个early console，即 static struct console early_serial8250_console __initdata = { .name = "uart", .write = early_serial8250_write, .flags = CON_PRINTBUFFER | CON_BOOT,//所用具有CON_BOOT属性的console都会在内核初始化到late initcall阶段被注销，相互消他们的函数是 .index = -1, }; */ if (console_drivers && newcon->flags & CON_BOOT) {//注册的是否是引导控制台。early console的CON_BOOT置位，表示只是一个引导控制台，以后会被注销 for_each_console(bcon) {////遍历全局console_drivers数组 if (!(bcon->flags & CON_BOOT)) {//判断是否已经有引导控制台了,有了的话就直接退出 printk(KERN_INFO "Too late to register bootconsole %s%d\n",newcon->name, newcon->index); return; } } } if (console_drivers && console_drivers->flags & CON_BOOT)//如果注册的是引导控制台 bcon = console_drivers;//让bcon指向全局console_drivers if (preferred_console < 0 || bcon || !console_drivers) preferred_console = selected_console;//设置preferred_console为uboot命令选择的selected_console(即索引) if (newcon->early_setup)//early console没有初始化early_setup字段，以下这个函数不执行 newcon->early_setup();//调用serial8250_console_early_setup() 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; } } } //传给内核参数: //Kernel command line: console=ttyS2,115200n8 rw root=/dev/ram0 initrd=0xc2000000,20M mem=128M ip=192.168.1.220::192.168.1.1:255.255.255.0::eth0:off //所以这里将根据传参console=ttyS2,115200来配置作为console的ttyS2串口 for (i = 0; i < MAX_CMDLINECONSOLES && console_cmdline[i].name[0];i++) {//遍历全局console_cmdline找到匹配的 if (strcmp(console_cmdline[i].name, newcon->name) != 0)//比较终端名称“ttyS” continue; if (newcon->index >= 0 &&newcon->index != console_cmdline[i].index)//console_cmdline[i].index=2。//比较次设备号 continue; if (newcon->index < 0) newcon->index = console_cmdline[i].index;//将终端号赋值给serial8250_console->index #ifdef CONFIG_A11Y_BRAILLE_CONSOLE//没有定义，下边不执行 if (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; }#endif //console_cmdline[i].options = "115200n8"，对于early console而言setup字段未被初始化，故下边的函数不执行 if (newcon->setup &&newcon->setup(newcon, console_cmdline[i].options) != 0)//调用serial8250_console_setup()对终端进行配置 break; newcon->flags |= CON_ENABLED; //设置标志为CON_ENABLE(这个在printk调用中使用到) newcon->index = console_cmdline[i].index;//设置索引号 if (i == selected_console) { //索引号和uboot指定的console的一样 newcon->flags |= CON_CONSDEV;//设置标志CON_CONSDEV(全局console_drivers链表中靠前) preferred_console = selected_console; } break; }//for循环作用大致是查看注册的console是否是uboot知道的引导console，是则设置相关标志和preferred_console if (!(newcon->flags & CON_ENABLED)) return; if (bcon && ((newcon->flags & (CON_CONSDEV | CON_BOOT)) == CON_CONSDEV))//防止重复打印 newcon->flags &= ~CON_PRINTBUFFER; acquire_console_sem(); if ((newcon->flags & CON_CONSDEV) || console_drivers == NULL) {//如果是preferred控制台 newcon->next = console_drivers; console_drivers = newcon;//添加进全局console_drivers链表前面位置（printk中会遍历该表调用合适的console的write方法打印信息） if (newcon->next) newcon->next->flags &= ~CON_CONSDEV; } else {//如果不是preferred控制台 newcon->next = console_drivers->next; console_drivers->next = newcon; //添加进全局console_drivers链表后面位置 } //主册console主要是刷选preferred_console放置在全局console_drivers链表前面,剩下的console放置链表靠后的位置,并设置相应的flags, //console_drivers最终会在printk函数的层层调用中遍历到,并调用console的write方法将信息打印出来 if (newcon->flags & CON_PRINTBUFFER) { spin_lock_irqsave(&logbuf_lock, flags); con_start = log_start; spin_unlock_irqrestore(&logbuf_lock, flags); } release_console_sem(); if (bcon && ((newcon->flags & (CON_CONSDEV | CON_BOOT)) == CON_CONSDEV)) { printk(KERN_INFO "console [%s%d] enabled, bootconsole disabled\n",newcon->name, newcon->index); for_each_console(bcon) if (bcon->flags & CON_BOOT) unregister_console(bcon); } else {//调用这里 printk(KERN_INFO "%sconsole [%s%d] enabled\n",(newcon->flags & CON_BOOT) ? "boot" : "" ,newcon->name, newcon->index); }}四、在未对console进行初始化之前，内核使用early console进行打印。之后内核进行真正的console初始化//console_init()在start_kernel()中调用，用来对控制台初始化，这个函数执行完成后，串口可以看到内核用printk()函数打印的信息void __init console_init(void){ initcall_t *call; /* Setup the default TTY line discipline. */ //此函数调用tty_register_ldisc(N_TTY, &tty_ldisc_N_TTY) //#define N_TTY 0 /*struct tty_ldisc_ops tty_ldisc_N_TTY = { .magic = TTY_LDISC_MAGIC, .name = "n_tty", .open = n_tty_open, .close = n_tty_close, .flush_buffer = n_tty_flush_buffer, .chars_in_buffer = n_tty_chars_in_buffer, .read = n_tty_read, .write = n_tty_write, .ioctl = n_tty_ioctl, .set_termios = n_tty_set_termios, .poll = n_tty_poll, .receive_buf = n_tty_receive_buf, .write_wakeup = n_tty_write_wakeup }; 内核定义一个tty_ldiscs数组，然后根据数组下标来存放对应的线路规程的操作集，而这里的数组下标表示的就是具体的协议，在头文件中已经通过宏定义好了。例如N_TTY 0。 所以可以发现:ldisc[0] 存放的是N_TTY对应的线路规程操作集 ldisc[1]存放的是N_SLIP对应的线路规程操作集 ldisc[2]存放的就是N_MOUSE对应的线路规程操作集 依次类推。此处就是ldisc[N_TTY] = tty_ldisc_N_TTY。 int tty_register_ldisc(int disc, struct tty_ldisc_ops *new_ldisc) { unsigned long flags; int ret = 0; if (disc < N_TTY || disc >= NR_LDISCS) return -EINVAL; spin_lock_irqsave(&tty_ldisc_lock, flags); tty_ldiscs[disc] = new_ldisc;//tty_ldiscs[0]存放的是N_TTY对应的线路规程操作集 new_ldisc->num = disc;//0 new_ldisc->refcount = 0; spin_unlock_irqrestore(&tty_ldisc_lock, flags); return ret; }*/ tty_ldisc_begin();//这段代码前面是注册了第0个(逻辑上1)线路规程 //依次调用从__con_initcall_start到__con_initcall_end之间的函数指针 //会调用两个函数就是con_init()和serial8250_console_init() call = __con_initcall_start; while (call < __con_initcall_end) { (*call)(); call++; }}static int __init serial8250_console_init(void){ if (nr_uarts > UART_NR)//串口数量不能大于3个 nr_uarts = UART_NR; serial8250_isa_init_ports();//对三个串口的uart_8250_port结构静态常量serial8250_ports结构进行初始化,主要是将up->port.ops = &serial8250_pops /* static struct console serial8250_console = { .name = "ttyS", .write = serial8250_console_write,//写方法 .device = uart_console_device,//tty驱动 .setup = serial8250_console_setup,//设置串口波特率，也就是设置串口。很重要，里面涉及到平台特性，波特率相关。 .early_setup = serial8250_console_early_setup, .flags = CON_PRINTBUFFER | CON_ANYTIME, .index = -1, .data = &serial8250_reg, }; */ register_console(&serial8250_console);//在这里注册serial8250_console真正的console终端 return 0;}console_initcall(serial8250_console_init);/*serial8250_console_init()函数会比serial8250_probe()先调用,所以调用register_console的时候,port还没有初始化,所以当register_console调用serial8250_console_setup()设置buad,parity bits的时候,serial8250_console_setup()会检测port->iobase和port->membase是否是有效值,如果不是就返回,放弃初始化console,所以实际上,console不是在serial8250_console_init()里边初始化,如果要在serial8250_console_init初始化,需要将port静态初始化.当serial8250_probe()调用uart_add_one_port->uart_configure_port:if (port->cons && !(port->cons->flags & CON_ENABLED)){ printk("%s retister console\n", __FUNCTION__); register_console(port->cons);}该函数会检查console有没有初始化,如果没有初始化,则调用register_console来初始化.所以console放在这里初始化也是比较好一些,可以将console_initcall(serial8250_console_init) comment.*///对三个串口的uart_8250_port结构静态常量serial8250_ports结构进行初始化,主要是将up->port.ops = &serial8250_popsstatic void __init serial8250_isa_init_ports(void){ struct uart_8250_port *up; static int first = 1; int i, irqflag = 0; if (!first)//静态变量，serial8250_console_init()第一次进入这个函数，之后serial8250_init()再进入这个函数就会直接返回 return; first = 0; //对三个串口的uart_8250_port结构serial8250_ports结构体进行初始化 for (i = 0; i < nr_uarts; i++) { struct uart_8250_port *up = &serial8250_ports[i]; up->port.line = i;//0代表串口0,1代表串口1 spin_lock_init(&up->port.lock); init_timer(&up->timer);//初始化定时器 up->timer.function = serial8250_timeout;//初始化定时器的超时函数 //ALPHA_KLUDGE_MCR needs to be killed. up->mcr_mask = ~ALPHA_KLUDGE_MCR; up->mcr_force = ALPHA_KLUDGE_MCR; //初始化uart_8250_port指向的uart_port字段port的操作 up->port.ops = &serial8250_pops; /* static struct uart_ops serial8250_pops = { .tx_empty = serial8250_tx_empty, .set_mctrl = serial8250_set_mctrl, .get_mctrl = serial8250_get_mctrl, .stop_tx = serial8250_stop_tx, .start_tx = serial8250_start_tx, .stop_rx = serial8250_stop_rx, .enable_ms = serial8250_enable_ms, .break_ctl = serial8250_break_ctl, .startup = serial8250_startup, .shutdown = serial8250_shutdown, .set_termios = serial8250_set_termios, .set_ldisc = serial8250_set_ldisc, .pm = serial8250_pm, .type = serial8250_type, .release_port = serial8250_release_port, .request_port = serial8250_request_port, .config_port = serial8250_config_port, .verify_port = serial8250_verify_port, #ifdef CONFIG_CONSOLE_POLL .poll_get_char = serial8250_get_poll_char, .poll_put_char = serial8250_put_poll_char, #endif }; */ } if (share_irqs)//中断是否共享（这里设置成不共享） irqflag = IRQF_SHARED; //条件不满足，不会进来初始化 for (i = 0, up = serial8250_ports;i < ARRAY_SIZE(old_serial_port) && i < nr_uarts;i++, up++) {/* up->port.iobase = old_serial_port[i].port; up->port.irq = irq_canonicalize(old_serial_port[i].irq); up->port.irqflags = old_serial_port[i].irqflags; up->port.uartclk = old_serial_port[i].baud_base * 16; up->port.flags = old_serial_port[i].flags; up->port.hub6 = old_serial_port[i].hub6; up->port.membase = old_serial_port[i].iomem_base; up->port.iotype = old_serial_port[i].io_type; up->port.regshift = old_serial_port[i].iomem_reg_shift; set_io_from_upio(&up->port); up->port.irqflags |= irqflag; if (serial8250_isa_config != NULL) serial8250_isa_config(i, &up->port, &up->capabilities);*/ }}//下边再次调用register_console()注册serial8250_console真正的console终端void register_console(struct console *newcon){ int i; unsigned long flags; struct console *bcon = NULL; /* 现在是注册一个serial8250_console，即 static struct console serial8250_console = { .name = "ttyS", .write = serial8250_console_write,//写方法 .device = uart_console_device,//tty驱动 .setup = serial8250_console_setup,//设置串口波特率，也就是设置串口。很重要，里面涉及到平台特性，波特率相关。 .early_setup = serial8250_console_early_setup, .flags = CON_PRINTBUFFER | CON_ANYTIME, .index = -1, .data = &serial8250_reg, }; */ if (console_drivers && newcon->flags & CON_BOOT) {//注册的是serial8250_console，CON_BOOT没有置位，不是引导控制台。下边不会进去遍历 for_each_console(bcon) {////遍历全局console_drivers数组 if (!(bcon->flags & CON_BOOT)) {//判断是否已经有引导控制台了,有了的话就直接退出 printk(KERN_INFO "Too late to register bootconsole %s%d\n",newcon->name, newcon->index); return; } } } if (console_drivers && console_drivers->flags & CON_BOOT)//如果注册的是引导控制台，serial8250_console不是引导控制台 bcon = console_drivers;//这里不执行 if (preferred_console < 0 || bcon || !console_drivers) preferred_console = selected_console;//设置preferred_console为uboot命令选择的selected_console(即在Uboot传入的参数“console=ttyS2,115200n8”在console_cmdline[]数组中的索引) //这里preferred_console =0 if (newcon->early_setup)//serial8250_console初始化early_setup字段 newcon->early_setup();//调用serial8250_console_early_setup() if (preferred_console < 0) {//由于preferred_console =0，不会进入下边 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; } } } //传给内核参数: //Kernel command line: console=ttyS2,115200n8 rw root=/dev/ram0 initrd=0xc2000000,20M mem=128M ip=192.168.1.220::192.168.1.1:255.255.255.0::eth0:off //所以这里将根据传参console=ttyS2,115200来配置作为console的ttyS2串口 for (i = 0; i < MAX_CMDLINECONSOLES && console_cmdline[i].name[0];i++) {//遍历全局console_cmdline找到匹配的，i=0就是匹配的“ttyS2” if (strcmp(console_cmdline[i].name, newcon->name) != 0)//比较终端名称“ttyS” continue; if (newcon->index >= 0 &&newcon->index != console_cmdline[i].index)//console_cmdline[i].index=2。//比较次设备号 continue; if (newcon->index < 0) newcon->index = console_cmdline[i].index;//将终端号赋值给serial8250_console->index，这里是2 //console_cmdline[i].options = "115200n8"，对于serial8250_console而言setup字段已初始化 if (newcon->setup && newcon->setup(newcon, console_cmdline[i].options) != 0)//调用serial8250_console_setup()对终端进行配置，调用不成功 break; //在这里注册serial8250_console时，调用serial8250_console_setup()由于port->iobase和port->membase不是有效值， //故返回错误，这样下边的操作不会执行，直接break跳出，从flag1出跳出函数。即在这里serial8250_console没有注册成功 //由于内核在下边的操作队串口进行初始化时，还会调用register_console（）来注册serial8250_console，在那时注册就会成功 newcon->flags |= CON_ENABLED; //设置标志为CON_ENABLE,表示console使能(这个在printk调用中使用到) newcon->index = console_cmdline[i].index;//设置索引号 if (i == selected_console) { //索引号和uboot指定的console的一样 newcon->flags |= CON_CONSDEV;//设置标志CON_CONSDEV(全局console_drivers链表中靠前) preferred_console = selected_console; } break; }//for循环作用大致是查看注册的console是否是uboot知道的引导console，是则设置相关标志和preferred_console //flag1： if (!(newcon->flags & CON_ENABLED))//若前边没有设置CON_ENABLED标志，就退出 return; if (bcon && ((newcon->flags & (CON_CONSDEV | CON_BOOT)) == CON_CONSDEV))//防止重复打印 newcon->flags &= ~CON_PRINTBUFFER; acquire_console_sem(); if ((newcon->flags & CON_CONSDEV) || console_drivers == NULL) {//如果是preferred控制台 newcon->next = console_drivers; console_drivers = newcon;//添加进全局console_drivers链表前面位置（printk中会遍历该表调用合适的console的write方法打印信息） if (newcon->next) newcon->next->flags &= ~CON_CONSDEV; } else {//如果不是preferred控制台 newcon->next = console_drivers->next; console_drivers->next = newcon; //添加进全局console_drivers链表后面位置 } //主册console主要是刷选preferred_console放置在全局console_drivers链表前面,剩下的console放置链表靠后的位置,并设置相应的flags, //console_drivers最终会在printk函数的层层调用中遍历到,并调用console的write方法将信息打印出来 if (newcon->flags & CON_PRINTBUFFER) { spin_lock_irqsave(&logbuf_lock, flags); con_start = log_start; spin_unlock_irqrestore(&logbuf_lock, flags); } release_console_sem(); if (bcon && ((newcon->flags & (CON_CONSDEV | CON_BOOT)) == CON_CONSDEV)) { printk(KERN_INFO "console [%s%d] enabled, bootconsole disabled\n",newcon->name, newcon->index); for_each_console(bcon) if (bcon->flags & CON_BOOT) unregister_console(bcon); } else {//调用这里 printk(KERN_INFO "%sconsole [%s%d] enabled\n",(newcon->flags & CON_BOOT) ? "boot" : "" ,newcon->name, newcon->index); }}//serial8250_console_early_setup()-->serial8250_find_port_for_earlycon()int serial8250_find_port_for_earlycon(void){ struct early_serial8250_device *device = &early_device;//early console初始化时对early_device结构的初始化 struct uart_port *port = &device->port; int line; int ret; if (!device->port.membase && !device->port.iobase)//early_device结构初始化时已经配置好 return -ENODEV; //early console注册时不会调用此函数。 //当真正的console初始化时，会调用此函数。 //真正的console初始化时，会查找early console注册时用的是哪一个串口号，从serial8250_ports[]中根据uart_port->mapbase地址来比对 line = serial8250_find_port(port);//根据uart_port结构找到串口号，比对没有找到串口号，line返回负值 if (line < 0) return -ENODEV;//从这里返回，下边的不再执行 //若找到early console用的串口号，更新当初传入内核参数使用的console_cmdline[i]，名称改成ttyS。。。。 ret = update_console_cmdline("uart", 8250, "ttyS", line, device->options); if (ret < 0) ret = update_console_cmdline("uart", 0,"ttyS", line, device->options); return ret;}static int __init serial8250_console_setup(struct console *co, char *options){ struct uart_port *port; int baud = 9600; int bits = 8; int parity = 'n'; int flow = 'n'; if (co->index >= nr_uarts)//console的索引，这里是2，即ttyS2 co->index = 0; port = &serial8250_ports[co->index].port;//找到对应的ttyS2的uart_port结构 //由于console_init在注册serial8250_console时调用的register_console()函数调用serial8250_console_setup() //进入这个函数时，由于ttyS2的uart_port结构没有初始化，port->iobase 和port->membase值都未设置，所以直接从下边返回 //当进行串口初始化时，还会回来注册serial8250_console，再调用到这里，由于设置了ttyS2的uart_port结构，所以下边的配置就会成功 if (!port->iobase && !port->membase)//第一次注册时，由于未设置，从这里直接返回 return -ENODEV; if (options)//如果options不为空，就将options里的数值写给baud, &parity, &bits, &flow uart_parse_options(options, &baud, &parity, &bits, &flow); //没有配置options，则使用缺省值，否则使用传下来的的参数options里的串口配置 return uart_set_options(port, co, baud, parity, bits, flow);}五、通过四知道，在对console注册时，没有成功，由于串口还没有配置。当对串口配置时再对console注册就能成功。serial8250_console就能注册到内核全局变量console_drivers中。这样终端打印时就通过注册的serial8250_console就能将信息打印到终端上。 //内核的打印函数asmlinkage int printk(const char *fmt, ...){ va_list args; //可变参数链表 int r;#ifdef CONFIG_KGDB_KDB if (unlikely(kdb_trap_printk)) { va_start(args, fmt); r = vkdb_printf(fmt, args); va_end(args); return r; }#endif va_start(args, fmt); //获取第一个可变参数 r = vprintk(fmt, args); //调用vprintk函数 va_end(args); //释放可变参数链表指针 return r;}//vprintk函数asmlinkage int vprintk(const char *fmt, va_list args){ int printed_len = 0; int current_log_level = default_message_loglevel; unsigned long flags; int this_cpu; char *p; boot_delay_msec(); printk_delay(); preempt_disable(); raw_local_irq_save(flags); this_cpu = smp_processor_id(); if (unlikely(printk_cpu == this_cpu)) { if (!oops_in_progress) { recursion_bug = 1; goto out_restore_irqs; } zap_locks(); } lockdep_off(); spin_lock(&logbuf_lock); printk_cpu = this_cpu; if (recursion_bug) { recursion_bug = 0; strcpy(printk_buf, recursion_bug_msg); printed_len = strlen(recursion_bug_msg); } printed_len += vscnprintf(printk_buf + printed_len,sizeof(printk_buf) - printed_len, fmt, args); p = printk_buf; if (p[0] == '<') {//处理打印级别字段 unsigned char c = p[1]; if (c && p[2] == '>') { switch (c) { case '0' ... '7': /* loglevel */ current_log_level = c - '0'; case 'd': /* KERN_DEFAULT */ if (!new_text_line) { emit_log_char('\n'); new_text_line = 1; } case 'c': /* KERN_CONT */ p += 3; break; } } } for ( ; *p; p++) { if (new_text_line) { /* Always output the token */ emit_log_char('<'); emit_log_char(current_log_level + '0'); emit_log_char('>'); printed_len += 3; new_text_line = 0; if (printk_time) { //打印时间信息 /* Follow the token with the time */ char tbuf[50], *tp; unsigned tlen; unsigned long long t; unsigned long nanosec_rem; t = cpu_clock(printk_cpu); nanosec_rem = do_div(t, 1000000000); tlen = sprintf(tbuf, "[%5lu.%06lu] ",(unsigned long) t,nanosec_rem / 1000); for (tp = tbuf; tp < tbuf + tlen; tp++) emit_log_char(*tp); printed_len += tlen; } if (!*p) break; } emit_log_char(*p); if (*p == '\n') new_text_line = 1; } if (acquire_console_semaphore_for_printk(this_cpu)) release_console_sem(); lockdep_on();out_restore_irqs: raw_local_irq_restore(flags); preempt_enable(); return printed_len;}//接着调用release_console_sem函数 void release_console_sem(void){ unsigned long flags; unsigned _con_start, _log_end; unsigned wake_klogd = 0; if (console_suspended) { up(&console_sem); return; } console_may_schedule = 0; for ( ; ; ) { spin_lock_irqsave(&logbuf_lock, flags); wake_klogd |= log_start - log_end; if (con_start == log_end) break; /* Nothing to print */ _con_start = con_start; _log_end = log_end; con_start = log_end; /* Flush */ spin_unlock(&logbuf_lock); stop_critical_timings(); /* don't trace print latency */ call_console_drivers(_con_start, _log_end); start_critical_timings(); local_irq_restore(flags); } console_locked = 0; up(&console_sem); spin_unlock_irqrestore(&logbuf_lock, flags); if (wake_klogd) wake_up_klogd();}EXPORT_SYMBOL(release_console_sem);//调用call_console_drivers函数static void call_console_drivers(unsigned start, unsigned end){ unsigned cur_index, start_print; static int msg_level = -1; BUG_ON(((int)(start - end)) > 0); cur_index = start; start_print = start; while (cur_index != end) { if (msg_level < 0 && ((end - cur_index) > 2) &&LOG_BUF(cur_index + 0) == '<' &&LOG_BUF(cur_index + 1) >= '0' &&LOG_BUF(cur_index + 1) <= '7' &&LOG_BUF(cur_index + 2) == '>') { msg_level = LOG_BUF(cur_index + 1) - '0'; cur_index += 3; start_print = cur_index; } while (cur_index != end) { char c = LOG_BUF(cur_index); cur_index++; if (c == '\n') { if (msg_level < 0) { msg_level = default_message_loglevel; } _call_console_drivers(start_print, cur_index, msg_level); msg_level = -1; start_print = cur_index; break; } } } _call_console_drivers(start_print, end, msg_level);}_call_console_drivers函数//调用console的写方法static void __call_console_drivers(unsigned start, unsigned end) { struct console *con; for_each_console(con) {//遍历console_drivers数组 #define for_each_console(con) for (con = console_drivers; con != NULL; con = con->next) if ((con->flags & CON_ENABLED) && con->write &&(cpu_online(smp_processor_id()) ||(con->flags & CON_ANYTIME))) con->write(con, &LOG_BUF(start), end - start); //调用console的写方法 } } //由于已经注册的终端是serial8250_console，这个终端的写方法是调用serial8250_console_write()函数--->uart_console_write()--->serial8250_console_putchar()//--->serial_out()最终打印在串口2终端上/* static struct console serial8250_console = { .name = "ttyS", .write = serial8250_console_write,//写方法 .device = uart_console_device,//tty驱动 .setup = serial8250_console_setup,//设置串口波特率，也就是设置串口。很重要，里面涉及到平台特性，波特率相关。 .early_setup = serial8250_console_early_setup, .flags = CON_PRINTBUFFER | CON_ANYTIME, .index = -1, .data = &serial8250_reg, }; */console_drivers链表在register_console中会设置联系客服
