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,0xff5e0000,115200n8
int __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_pops
static 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