串口的open操作(tty_open)
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根据前面的操作，串口作为字符驱动也已经注册到系统了，/dev目录下也有设备文件节点了。
那接下来uart的操作是如何进行的呢?
操作硬件之前都是要先open设备，先来分析下这里的open函数具体做了那些工作(做了大量工作 ，真的！)。
应用层通过open系统调用open(“/dev/s3c2410_serial0”，)一层一层调用到会调用到tty_open。
因为串口在linux下是作为tty设备的，结合前面的注册过程可以分析这里首先调用的就是tty_open这个函数。
cdev_init(&driver->cdev, &tty_fops);static const struct file_operations tty_fops = {
.llseek = no_llseek,
.read = tty_read,
.write = tty_write,
.poll = tty_poll,
.unlocked_ioctl = tty_ioctl,
.compat_ioctl = tty_compat_ioctl,
.open = tty_open,
.release = tty_release,
.fasync = tty_fasync,
};所以这里就调用的是tty_open函数。
下面具体分析tty_open
static int tty_open(struct inode *inode, struct file *filp)
{
struct tty_struct *tty = NULL;
int noctty, retval;
struct tty_driver *driver;
int index;
dev_t device = inode->i_rdev;
unsigned saved_flags = filp->f_flags;
nonseekable_open(inode, filp);
retry_open:
noctty = filp->f_flags & O_NOCTTY;
index = -1;
retval = 0;
mutex_lock(&tty_mutex);
tty_lock();
if (device == MKDEV(TTYAUX_MAJOR, 0)) {
tty = get_current_tty();
if (!tty) {
tty_unlock();
mutex_unlock(&tty_mutex);
return -ENXIO;
}
driver = tty_driver_kref_get(tty->driver);
index = tty->index;
filp->f_flags |= O_NONBLOCK; /* Don't let /dev/tty block */
/* noctty = 1; */
/* FIXME: Should we take a driver reference ? */
tty_kref_put(tty);
goto got_driver;
}
#ifdef CONFIG_VT
if (device == MKDEV(TTY_MAJOR, 0)) {
extern struct tty_driver *console_driver;
driver = tty_driver_kref_get(console_driver);
index = fg_console;
noctty = 1;
goto got_driver;
}
#endif
if (device == MKDEV(TTYAUX_MAJOR, 1)) {
struct tty_driver *console_driver = console_device(&index);
if (console_driver) {
driver = tty_driver_kref_get(console_driver);
if (driver) {
/* Don't let /dev/console block */
filp->f_flags |= O_NONBLOCK;
noctty = 1;
goto got_driver;
}
}
tty_unlock();
mutex_unlock(&tty_mutex);
return -ENODEV;
}
driver = get_tty_driver(device, &index);
if (!driver) {
tty_unlock();
mutex_unlock(&tty_mutex);
return -ENODEV;
}
got_driver:
if (!tty) {
/* check whether we're reopening an existing tty */
tty = tty_driver_lookup_tty(driver, inode, index);
if (IS_ERR(tty)) {
tty_unlock();
mutex_unlock(&tty_mutex);
return PTR_ERR(tty);
}
}
if (tty) {
retval = tty_reopen(tty);
if (retval)
tty = ERR_PTR(retval);
} else
tty = tty_init_dev(driver, index, 0);
mutex_unlock(&tty_mutex);
tty_driver_kref_put(driver);
if (IS_ERR(tty)) {
tty_unlock();
return PTR_ERR(tty);
}
retval = tty_add_file(tty, filp);
if (retval) {
tty_unlock();
return retval;
}
check_tty_count(tty, "tty_open");
if (tty->driver->type == TTY_DRIVER_TYPE_PTY &&
tty->driver->subtype == PTY_TYPE_MASTER)
noctty = 1;
#ifdef TTY_DEBUG_HANGUP
printk(KERN_DEBUG "opening %s...", tty->name);
#endif
if (!retval) {
if (tty->ops->open)
retval = tty->ops->open(tty, filp);
else
retval = -ENODEV;
}
filp->f_flags = saved_flags;
if (!retval && test_bit(TTY_EXCLUSIVE, &tty->flags) &&
!capable(CAP_SYS_ADMIN))
retval = -EBUSY;
if (retval) {
#ifdef TTY_DEBUG_HANGUP
printk(KERN_DEBUG "error %d in opening %s...", retval,
tty->name);
#endif
tty_unlock(); /* need to call tty_release without BTM */
tty_release(inode, filp);
if (retval != -ERESTARTSYS)
return retval;
if (signal_pending(current))
return retval;
schedule();
/*
* Need to reset f_op in case a hangup happened.
*/
tty_lock();
if (filp->f_op == &hung_up_tty_fops)
filp->f_op = &tty_fops;
tty_unlock();
goto retry_open;
}
tty_unlock();
mutex_lock(&tty_mutex);
tty_lock();
spin_lock_irq(¤t->sighand->siglock);
if (!noctty &&
current->signal->leader &&
!current->signal->tty &&
tty->session == NULL)
__proc_set_tty(current, tty);
spin_unlock_irq(¤t->sighand->siglock);
tty_unlock();
mutex_unlock(&tty_mutex);
return 0;
}
5 0(/dev/tty)
5 1(/dev/console)
4 0(/dev/tty0)
此处打开的是/dev/s3c2410_serial0设备号为204 64
所以前面的判断全部失败，直接执行标红的那条语句
driver = get_tty_driver(device, &index);static struct tty_driver *get_tty_driver(dev_t device, int *index)
{
struct tty_driver *p;
list_for_each_entry(p, &tty_drivers, tty_drivers) {
dev_t base = MKDEV(p->major, p->minor_start);
if (device < base || device >= base + p->num)
continue;
*index = device - base;
return tty_driver_kref_get(p);
}
return NULL;
}因为一个tty_driver对应的是所有此种类型的tty设备，比如所有的串口设备，所以需要通过这个索引号
index来判断打开的是具体哪个设备。并且每个具体的设备对应着一个用来描述自己的tty_struct。
而系统后面的操作全部和这个tty_struct相关。
然后接着open函数会判断是否有tty_struct，假如不存在则创建并初始化一个tty_struct。
tty_struct是tty结构体中最重要的一个数据结构，内核通过tty-struct这个结构体来描述一个具体的tty设备在内核中
的活动状况的，后面对设备的write、read中都需要使用到这个tty_struct，并且严重依赖这个结构体。
由于此处是不存在tty_struct,所以直接调用函数初始化tty_struct
tty = tty_init_dev(driver, index, 0);参数driver为tty_driver，是前面通过get_tty_driver获得的，index此处为0，也是通过前面get_tty_driver获得的。
tty_init_dev()函数具体如下
struct tty_struct *tty_init_dev(struct tty_driver *driver, int idx,
int first_ok)
{
struct tty_struct *tty;
int retval;
/* Check if pty master is being opened multiple times */
if (driver->subtype == PTY_TYPE_MASTER &&
(driver->flags & TTY_DRIVER_DEVPTS_MEM) && !first_ok) {
return ERR_PTR(-EIO);
}
/*
* First time open is complex, especially for PTY devices.
* This code guarantees that either everything succeeds and the
* TTY is ready for operation, or else the table slots are vacated
* and the allocated memory released. (Except that the termios
* and locked termios may be retained.)
*/
if (!try_module_get(driver->owner))
return ERR_PTR(-ENODEV);
tty = alloc_tty_struct();
if (!tty)
goto fail_no_mem;
initialize_tty_struct(tty, driver, idx);
retval = tty_driver_install_tty(driver, tty);
if (retval < 0) {
free_tty_struct(tty);
module_put(driver->owner);
return ERR_PTR(retval);
}
/*
* Structures all installed ... call the ldisc open routines.
* If we fail here just call release_tty to clean up. No need
* to decrement the use counts, as release_tty doesn't care.
*/
retval = tty_ldisc_setup(tty, tty->link);
if (retval)
goto release_mem_out;
return tty;
fail_no_mem:
module_put(driver->owner);
return ERR_PTR(-ENOMEM);
/* call the tty release_tty routine to clean out this slot */
release_mem_out:
if (printk_ratelimit())
printk(KERN_INFO "tty_init_dev: ldisc open failed, "
"clearing slot %d\n", idx);
release_tty(tty, idx);
return ERR_PTR(retval);
}这个函数首先是给tty_struct分配好内存，然后将其初始化，初始化的工作在函数
initialize_tty_struct(tty, driver, idx);初始化的tty_struct部分信息来自于tty_driver，所以将tty_driver传递进此函数。
具体的initialize_tty_struct函数如下
void initialize_tty_struct(struct tty_struct *tty,
struct tty_driver *driver, int idx)
{
memset(tty, 0, sizeof(struct tty_struct));
kref_init(&tty->kref);
tty->magic = TTY_MAGIC;
tty_ldisc_init(tty);
tty->session = NULL;
tty->pgrp = NULL;
tty->overrun_time = jiffies;
tty->buf.head = tty->buf.tail = NULL;
tty_buffer_init(tty);
mutex_init(&tty->termios_mutex);
mutex_init(&tty->ldisc_mutex);
init_waitqueue_head(&tty->write_wait);
init_waitqueue_head(&tty->read_wait);
INIT_WORK(&tty->hangup_work, do_tty_hangup);
mutex_init(&tty->atomic_read_lock);
mutex_init(&tty->atomic_write_lock);
mutex_init(&tty->output_lock);
mutex_init(&tty->echo_lock);
spin_lock_init(&tty->read_lock);
spin_lock_init(&tty->ctrl_lock);
INIT_LIST_HEAD(&tty->tty_files);
INIT_WORK(&tty->SAK_work, do_SAK_work);
tty->driver = driver;
tty->ops = driver->ops;
tty->index = idx;
tty_line_name(driver, idx, tty->name);
tty->dev = tty_get_device(tty);
}而这里的初始化就是根据数组的索引号，从前面初始化好的tty线路规程操作方法数组中获取对应的操作方法，然后将其填充到
tty_struct中对应的域中。具体如下
void tty_ldisc_init(struct tty_struct *tty)
{
struct tty_ldisc *ld = tty_ldisc_get(N_TTY);
if (IS_ERR(ld))
panic("n_tty: init_tty");
tty_ldisc_assign(tty, ld);
} 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
}; 最后通过函数
tty_ldisc_assign(tty, ld);在这个tty_struct的初始化函数还需要注意的是
tty->ops = driver->ops;中的ops
此处的tty_driver的ops在uart_register_driver中被设置成如下
static const struct tty_operations uart_ops = {
.open = uart_open,
.close = uart_close,
.write = uart_write,
.put_char = uart_put_char,
.flush_chars = uart_flush_chars,
.write_room = uart_write_room,
.chars_in_buffer= uart_chars_in_buffer,
.flush_buffer = uart_flush_buffer,
.ioctl = uart_ioctl,
.throttle = uart_throttle,
.unthrottle = uart_unthrottle,
.send_xchar = uart_send_xchar,
.set_termios = uart_set_termios,
.set_ldisc = uart_set_ldisc,
.stop = uart_stop,
.start = uart_start,
.hangup = uart_hangup,
.break_ctl = uart_break_ctl,
.wait_until_sent= uart_wait_until_sent,
#ifdef CONFIG_PROC_FS
.proc_fops = &uart_proc_fops,
#endif
.tiocmget = uart_tiocmget,
.tiocmset = uart_tiocmset,
.get_icount = uart_get_icount,
#ifdef CONFIG_CONSOLE_POLL
.poll_init = uart_poll_init,
.poll_get_char = uart_poll_get_char,
.poll_put_char = uart_poll_put_char,
#endif
};接着函数返回到tty_init_dev函数中继续往下执行。
调用函数
retval = tty_driver_install_tty(driver, tty);tty_struct存放到tty_driver的tty_structs[]数组中，存放的位置依据tty-struct->index
这样可以tty_driver中每个设备都可以根据index找到对应的tty_struct了。
最后再次返回tty_init_dev函数中执行
retval = tty_ldisc_setup(tty, tty->link);这个函数会调用tty线路规程中的open方法n_tty_open函数
n_tty_open中主要完成buff缓冲区大小设置的工作，具体不展开了(其实我也没仔细分析)。
最后这个tty_struct的初始化函数彻底执行完毕，退到tty_open函数中继续执行后续代码。
在tty_open中会被执行到的代码是
retval = tty->ops->open(tty, filp);这里就是tty-struct中的ops，在tty_struct初始化的时候被赋值成tty_driver的ops，所以这里调用到的就是
uart_open函数。uart_open函数的重要作用是找到之前初始化的时候保存在tty_driver中的uart_state，因为这里面还有uart_port的重要信息。
找到这个uart_state后将其赋值给tty_struct。具体的uart_open函数如下
static int uart_open(struct tty_struct *tty, struct file *filp)
{
struct uart_driver *drv = (struct uart_driver *)tty->driver->driver_state;
struct uart_state *state;
struct tty_port *port;
int retval, line = tty->index;
BUG_ON(!tty_locked());
pr_debug("uart_open(%d) called\n", line);
/*
* tty->driver->num won't change, so we won't fail here with
* tty->driver_data set to something non-NULL (and therefore
* we won't get caught by uart_close()).
*/
retval = -ENODEV;
if (line >= tty->driver->num)
goto fail;
/*
* We take the semaphore inside uart_get to guarantee that we won't
* be re-entered while allocating the state structure, or while we
* request any IRQs that the driver may need. This also has the nice
* side-effect that it delays the action of uart_hangup, so we can
* guarantee that state->port.tty will always contain something
* reasonable.
*/
state = uart_get(drv, line);
if (IS_ERR(state)) {
retval = PTR_ERR(state);
goto fail;
}
port = &state->port;
/*
* Once we set tty->driver_data here, we are guaranteed that
* uart_close() will decrement the driver module use count.
* Any failures from here onwards should not touch the count.
*/
tty->driver_data = state;
state->uart_port->state = state;
tty->low_latency = (state->uart_port->flags & UPF_LOW_LATENCY) ? 1 : 0;
tty->alt_speed = 0;
tty_port_tty_set(port, tty);
/*
* If the port is in the middle of closing, bail out now.
*/
if (tty_hung_up_p(filp)) {
retval = -EAGAIN;
port->count--;
mutex_unlock(&port->mutex);
goto fail;
}
/*
* Make sure the device is in D0 state.
*/
if (port->count == 1)
uart_change_pm(state, 0);
/*
* Start up the serial port.
*/
retval = uart_startup(tty, state, 0);
/*
* If we succeeded, wait until the port is ready.
*/
mutex_unlock(&port->mutex);
if (retval == 0)
retval = tty_port_block_til_ready(port, tty, filp);
fail:
return retval;
} state = uart_get(drv, line);来找到保存在tty_driver中的uart_state。
最后将其值赋值给tty_struct。此处特别注意一下，这个uart_state会被放到tty_struct的driver_data中的！因为后面的write、read都是从driver_data中找到这个uar_state的！
最后函数条用uart_startup函数来初始化串口硬件
retval = uart_startup(tty, state, 0);具体的uart_startup函数如下
static int uart_startup(struct tty_struct *tty, struct uart_state *state, int init_hw)
{
struct uart_port *uport = state->uart_port;
struct tty_port *port = &state->port;
unsigned long page;
int retval = 0;
if (port->flags & ASYNC_INITIALIZED)
return 0;
/*
* Set the TTY IO error marker - we will only clear this
* once we have successfully opened the port. Also set
* up the tty->alt_speed kludge
*/
set_bit(TTY_IO_ERROR, &tty->flags);
if (uport->type == PORT_UNKNOWN)
return 0;
/*
* Initialise and allocate the transmit and temporary
* buffer.
*/
if (!state->xmit.buf) {
/* This is protected by the per port mutex */
page = get_zeroed_page(GFP_KERNEL);
if (!page)
return -ENOMEM;
state->xmit.buf = (unsigned char *) page;
uart_circ_clear(&state->xmit);
}
retval = uport->ops->startup(uport);
if (retval == 0) {
if (init_hw) {
/*
* Initialise the hardware port settings.
*/
uart_change_speed(tty, state, NULL);
/*
* Setup the RTS and DTR signals once the
* port is open and ready to respond.
*/
if (tty->termios->c_cflag & CBAUD)
uart_set_mctrl(uport, TIOCM_RTS | TIOCM_DTR);
}
if (port->flags & ASYNC_CTS_FLOW) {
spin_lock_irq(&uport->lock);
if (!(uport->ops->get_mctrl(uport) & TIOCM_CTS))
tty->hw_stopped = 1;
spin_unlock_irq(&uport->lock);
}
set_bit(ASYNCB_INITIALIZED, &port->flags);
clear_bit(TTY_IO_ERROR, &tty->flags);
}
if (retval && capable(CAP_SYS_ADMIN))
retval = 0;
return retval;
}
即uart_port的ops，此ops在初始化的时候被初始化为
static struct uart_ops s3c24xx_serial_ops = {
.pm = s3c24xx_serial_pm,
.tx_empty = s3c24xx_serial_tx_empty,
.get_mctrl = s3c24xx_serial_get_mctrl,
.set_mctrl = s3c24xx_serial_set_mctrl,
.stop_tx = s3c24xx_serial_stop_tx,
.start_tx = s3c24xx_serial_start_tx,
.stop_rx = s3c24xx_serial_stop_rx,
.enable_ms = s3c24xx_serial_enable_ms,
.break_ctl = s3c24xx_serial_break_ctl,
.startup = s3c24xx_serial_startup,
.shutdown = s3c24xx_serial_shutdown,
.set_termios = s3c24xx_serial_set_termios,
.type = s3c24xx_serial_type,
.release_port = s3c24xx_serial_release_port,
.request_port = s3c24xx_serial_request_port,
.config_port = s3c24xx_serial_config_port,
.verify_port = s3c24xx_serial_verify_port,
};static int s3c24xx_serial_startup(struct uart_port *port)
{
struct s3c24xx_uart_port *ourport = to_ourport(port);
int ret;
dbg("s3c24xx_serial_startup: port=%p (%08lx,%p)\n",
port->mapbase, port->membase);
rx_enabled(port) = 1;
ret = request_irq(ourport->rx_irq, s3c24xx_serial_rx_chars, 0,
s3c24xx_serial_portname(port), ourport);
if (ret != 0) {
printk(KERN_ERR "cannot get irq %d\n", ourport->rx_irq);
return ret;
}
ourport->rx_claimed = 1;
dbg("requesting tx irq...\n");
tx_enabled(port) = 1;
ret = request_irq(ourport->tx_irq, s3c24xx_serial_tx_chars, 0,
s3c24xx_serial_portname(port), ourport);
if (ret) {
printk(KERN_ERR "cannot get irq %d\n", ourport->tx_irq);
goto err;
}
ourport->tx_claimed = 1;
dbg("s3c24xx_serial_startup ok\n");
/* the port reset code should have done the correct
* register setup for the port controls */
return ret;
err:
s3c24xx_serial_shutdown(port);
return ret;
}
其作用还是初始化，主要初始化的是s3c24xx的uart的中断。
设置uart的发送中断，接收中断处理函数。
至此，通过open函数后s3c24xx的uart硬件部分也初始化好了，接着可以通过write、read函数收发数据了。
总结下
open的主要作用是 在内核通过创建并初始化一个tty_struct来描述具体对应的一个硬件设备，比如这里就是用一个tty_struct来描述s3c24xx上的uart0的，然后找到uart_port
中ops的startup方法初始化uart的硬件。
具体的tty_struct初始化过程中最重要的几步如下
1.初始化tty-struct的ops，就是将tty_driver中的ops赋值给tty_struct
2.初始化tty线路规程操作集
3.初始化tty_struct中的uart_state，uart_state中包含uart_port信息，这一步通过步骤1中ops中的open方法来完成。
4.根据步骤3中找到的uart_state，找到里面的uart_port的ops中的startup方法来初始化uart硬件。
open的流程大致如下：
open
--tty_open
|
--get_tty_driver
--tty_init_dev
--tty->ops->open(uart_open)
|
--uart_startup
|
--uport->ops->startup(s3c24xx_serial_startup())
|
--request_irq(rx_irq)
--request_irq(tx_irq)
中断处理函数如下：
s3c24xx_serial_rx_chars
s3c24xx_serial_tx_chars
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