分类： 嵌入式
2013-06-03 10:55:33
水平有限，描述不当之处还请之处，转载请注明出处http://blog.csdn.net/vanbreaker/article/details/7743184
本节结合i2cdev，来阐述Linux下的IIC是如何进行数据传输的。和spidev类似，i2cdev也是一个通用的设备驱动，但是又有些不同。在 spidev中，spidev驱动注册会和相应的从设备绑定，也就是说spidev对应的是一个实际的从设备，而i2cdev只是一个虚拟的从设备，因为 它并不对应一个实际的从设备，而是和IIC控制器adapter绑定，只有当用户调用open()打开一个设备文件时，才会创建一个虚拟的client。 那么怎么知道该i2cdev对应哪个从设备呢？其实很简单，只需要用户在用户空间多做一个工作，就是通过Iocntl()函数来设定从设备的地址，当然这 个从设备必须是挂接在对应的adapter下的。所以i2cdev作为一个虚拟设备，本身并不对应任何设备，但又可以对应任意一个挂接在其依附的 adapter下的设备。
先来看下i2cdev的定义
struct i2c_dev {
struct list_head list;//用于链入i2c_dev_list
struct i2c_adapter *adap;//依附的IIC主控制器
struct device *dev;
};
第一步，当然是初始化i2cdev模块，将i2cdev作为字符设备注册，创建一个i2cdev类，并且注册i2cdev的驱动
static int __init i2c_dev_init(void)
{
int res;
printk(KERN_INFO "i2c /dev entries driver\n");
/*注册i2cdev字符设备*/
res = register_chrdev(I2C_MAJOR, "i2c", &i2cdev_fops);
if (res)
goto out;
/*创建i2cdev类*/
i2c_dev_class = class_create(THIS_MODULE, "i2c-dev");
if (IS_ERR(i2c_dev_class)) {
res = PTR_ERR(i2c_dev_class);
goto out_unreg_chrdev;
}
/*添加i2cdev驱动*/
res = i2c_add_driver(&i2cdev_driver);
if (res)
goto out_unreg_class;
return 0;
out_unreg_class:
class_destroy(i2c_dev_class);
out_unreg_chrdev:
unregister_chrdev(I2C_MAJOR, "i2c");
out:
printk(KERN_ERR "%s: Driver Initialisation failed\n", __FILE__);
return res;
}
i2c_add_driver()只是i2c_register_driver()的一个封装，进入i2c_register_driver()
int i2c_register_driver(struct module *owner, struct i2c_driver *driver)
{
...
...
...
driver->driver.bus = &i2c_bus_type;
...
res = driver_register(&driver->driver);
...
/*遍历i2c_adapter_class下的i2c_adapter,并调用__attach_adapter*/
class_for_each_device(&i2c_adapter_class, NULL, driver,
__attach_adapter);
...
return 0;
}
我们可以看到主要操作有两个，一个是driver_register(),还有一个就是遍历adapter,并调用__attach_adapter()
实际上这里的driver_register()并没有太多实际的作用，因为它无法完成i2cdev_driver和从设备的匹配，为什么呢？我们来看一下i2c_bus_type中定义的match函数
static int i2c_device_match(struct device *dev, struct device_driver *drv)
{
struct i2c_client *client = to_i2c_client(dev);
struct i2c_driver *driver = to_i2c_driver(drv);
/* make legacy i2c drivers bypass driver model probing entirely;
* such drivers scan each i2c adapter/bus themselves.
*/
if (!is_newstyle_driver(driver))
return 0;
/* match on an id table if there is one */
if (driver->id_table)
return i2c_match_id(driver->id_table, client) != NULL;
return 0;
}
static const struct i2c_device_id *i2c_match_id(const struct i2c_device_id *id,
const struct i2c_client *client)
{
while (id->name[0]) {
if (strcmp(client->name, id->name) == 0)
return id;
id++;
}
return NULL;
}
可以看到，只有当driver->id_table不为空时，才会调用实际的匹配函数i2c_match_id(),而i2cdev_driver中并没有定义id_table,因此是无法和任何从设备匹配的。
static struct i2c_driver i2cdev_driver = {
.driver = {
.name = "dev_driver",
},
.attach_adapter = i2cdev_attach_adapter,
.detach_adapter = i2cdev_detach_adapter,
};
再来看__attach_adapter()
static int i2cdev_attach_adapter(struct i2c_adapter *adap)
{
struct i2c_dev *i2c_dev;
...
i2c_dev = get_free_i2c_dev(adap);
/*注册i2c_dev设备*/
i2c_dev->dev = device_create(i2c_dev_class, &adap->dev,
MKDEV(I2C_MAJOR, adap->nr), NULL,
"i2c-%d", adap->nr);
...
res = device_create_file(i2c_dev->dev, &dev_attr_name);
...
...
}
跟踪get_free_i2c_dev()
static struct i2c_dev *get_free_i2c_dev(struct i2c_adapter *adap)
{
struct i2c_dev *i2c_dev;
if (adap->nr >= I2C_MINORS) {
printk(KERN_ERR "i2c-dev: Out of device minors (%d)\n",
adap->nr);
return ERR_PTR(-ENODEV);
}
/*为i2c_dev申请空间*/
i2c_dev = kzalloc(sizeof(*i2c_dev), GFP_KERNEL);
if (!i2c_dev)
return ERR_PTR(-ENOMEM);
i2c_dev->adap = adap;//设置i2c_dev依附的i2c_adapter
spin_lock(&i2c_dev_list_lock);
list_add_tail(&i2c_dev->list, &i2c_dev_list);//将i2c_dev添加到i2c_dev_list
spin_unlock(&i2c_dev_list_lock);
return i2c_dev;
}
到这里就完成了i2cdev_driver的注册，主要任务就是在每个i2c_adapter下创建一个绑定的i2c_dev。
然后就是通过open来打开一个i2cdev
static int i2cdev_open(struct inode *inode, struct file *file)
{
unsigned int minor = iminor(inode);
struct i2c_client *client;
struct i2c_adapter *adap;
struct i2c_dev *i2c_dev;
int ret = 0;
lock_kernel();
/*遍历i2c_dev_list，通过次设备号来定位i2c_dev*/
i2c_dev = i2c_dev_get_by_minor(minor);
if (!i2c_dev) {
ret = -ENODEV;
goto out;
}
/*获取i2c_dev对应的i2c_adapter*/
adap = i2c_get_adapter(i2c_dev->adap->nr);
if (!adap) {
ret = -ENODEV;
goto out;
}
/* This creates an anonymous i2c_client, which may later be
* pointed to some address using I2C_SLAVE or I2C_SLAVE_FORCE.
*
* This client is ** NEVER REGISTERED ** with the driver model
* or I2C core code!! It just holds private copies of addressing
* information and maybe a PEC flag.
*/
/*创建一个client虚拟从设备*/
client = kzalloc(sizeof(*client), GFP_KERNEL);
if (!client) {
i2c_put_adapter(adap);
ret = -ENOMEM;
goto out;
}
snprintf(client->name, I2C_NAME_SIZE, "i2c-dev %d", adap->nr);
client->driver = &i2cdev_driver;//设定client所依附的driver
client->adapter = adap; //设定client所依附的i2c控制器
file->private_data = client;
out:
unlock_kernel();
return ret;
}
此时已经建立了一个虚拟的client,下一步就是要让其对应一个实际的从设备，那么就要在用户空间通过iocntl()指定从设备的地址
static long i2cdev_ioctl(struct file *file, unsigned int cmd, unsigned long arg)
{
...
switch ( cmd ) {
case I2C_SLAVE:
case I2C_SLAVE_FORCE:
if ((arg > 0x3ff) ||
(((client->flags & I2C_M_TEN) == 0) && arg > 0x7f))
return -EINVAL;
if (cmd == I2C_SLAVE && i2cdev_check_addr(client->adapter, arg))
return -EBUSY;
/* REVISIT: address could become busy later */
client->addr = arg;
return 0;
...
}
然后还需要设置超时等待的时间，和重试次数等等……就不赘述了。
然后就可以通过read()/write()函数来进行一个message的读写操作，也可以通过iocntl()函数控制进行一个消息组的传输，我们通过write()来分析消息传输的机制
static ssize_t i2cdev_write (struct file *file, const char __user *buf, size_t count,
loff_t *offset)
{
int ret;
char *tmp;
struct i2c_client *client = (struct i2c_client *)file->private_data;
if (count > 8192)
count = 8192;
tmp = kmalloc(count,GFP_KERNEL);
if (tmp==NULL)
return -ENOMEM;
if (copy_from_user(tmp,buf,count)) {//从用户空间获取数据
kfree(tmp);
return -EFAULT;
}
pr_debug("i2c-dev: i2c-%d writing %zu bytes.\n",
iminor(file->f_path.dentry->d_inode), count);
ret = i2c_master_send(client,tmp,count);//调用IIC核心层函数进行发送处理
kfree(tmp);
return ret;
}
int i2c_master_send(struct i2c_client *client,const char *buf ,int count)
{
int ret;
struct i2c_adapter *adap=client->adapter;
struct i2c_msg msg;
msg.addr = client->addr;//从设备地址
msg.flags = client->flags & I2C_M_TEN;
msg.len = count; //数据传输长度
msg.buf = (char *)buf;//数据缓冲区
ret = i2c_transfer(adap, &msg, 1);
/* If everything went ok (i.e. 1 msg transmitted), return #bytes
transmitted, else error code. */
return (ret == 1) ? count : ret;
}
IIC核心层函数i2c_transfer会调用相应的adap->algo中定义的
传输函数master_xfer()进行传输，s3c24xx中的master_xfer()函数为
s3c24xx_i2c_xfer()，s3c24xx_i2c_xfer()会调用s3c24xx_i2c_doxfer()进行实际的传输处理
static int s3c24xx_i2c_doxfer(struct s3c24xx_i2c *i2c,
struct i2c_msg *msgs, int num)
{
unsigned long timeout;
int ret;
if (i2c->suspended)
return -EIO;
ret = s3c24xx_i2c_set_master(i2c);
if (ret != 0) {
dev_err(i2c->dev, "cannot get bus (error %d)\n", ret);
ret = -EAGAIN;
goto out;
}
spin_lock_irq(&i2c->lock);
i2c->msg = msgs;//保存msg起始地址
i2c->msg_num = num; //保存msg的数量
i2c->msg_ptr = 0; //已传输的msg数量为0
i2c->msg_idx = 0; //单个msg中已传输的字节为0
i2c->state = STATE_START;//设置状态为start
s3c24xx_i2c_enable_irq(i2c);//使能i2c中断
/*确定是读还是写，并设定相关寄存器，准备开始传输数据*/
s3c24xx_i2c_message_start(i2c, msgs);
spin_unlock_irq(&i2c->lock);
/*阻塞进程，直到i2c中断完成传输任务或超时*/
timeout = wait_event_timeout(i2c->wait, i2c->msg_num == 0, HZ * 5);
ret = i2c->msg_idx;
/* having these next two as dev_err() makes life very
* noisy when doing an i2cdetect */
if (timeout == 0)
dev_dbg(i2c->dev, "timeout\n");
else if (ret != num)
dev_dbg(i2c->dev, "incomplete xfer (%d)\n", ret);
/* ensure the stop has been through the bus */
msleep(1);
out:
return ret;
}
到这里，传输工作就交给IIC中断函数进行处理了
static irqreturn_t s3c24xx_i2c_irq(int irqno, void *dev_id)
{
struct s3c24xx_i2c *i2c = dev_id;
unsigned long status;
unsigned long tmp;
status = readl(i2c->regs + S3C2410_IICSTAT);
if (status & S3C2410_IICSTAT_ARBITR) {
/* deal with arbitration loss */
dev_err(i2c->dev, "deal with arbitration loss\n");
}
if (i2c->state == STATE_IDLE) {
dev_dbg(i2c->dev, "IRQ: error i2c->state == IDLE\n");
tmp = readl(i2c->regs + S3C2410_IICCON);
tmp &= ~S3C2410_IICCON_IRQPEND;
writel(tmp, i2c->regs + S3C2410_IICCON);
goto out;
}
/* pretty much this leaves us with the fact that we've
* transmitted or received whatever byte we last sent */
i2s_s3c_irq_nextbyte(i2c, status);
out:
return IRQ_HANDLED;
}
我们来看中断的实际处理函数i2s_s3c_irq_nextbyte(),不知道这里为什么是i2s-.-,难道是内核开发者的笔误?
static int i2s_s3c_irq_nextbyte(struct s3c24xx_i2c *i2c, unsigned long iicstat)
{
unsigned long tmp;
unsigned char byte;
int ret = 0;
switch (i2c->state) {
case STATE_IDLE:
dev_err(i2c->dev, "%s: called in STATE_IDLE\n", __func__);
goto out;
break;
case STATE_STOP: //停止传输，禁止中断
dev_err(i2c->dev, "%s: called in STATE_STOP\n", __func__);
s3c24xx_i2c_disable_irq(i2c);
goto out_ack;
case STATE_START:
/* last thing we did was send a start condition on the
* bus, or started a new i2c message
*/
if (iicstat & S3C2410_IICSTAT_LASTBIT &&
!(i2c->msg->flags & I2C_M_IGNORE_NAK)) {
/* ack was not received... */
dev_dbg(i2c->dev, "ack was not received\n");
s3c24xx_i2c_stop(i2c, -ENXIO);
goto out_ack;
}
/*确定读写状态*/
if (i2c->msg->flags & I2C_M_RD)
i2c->state = STATE_READ;
else
i2c->state = STATE_WRITE;
/* terminate the transfer if there is nothing to do
* as this is used by the i2c probe to find devices. */
/*如果已经是最后一条消息并且该消息已经发送完，则停止传输*/
if (is_lastmsg(i2c) && i2c->msg->len == 0) {
s3c24xx_i2c_stop(i2c, 0);
goto out_ack;
}
if (i2c->state == STATE_READ)
goto prepare_read;
/* fall through to the write state, as we will need to
* send a byte as well */
case STATE_WRITE:
/* we are writing data to the device... check for the
* end of the message, and if so, work out what to do
*/
if (!(i2c->msg->flags & I2C_M_IGNORE_NAK)) {
if (iicstat & S3C2410_IICSTAT_LASTBIT) {
dev_dbg(i2c->dev, "WRITE: No Ack\n");
s3c24xx_i2c_stop(i2c, -ECONNREFUSED);
goto out_ack;
}
}
retry_write:
if (!is_msgend(i2c)) {//该消息还未发送完毕
byte = i2c->msg->buf[i2c->msg_ptr++];
writeb(byte, i2c->regs + S3C2410_IICDS);//将数据写入寄存器发送
/* delay after writing the byte to allow the
* data setup time on the bus, as writing the
* data to the register causes the first bit
* to appear on SDA, and SCL will change as
* soon as the interrupt is acknowledged */
ndelay(i2c->tx_setup);
} else if (!is_lastmsg(i2c)) {//不是最后一条消息，则取下一条消息
/* we need to go to the next i2c message */
dev_dbg(i2c->dev, "WRITE: Next Message\n");
i2c->msg_ptr = 0;
i2c->msg_idx++;
i2c->msg++; //取下一条消息进行发送
/* check to see if we need to do another message */
if (i2c->msg->flags & I2C_M_NOSTART) {
if (i2c->msg->flags & I2C_M_RD) {
/* cannot do this, the controller
* forces us to send a new START
* when we change direction */
s3c24xx_i2c_stop(i2c, -EINVAL);
}
goto retry_write;
} else {/*开始新一轮的发送*/
/* send the new start */
s3c24xx_i2c_message_start(i2c, i2c->msg);
i2c->state = STATE_START;
}
} else {
/* send stop */
s3c24xx_i2c_stop(i2c, 0);
}
break;
case STATE_READ:
/* we have a byte of data in the data register, do
* something with it, and then work out wether we are
* going to do any more read/write
*/
byte = readb(i2c->regs + S3C2410_IICDS);//读取数据
i2c->msg->buf[i2c->msg_ptr++] = byte; //存放数据
prepare_read:
if (is_msglast(i2c)) {//如果是最后一条消息
/* last byte of buffer */
if (is_lastmsg(i2c))//如果消息发送完毕
s3c24xx_i2c_disable_ack(i2c);//失能ack
} else if (is_msgend(i2c)) {//消息读取完毕
/* ok, we've read the entire buffer, see if there
* is anything else we need to do */
if (is_lastmsg(i2c)) {//如果是最后一条消息则停止传输
/* last message, send stop and complete */
dev_dbg(i2c->dev, "READ: Send Stop\n");
s3c24xx_i2c_stop(i2c, 0);
} else {//否则取下一个消息存放读取的数据
/* go to the next transfer */
dev_dbg(i2c->dev, "READ: Next Transfer\n");
i2c->msg_ptr = 0;
i2c->msg_idx++;
i2c->msg++;
}
}
break;
}
/* acknowlegde the IRQ and get back on with the work */
out_ack:
tmp = readl(i2c->regs + S3C2410_IICCON);
tmp &= ~S3C2410_IICCON_IRQPEND;
writel(tmp, i2c->regs + S3C2410_IICCON);
out:
return ret;
}
IIC的框架介绍到此为止