Linux I2C子系统分析-I2C设备驱动

接下来以一个实际的例子来看I2C设备驱动，就以drivers/i2c/i2c-dev.c为例。

先看它的初始化和注销函数

[cpp] view plaincopy

static int __init i2c_dev_init(void)
{
int res;
printk(KERN_INFO "i2c /dev entries driver\n");
res = register_chrdev(I2C_MAJOR, "i2c", &i2cdev_fops);
if (res)
goto out;
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;
}
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;
}
static void __exit i2c_dev_exit(void)
{
i2c_del_driver(&i2cdev_driver);
class_destroy(i2c_dev_class);
unregister_chrdev(I2C_MAJOR,"i2c");
}

首先调用register_chrdev注册了一个字符设备，这是老的字符驱动注册方式。然后到了接下来的主角，i2c_add_driver，在I2C子系统中，I2C设备驱动就是采用这个函数注册，注销一个I2C设备驱动使用下面的i2c_del_driver函数，那就具体看看这个I2C设备驱动注册函数。

[cpp] view plaincopy

static inline int i2c_add_driver(struct i2c_driver *driver)
{
return i2c_register_driver(THIS_MODULE, driver);
}
int i2c_register_driver(struct module *owner, struct i2c_driver *driver)
{
int res;
/* Can't register until after driver model init */
if (unlikely(WARN_ON(!i2c_bus_type.p)))
return -EAGAIN;
/* add the driver to the list of i2c drivers in the driver core */
driver->driver.owner = owner;
driver->driver.bus = &i2c_bus_type; /*指定驱动的总线类型*/
/* When registration returns, the driver core
* will have called probe() for all matching-but-unbound devices.
*/
res = driver_register(&driver->driver); /*注册驱动*/
if (res)
return res;
pr_debug("i2c-core: driver [%s] registered\n", driver->driver.name);
INIT_LIST_HEAD(&driver->clients);
/* Walk the adapters that are already present */
mutex_lock(&core_lock);
bus_for_each_dev(&i2c_bus_type, NULL, driver, __attach_adapter);
mutex_unlock(&core_lock);
return 0;
}

再来看看i2c设备驱动注销函数

[cpp] view plaincopy

void i2c_del_driver(struct i2c_driver *driver)
{
mutex_lock(&core_lock);
bus_for_each_dev(&i2c_bus_type, NULL, driver, __detach_adapter);
mutex_unlock(&core_lock);
driver_unregister(&driver->driver);
pr_debug("i2c-core: driver [%s] unregistered\n", driver->driver.name);
}

也没什么，最后调用的就是驱动的注销函数driver_unregister函数。

来看传递给注册和注销i2c驱动函数的参数什么，i2cdev_driver它是structi2c_driver结构类型，i2c设备驱动就是使用这个结构类型描述，这个结构类型定义在include/linux/i2c.h

[cpp] view plaincopy

struct i2c_driver {
unsigned int class;
/* Notifies the driver that a new bus has appeared or is about to be
* removed. You should avoid using this if you can, it will probably
* be removed in a near future.
*/
int (*attach_adapter)(struct i2c_adapter *);
int (*detach_adapter)(struct i2c_adapter *);
/* Standard driver model interfaces */
int (*probe)(struct i2c_client *, const struct i2c_device_id *);
int (*remove)(struct i2c_client *);
/* driver model interfaces that don't relate to enumeration */
void (*shutdown)(struct i2c_client *);
int (*suspend)(struct i2c_client *, pm_message_t mesg);
int (*resume)(struct i2c_client *);
/* a ioctl like command that can be used to perform specific functions
* with the device.
*/
int (*command)(struct i2c_client *client, unsigned int cmd, void *arg);
struct device_driver driver;
const struct i2c_device_id *id_table;
/* Device detection callback for automatic device creation */
int (*detect)(struct i2c_client *, int kind, struct i2c_board_info *);
const struct i2c_client_address_data *address_data;
struct list_head clients;
};

来看i2c-dev.c中是怎么定义的

[cpp] view plaincopy

static struct i2c_driver i2cdev_driver = {
.driver = {
.name = "dev_driver",
},
.attach_adapter = i2cdev_attach_adapter,
.detach_adapter = i2cdev_detach_adapter,
};

这是老的方式，所以它只是给attach_adapter和detach_adapter赋了值，由于这里是老的方式，所以我们也就不去具体看这个函数了，我们直接去看它的数据传输部分吧。

[cpp] view plaincopy

static ssize_t i2cdev_read (struct file *file, char __user *buf, size_t count,
loff_t *offset)
{
char *tmp;
int ret;
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;
pr_debug("i2c-dev: i2c-%d reading %zu bytes.\n",
iminor(file->f_path.dentry->d_inode), count);
ret = i2c_master_recv(client,tmp,count);
if (ret >= 0)
ret = copy_to_user(buf,tmp,count)?-EFAULT:ret;
kfree(tmp);
return ret;
}

这是i2c设备读函数，我们看它是调用的i2c_master_recv函数去操作的，去看这个函数

[cpp] view plaincopy

int i2c_master_recv(struct i2c_client *client, char *buf ,int count)
{
struct i2c_adapter *adap=client->adapter;
struct i2c_msg msg;
int ret;
msg.addr = client->addr;
msg.flags = client->flags & I2C_M_TEN;
msg.flags |= I2C_M_RD;
msg.len = count;
msg.buf = buf;
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;
}

i2c设备写函数

[cpp] view plaincopy

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)) {
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);
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;
msg.flags = client->flags & I2C_M_TEN;
msg.len = count;
msg.buf = (char *)buf;
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;
}

这两个函数最终都是调用的i2c_transfer函数去完成数据的传输，只是他们的msg的flags不一样，读操作的flags要加上I2C_M_RD这个标志。

再看它们两个共同的i2c_transfer函数

[cpp] view plaincopy

int i2c_transfer(struct i2c_adapter *adap, struct i2c_msg *msgs, int num)
{
unsigned long orig_jiffies;
int ret, try;
/* REVISIT the fault reporting model here is weak:
*
* - When we get an error after receiving N bytes from a slave,
* there is no way to report "N".
*
* - When we get a NAK after transmitting N bytes to a slave,
* there is no way to report "N" ... or to let the master
* continue executing the rest of this combined message, if
* that's the appropriate response.
*
* - When for example "num" is two and we successfully complete
* the first message but get an error part way through the
* second, it's unclear whether that should be reported as
* one (discarding status on the second message) or errno
* (discarding status on the first one).
*/
if (adap->algo->master_xfer) {
#ifdef DEBUG
for (ret = 0; ret < num; ret++) {
dev_dbg(&adap->dev, "master_xfer[%d] %c, addr=0x%02x, "
"len=%d%s\n", ret, (msgs[ret].flags & I2C_M_RD)
? 'R' : 'W', msgs[ret].addr, msgs[ret].len,
(msgs[ret].flags & I2C_M_RECV_LEN) ? "+" : "");
}
#endif
if (in_atomic() || irqs_disabled()) {
ret = mutex_trylock(&adap->bus_lock);
if (!ret)
/* I2C activity is ongoing. */
return -EAGAIN;
} else {
mutex_lock_nested(&adap->bus_lock, adap->level);
}
/* Retry automatically on arbitration loss */
orig_jiffies = jiffies;
for (ret = 0, try = 0; try <= adap->retries; try++) {
ret = adap->algo->master_xfer(adap, msgs, num);
if (ret != -EAGAIN)
break;
if (time_after(jiffies, orig_jiffies + adap->timeout))
break;
}
mutex_unlock(&adap->bus_lock);
return ret;
} else {
dev_dbg(&adap->dev, "I2C level transfers not supported\n");
return -EOPNOTSUPP;
}
}

我们看就是调用总线的master_xfer方法，我们在前面分析使用gpio模拟i2c总线时，看过这样一句 .master_xfer =bit_xfer, ，所以最终调用的是这个函数来完成数据传输。使用i2c_master_recv和i2c_master_send函数一次只能传输一个msg，由于它一次只能传输一个msg，所以它的传输方向不能改变，也就是一次只能完成读或写操作，并且读操作时还不能传递设备的基地址，所以通常是不会用这两个函数的，直接的做法时，构造两个msg，一个msg的数据为操作设备基地址，另外一个msg才是我们真正要读写的数据，最后调用i2c_transfer函数去完成数据的传送。

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