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

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。

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

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

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

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。

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

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

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

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 DEBUGfor (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) ? "+" : "");}#endifif (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。