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本节不需要编写驱动，直接可以使用i2c-dev中编写号的驱动，来进行使用。
也就是说这是一个完全的应用程序。
程序在后面的代码仓库。这里说一下原理。
正常情况下，每一个adaptor，都会在i2c_dev_init函数调用时创建一个设备在/dev/i2c-x
这要求一个前提，adaptor必须在，i2c_dev_init执行之前先执行。
内核的策略就是adaptor使用subsys_initcall修饰，而i2c_dev_init则用module_init修饰。
static int __init i2c_adap_s3c_init(void)
{
return platform_driver_register(&s3c24xx_i2c_driver);
}
subsys_initcall(i2c_adap_s3c_init);
/* ------------------------------------------------------------------------- */
/*
* module load/unload record keeping
*/
static int __init i2c_dev_init(void)
{
int res;
printk(KERN_INFO "i2c /dev entries driver\n");
/* 获取i2c的主设备号 */
res = register_chrdev_region(MKDEV(I2C_MAJOR, 0), I2C_MINORS, "i2c");
if (res)
goto out;
/* 创建一个类 */
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;
}
i2c_dev_class->dev_groups = i2c_groups;
/* Keep track of adapters which will be added or removed later 通知i2cdev相关的东西 */
res = bus_register_notifier(&i2c_bus_type, &i2cdev_notifier);
if (res)
goto out_unreg_class;
/* Bind to already existing adapters right away 遍历一遍adaptor */
i2c_for_each_dev(NULL, i2cdev_attach_adapter);
return 0;
out_unreg_class:
class_destroy(i2c_dev_class);
out_unreg_chrdev:
unregister_chrdev_region(MKDEV(I2C_MAJOR, 0), I2C_MINORS);
out:
printk(KERN_ERR "%s: Driver Initialisation failed\n", __FILE__);
return res;
}
module_init(i2c_dev_init);
遍历函数，就是为每个i2c_adaptor创建一个设备
/*
* 申请一个i2c_dev设备
*/
static int i2cdev_attach_adapter(struct device *dev, void *dummy)
{
struct i2c_adapter *adap;
struct i2c_dev *i2c_dev;
int res;
if (dev->type != &i2c_adapter_type)
return 0;
adap = to_i2c_adapter(dev); /* 获取adaptor */
i2c_dev = get_free_i2c_dev(adap); /* 申请一个i2c_dev */
if (IS_ERR(i2c_dev))
return PTR_ERR(i2c_dev);
/* 初始化设备,这里已经绑定了操作方法 */
cdev_init(&i2c_dev->cdev, &i2cdev_fops);
i2c_dev->cdev.owner = THIS_MODULE;
/* 申请的字符设备加入内核 */
res = cdev_add(&i2c_dev->cdev, MKDEV(I2C_MAJOR, adap->nr), 1);
if (res)
goto error_cdev;
/* register this i2c device with the driver core */
/* 创建设备,这样就会通过class,自动在/dev/下创建设备了,名字如i2c-%d */
i2c_dev->dev = device_create(i2c_dev_class, &adap->dev,
MKDEV(I2C_MAJOR, adap->nr), NULL,
"i2c-%d", adap->nr);
if (IS_ERR(i2c_dev->dev)) {
res = PTR_ERR(i2c_dev->dev);
goto error;
}
pr_debug("i2c-dev: adapter [%s] registered as minor %d\n",
adap->name, adap->nr);
return 0;
error:
cdev_del(&i2c_dev->cdev);
error_cdev:
put_i2c_dev(i2c_dev);
return res;
}
static struct i2c_dev *get_free_i2c_dev(struct i2c_adapter *adap)
{
struct i2c_dev *i2c_dev;
/* 确定没超出adaptor最大数量 */
if (adap->nr >= I2C_MINORS) {
printk(KERN_ERR "i2c-dev: Out of device minors (%d)\n",
adap->nr);
return ERR_PTR(-ENODEV);
}
/* 申请一个i2c_dev设备 */
i2c_dev = kzalloc(sizeof(*i2c_dev), GFP_KERNEL);
if (!i2c_dev)
return ERR_PTR(-ENOMEM);
i2c_dev->adap = adap; /* 一个i2c_dev,对应一个adap,所以绑定 */
/* 加入设备链表 */
spin_lock(&i2c_dev_list_lock);
list_add_tail(&i2c_dev->list, &i2c_dev_list);
spin_unlock(&i2c_dev_list_lock);
return i2c_dev;
}
具体的操作还得看，具体的操作方法。其中open函数是主要的精髓
static const struct file_operations i2cdev_fops = {
.owner = THIS_MODULE,
.llseek = no_llseek,
.read = i2cdev_read,
.write = i2cdev_write,
.unlocked_ioctl = i2cdev_ioctl,
.compat_ioctl = compat_i2cdev_ioctl,
.open = i2cdev_open,
.release = i2cdev_release,
};
static int i2cdev_open(struct inode *inode, struct file *file)
{
unsigned int minor = iminor(inode);
struct i2c_client *client;
struct i2c_adapter *adap;
/*
* 根据次设备号,从adaptor链表获取这个设备的adaptor
*/
adap = i2c_get_adapter(minor);
if (!adap)
return -ENODEV;
/* 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
*/
client = kzalloc(sizeof(*client), GFP_KERNEL);
if (!client) {
i2c_put_adapter(adap);
return -ENOMEM;
}
snprintf(client->name, I2C_NAME_SIZE, "i2c-dev %d", adap->nr);
/* client绑定,adaptor,同时把这个client绑定到这个文件的私有数据上,
* 这样read,write使用的时候直接就可以找到client
*/
client->adapter = adap;
file->private_data = client;
return 0;
}因为上面open打开时创建的的这个client是一个临时的，在close掉这个文件后会删除。默认也是没有设置地址的，这里就只能使用ioctrl来设置地址。
static long i2cdev_ioctl(struct file *file, unsigned int cmd, unsigned long arg)
{
struct i2c_client *client = file->private_data;
unsigned long funcs;
dev_dbg(&client->adapter->dev, "ioctl, cmd=0x%02x, arg=0x%02lx\n",
cmd, 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;
case I2C_TENBIT:
if (arg)
client->flags |= I2C_M_TEN;
else
client->flags &= ~I2C_M_TEN;
return 0;
case I2C_PEC:
/*
* Setting the PEC flag here won't affect kernel drivers,
* which will be using the i2c_client node registered with
* the driver model core. Likewise, when that client has
* the PEC flag already set, the i2c-dev driver won't see
* (or use) this setting.
*/
if (arg)
client->flags |= I2C_CLIENT_PEC;
else
client->flags &= ~I2C_CLIENT_PEC;
return 0;
case I2C_FUNCS:
funcs = i2c_get_functionality(client->adapter);
return put_user(funcs, (unsigned long __user *)arg);
case I2C_RDWR: {
struct i2c_rdwr_ioctl_data rdwr_arg;
struct i2c_msg *rdwr_pa;
if (copy_from_user(&rdwr_arg,
(struct i2c_rdwr_ioctl_data __user *)arg,
sizeof(rdwr_arg)))
return -EFAULT;
/* Put an arbitrary limit on the number of messages that can
* be sent at once */
if (rdwr_arg.nmsgs > I2C_RDWR_IOCTL_MAX_MSGS)
return -EINVAL;
rdwr_pa = memdup_user(rdwr_arg.msgs,
rdwr_arg.nmsgs * sizeof(struct i2c_msg));
if (IS_ERR(rdwr_pa))
return PTR_ERR(rdwr_pa);
return i2cdev_ioctl_rdwr(client, rdwr_arg.nmsgs, rdwr_pa);
}
case I2C_SMBUS: {
struct i2c_smbus_ioctl_data data_arg;
if (copy_from_user(&data_arg,
(struct i2c_smbus_ioctl_data __user *) arg,
sizeof(struct i2c_smbus_ioctl_data)))
return -EFAULT;
return i2cdev_ioctl_smbus(client, data_arg.read_write,
data_arg.command,
data_arg.size,
data_arg.data);
}
case I2C_RETRIES:
client->adapter->retries = arg;
break;
case I2C_TIMEOUT:
/* For historical reasons, user-space sets the timeout
* value in units of 10 ms.
*/
client->adapter->timeout = msecs_to_jiffies(arg * 10);
break;
default:
/* NOTE: returning a fault code here could cause trouble
* in buggy userspace code. Some old kernel bugs returned
* zero in this case, and userspace code might accidentally
* have depended on that bug.
*/
return -ENOTTY;
}
return 0;
}从上面可以看到ioctrl的功能是非常丰富的。
可以设置从机地址，可以读取这个adaptor的功能，可以进行读写操作，设置读写标志等等。
/* ------------------------------------------------------------------------- */
/*
* After opening an instance of this character special file, a file
* descriptor starts out associated only with an i2c_adapter (and bus).
*
* Using the I2C_RDWR ioctl(), you can then *immediately* issue i2c_msg
* traffic to any devices on the bus used by that adapter. That's because
* the i2c_msg vectors embed all the addressing information they need, and
* are submitted directly to an i2c_adapter. However, SMBus-only adapters
* don't support that interface.
*
* To use read()/write() system calls on that file descriptor, or to use
* SMBus interfaces (and work with SMBus-only hosts!), you must first issue
* an I2C_SLAVE (or I2C_SLAVE_FORCE) ioctl. That configures an anonymous
* (never registered) i2c_client so it holds the addressing information
* needed by those system calls and by this SMBus interface.
*/
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 = file->private_data;
if (count > 8192)
count = 8192;
/* 申请临时空间 */
tmp = kmalloc(count, GFP_KERNEL);
if (tmp == NULL)
return -ENOMEM;
pr_debug("i2c-dev: i2c-%d reading %zu bytes.\n",
iminor(file_inode(file)), count);
/* 读操作 */
ret = i2c_master_recv(client, tmp, count);
if (ret >= 0)
/* 把读取的数据拷贝的用户buf */
ret = copy_to_user(buf, tmp, count) ? -EFAULT : ret;
kfree(tmp);
return ret;
}
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 = file->private_data;
if (count > 8192)
count = 8192;
/* 写的数据比较多,采用memap方式,比较少,则申请一段内核内存,拷贝过来 */
tmp = memdup_user(buf, count);
if (IS_ERR(tmp))
return PTR_ERR(tmp);
pr_debug("i2c-dev: i2c-%d writing %zu bytes.\n",
iminor(file_inode(file)), count);
/* 写操作 */
ret = i2c_master_send(client, tmp, count);
kfree(tmp);
return ret;
}
上面已经说得很清楚了。
上面的读只具有读功能，不具有设置 读的寄存器地址的功能，所以读数据之前要设置读的寄存器（用写）。
代码就不贴了，给出代码仓库位置如下
https://github.com/To-run-away/linux-i2c-driver/tree/master/i2c_cdev
测试过程如下：
最后一种，就是，驱动中使用下面三个函数中的任意一个创建一个client，这样也就代表在注册驱动时，设备也同时创建了。
struct i2c_client *
i2c_new_probed_device(struct i2c_adapter *adap,
struct i2c_board_info *info,
unsigned short const *addr_list,
int (*probe)(struct i2c_adapter *, unsigned short addr));
struct i2c_client *
i2c_new_device(struct i2c_adapter *adap, struct i2c_board_info const *info);
struct i2c_client *i2c_new_dummy(struct i2c_adapter *adapter, u16 address);
这样有驱动，肯定可以匹配到设备（毕竟驱动和设备是一个人编写，名字肯定会一致）。
这种情况大多用于，i2c设备是作为一个其它类设备。比如触摸屏IC，这个i2c作为输入子系统注册。
这种情况就不再细说了。