一:前言
I2c是philips提出的外设总线.I2C只有两条线,一条串行数据线:SDA,一条是时钟线SCL.正因为这样,它方便了工程人员的布线.另外,I2C是一种多主机控制总线.它和USB总线不同,USB是基于master-slave机制,任何设备的通信必须由主机发起才可以.而I2C是基于multi master机制.一同总线上可允许多个master.关于I2C协议的知识,这里不再赘述.可自行下载spec阅读即可.
二:I2C架构概述
在linux中,I2C驱动架构如下所示:
如上图所示,每一条I2C对应一个adapter.在kernel中,每一个adapter提供了一个描述的结构(struct i2c_adapter),也定义了adapter支持的操作(struct i2c_adapter).再通过i2c core层将i2c设备与i2c adapter关联起来.
这个图只是提供了一个大概的框架.在下面的代码分析中,从下至上的来分析这个框架图.以下的代码分析是基于linux 2.6.26.分析的代码基本位于:
linux-2.6.26.3/drivers/i2c/位置.
三:adapter注册
在kernel中提供了两个adapter注册接口,分别为i2c_add_adapter()和i2c_add_numbered_adapter().由于在系统中可能存在多个adapter,因为将每一条I2C总线对应一个编号,下文中称为I2C总线号.这个总线号的PCI中的总线号不同.它和硬件无关,只是软件上便于区分而已.
对于i2c_add_adapter()而言,它使用的是动态总线号,即由系统给其分析一个总线号,而i2c_add_numbered_adapter()则是自己指定总线号,如果这个总线号非法或者是被占用,就会注册失败.
分别来看一下这两个函数的代码:
int i2c_add_adapter(struct i2c_adapter *adapter)
{
int id, res = 0;
retry:
if (idr_pre_get(&i2c_adapter_idr, GFP_KERNEL) == 0)
return -ENOMEM;
mutex_lock(&core_lock);
/* "above" here means "above or equal to", sigh */
res = idr_get_new_above(&i2c_adapter_idr, adapter,
__i2c_first_dynamic_bus_num, &id);
mutex_unlock(&core_lock);
if (res < 0) {
if (res == -EAGAIN)
goto retry;
return res;
}
adapter->nr = id;
return i2c_register_adapter(adapter);
}
在这里涉及到一个idr结构.idr结构本来是为了配合page cache中的radix tree而设计的.在这里我们只需要知道,它是一种高效的搜索树,且这个树预先存放了一些内存.避免在内存不够的时候出现问题.所在,在往idr中插入结构的时候,首先要调用idr_pre_get()为它预留足够的空闲内存,然后再调用idr_get_new_above()将结构插入idr中,该函数以参数的形式返回一个id.以后凭这个id就可以在idr中找到相对应的结构了.对这个数据结构操作不太理解的可以查阅本站<<
linux文件系统之文件的读写>>中有关radix tree的分析.
注意一下idr_get_new_above(&i2c_adapter_idr, adapter,__i2c_first_dynamic_bus_num, &id)的参数的含义,它是将adapter结构插入到i2c_adapter_idr中,存放位置的id必须要大于或者等于__i2c_first_dynamic_bus_num,
然后将对应的id号存放在adapter->nr中.调用i2c_register_adapter(adapter)对这个adapter进行进一步注册.
看一下另外一人注册函数: i2c_add_numbered_adapter( ),如下所示:
int i2c_add_numbered_adapter(struct i2c_adapter *adap)
{
int id;
int status;
if (adap->nr & ~MAX_ID_MASK)
return -EINVAL;
retry:
if (idr_pre_get(&i2c_adapter_idr, GFP_KERNEL) == 0)
return -ENOMEM;
mutex_lock(&core_lock);
/* "above" here means "above or equal to", sigh;
* we need the "equal to" result to force the result
*/
status = idr_get_new_above(&i2c_adapter_idr, adap, adap->nr, &id);
if (status == 0 && id != adap->nr) {
status = -EBUSY;
idr_remove(&i2c_adapter_idr, id);
}
mutex_unlock(&core_lock);
if (status == -EAGAIN)
goto retry;
if (status == 0)
status = i2c_register_adapter(adap);
return status;
}
对比一下就知道差别了,在这里它已经指定好了adapter->nr了.如果分配的id不和指定的相等,便返回错误.
过一步跟踪i2c_register_adapter().代码如下:
static int i2c_register_adapter(struct i2c_adapter *adap)
{
int res = 0, dummy;
mutex_init(&adap->bus_lock);
mutex_init(&adap->clist_lock);
INIT_LIST_HEAD(&adap->clients);
mutex_lock(&core_lock);
/* Add the adapter to the driver core.
* If the parent pointer is not set up,
* we add this adapter to the host bus.
*/
if (adap->dev.parent == NULL) {
adap->dev.parent = &platform_bus;
pr_debug("I2C adapter driver [%s] forgot to specify "
"physical device/n", adap->name);
}
sprintf(adap->dev.bus_id, "i2c-%d", adap->nr);
adap->dev.release = &i2c_adapter_dev_release;
adap->dev.class = &i2c_adapter_class;
res = device_register(&adap->dev);
if (res)
goto out_list;
dev_dbg(&adap->dev, "adapter [%s] registered/n", adap->name);
/* create pre-declared device nodes for new-style drivers */
if (adap->nr < __i2c_first_dynamic_bus_num)
i2c_scan_static_board_info(adap);
/* let legacy drivers scan this bus for matching devices */
dummy = bus_for_each_drv(&i2c_bus_type, NULL, adap,
i2c_do_add_adapter);
out_unlock:
mutex_unlock(&core_lock);
return res;
out_list:
idr_remove(&i2c_adapter_idr, adap->nr);
goto out_unlock;
}
首先对adapter和adapter中内嵌的struct device结构进行必须的初始化.之后将adapter内嵌的struct device注册.
在这里注意一下adapter->dev的初始化.它的类别为i2c_adapter_class,如果没有父结点,则将其父结点设为platform_bus.adapter->dev的名字为i2c + 总线号.
测试一下:
[eric@mochow i2c]$ cd /sys/class/i2c-adapter/
[eric@mochow i2c-adapter]$ ls
i2c-0
可以看到,在我的PC上,有一个I2C adapter,看下详细信息:
[eric@mochow i2c-adapter]$ tree
.
`-- i2c-0
|-- device -> ../../../devices/pci0000:00/0000:00:1f.3/i2c-0
|-- name
|-- subsystem -> ../../../class/i2c-adapter
`-- uevent
3 directories, 2 files
可以看到,该adapter是一个PCI设备.
继续往下看:
之后,在注释中看到,有两种类型的driver,一种是new-style drivers,另外一种是legacy drivers
New-style drivers是在2.6近版的kernel加入的.它们最主要的区别是在adapter和i2c driver的匹配上.
{
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))
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;
}
首先为主册了一个主设备号为I2C_MAJOR(89),操作集为i2cdev_fops的字符设备.然后注册了一个名为”i2c-dev”的class.之后再注册了一个i2c的driver.如下所示:
res = i2c_add_driver(&i2cdev_driver);
if (res)
goto out_unreg_class;
i2cdev_driver定义如下:
static struct i2c_driver i2cdev_driver = {
.driver = {
.name = "dev_driver",
},
.id = I2C_DRIVERID_I2CDEV,
.attach_adapter = i2cdev_attach_adapter,
.detach_adapter = i2cdev_detach_adapter,
.detach_client = i2cdev_detach_client,
};
也就是说,当它注册或者有新的adapter注册后,就会它的attach_adapter()函数.该函数代码如下:
static int i2cdev_attach_adapter(struct i2c_adapter *adap)
{
struct i2c_dev *i2c_dev;
int res;
i2c_dev = get_free_i2c_dev(adap);
if (IS_ERR(i2c_dev))
return PTR_ERR(i2c_dev);
/* register this i2c device with the driver core */
i2c_dev->dev = device_create(i2c_dev_class, &adap->dev,
MKDEV(I2C_MAJOR, adap->nr),
"i2c-%d", adap->nr);
if (IS_ERR(i2c_dev->dev)) {
res = PTR_ERR(i2c_dev->dev);
goto error;
}
res = device_create_file(i2c_dev->dev, &dev_attr_name);
if (res)
goto error_destroy;
pr_debug("i2c-dev: adapter [%s] registered as minor %d/n",
adap->name, adap->nr);
return 0;
error_destroy:
device_destroy(i2c_dev_class, MKDEV(I2C_MAJOR, adap->nr));
error:
return_i2c_dev(i2c_dev);
return res;
}
这个函数也很简单,首先调用get_free_i2c_dev()分配并初始化了一个struct i2c_dev结构,使i2c_dev->adap指向操作的adapter.之后,该i2c_dev会被链入链表i2c_dev_list中.再分别以I2C_MAJOR,
adap->nr为主次设备号创建了一个device.如果此时系统配置了udev或者是hotplug,那么就么在/dev下自动创建相关的设备节点了.
刚才我们说过,所有主设备号为I2C_MAJOR的设备节点的操作函数是i2cdev_fops.它的定义如下所示:
static const struct file_operations i2cdev_fops = {
.owner = THIS_MODULE,
.llseek = no_llseek,
.read = i2cdev_read,
.write = i2cdev_write,
.ioctl = i2cdev_ioctl,
.open = i2cdev_open,
.release = i2cdev_release,
};
7.1:i2c dev的open操作
Open操作对应的函数为i2cdev_open().代码如下:
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;
//以次设备号从i2c_dev_list链表中取得i2c_dev
i2c_dev = i2c_dev_get_by_minor(minor);
if (!i2c_dev)
return -ENODEV;
//以apapter的总线号从i2c_adapter_idr中找到adapter
adap = i2c_get_adapter(i2c_dev->adap->nr);
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.
*/
//分配并初始化一个i2c_client结构
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->driver = &i2cdev_driver;
//clinet->adapter指向操作的adapter
client->adapter = adap;
//关联到file
file->private_data = client;
return 0;
}
注意这里分配并初始化了一个struct i2c_client结构.但是没有注册这个clinet.此外,这个函数中还有一个比较奇怪的操作.不是在前面已经将i2c_dev->adap指向要操作的adapter么?为什么还要以adapter->nr为关键字从i2c_adapter_idr去找这个操作的adapter呢?注意了,调用i2c_get_adapter()从总线号nr找到操作的adapter的时候,还会增加module的引用计数.这样可以防止模块意外被释放掉.也许有人会有这样的疑问,那 i2c_dev->adap->nr操作,如果i2c_dev->adap被释放掉的话,不是一样会引起系统崩溃么?这里因为,在i2cdev_attach_adapter()间接的增加了一次adapter的一次引用计数.如下:
tatic int i2cdev_attach_adapter(struct i2c_adapter *adap)
{
．．．．．．
i2c_dev->dev = device_create(i2c_dev_class, &adap->dev,
MKDEV(I2C_MAJOR, adap->nr),
"i2c-%d", adap->nr);
．．．．．．
}
看到了么，i2c_dev内嵌的device是以adap->dev为父结点,在device_create()中会增次adap->dev的一次引用计数.
好了,open()操作到此就完成了.
7.2:read操作
Read操作对应的操作函数如下示:
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 %zd 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;
}
首先从file结构中取得struct i2c_clinet.然后在kernel同分配相同长度的缓存区,随之调用i2c_master_recv()从设备中读取数据.再将读取出来的数据copy到用户空间中.
I2c_master_recv()代码如下:
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;
}
看完前面的代码之后,这个函数应该很简单了,就是为读操作初始化了一个i2c_msg.然后调用i2c_tanster().代码中的client->flags & I2C_M_TEN表示adapter是否采用10位寻址的方式.在这里就不再详细分析了.
另外,有人可能看出了一个问题.这里clinet->addr是从哪来的呢?对,在read之前应该还要有一步操作来设置clinet->addr的值.这个过程是ioctl的操作.ioctl可以设置PEC标志,重试次数,超时时间,和发送接收数据等,我们在这里只看一下clinet->addr的设置.代码片段如下示:
static int i2cdev_ioctl(struct inode *inode, struct file *file,
unsigned int cmd, unsigned long arg)
{
．．．．．．
．．．．．．
switch ( cmd ) {
case I2C_SLAVE:
case I2C_SLAVE_FORCE:
/* NOTE: devices set up to work with "new style" drivers
* can't use I2C_SLAVE, even when the device node is not
* bound to a driver. Only I2C_SLAVE_FORCE will work.
*
* 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 > 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;
．．．．．．
．．．．．．
}
由此可见,调用I2C_SLAVE或者I2C_SLAVE_FORCE的Ioctl就会设置clinet->addr.另外,注释中也说得很清楚了.如果是I2C_SLAVE的话,还会调用其所长i2cdev_check_addr().进行地址检查,如果adapter已经关联到这个地址的设备,就会检查失败.
7.2:write操作
Write操作如下所示:
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 %zd bytes./n",
iminor(file->f_path.dentry->d_inode), count);
ret = i2c_master_send(client,tmp,count);
kfree(tmp);
return ret;
}
该操作比较简单,就是将用户空间的数据发送到i2c 设备.
八:小结
在本节中,分析了i2c的框架设计.这个框架大体上沿用了Linux的设备驱动框架,不过之中又做了很多变通.在之后的分析中,会分别举一个adapter和i2c device的例子来详细描述一下有关i2c driver的设计.