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.
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.
3.1: new-style 形式的adapter注册
对于第一种,也就是new-style drivers,将相关代码再次列出如下:
if (adap->nr < __i2c_first_dynamic_bus_num)
i2c_scan_static_board_info(adap);
如果adap->nr 小于__i2c_first_dynamic_bus_num的话,就会进入到i2c_scan_static_board_info().
结合我们之前分析的adapter的两种注册分式: i2c_add_adapter()所分得的总线号肯会不会小于__i2c_first_dynamic_bus_num.只有i2c_add_numbered_adapter()才有可能满足:
(adap->nr < __i2c_first_dynamic_bus_num)
而且必须要调用i2c_register_board_info()将板子上的I2C设备信息预先注册时才会更改__i2c_first_dynamic_bus_num的值.在x86上只没有使用i2c_register_board_info()的.因此,x86平台上的分析可以忽略掉new-style driver的方式.不过,还是详细分析这种情况下.
首先看一下i2c_register_board_info(),如下:
int __init
i2c_register_board_info(int busnum,
struct i2c_board_info const *info, unsigned len)
{
int status;
mutex_lock(&__i2c_board_lock);
/* dynamic bus numbers will be assigned after the last static one */
if (busnum >= __i2c_first_dynamic_bus_num)
__i2c_first_dynamic_bus_num = busnum + 1;
for (status = 0; len; len--, info++) {
struct i2c_devinfo *devinfo;
devinfo = kzalloc(sizeof(*devinfo), GFP_KERNEL);
if (!devinfo) {
pr_debug("i2c-core: can't register boardinfo!\n");
status = -ENOMEM;
break;
}
devinfo->busnum = busnum;
devinfo->board_info = *info;
list_add_tail(&devinfo->list, &__i2c_board_list);
}
mutex_unlock(&__i2c_board_lock);
return status;
}
这个函数比较简单, struct i2c_board_info用来表示I2C设备的一些情况,比如所在的总线.名称,地址,中断号等.最后,这些信息会被存放到__i2c_board_list链表.
跟踪i2c_scan_static_board_info():代码如下:
static void i2c_scan_static_board_info(struct i2c_adapter *adapter)
{
struct i2c_devinfo *devinfo;
mutex_lock(&__i2c_board_lock);
list_for_each_entry(devinfo, &__i2c_board_list, list) {
if (devinfo->busnum == adapter->nr
&& !i2c_new_device(adapter,
&devinfo->board_info))
printk(KERN_ERR "i2c-core: can't create i2c%d-%04x\n",
i2c_adapter_id(adapter),
devinfo->board_info.addr);
}
mutex_unlock(&__i2c_board_lock);
}
该函数遍历挂在__i2c_board_list链表上面的i2c设备的信息,也就是我们在启动的时候指出的i2c设备的信息.
如果指定设备是位于adapter所在的I2C总线上,那么,就调用i2c_new_device().代码如下:
struct i2c_client *
i2c_new_device(struct i2c_adapter *adap, struct i2c_board_info const *info)
{
struct i2c_client *client;
int status;
client = kzalloc(sizeof *client, GFP_KERNEL);
if (!client)
return NULL;
client->adapter = adap;
client->dev.platform_data = info->platform_data;
device_init_wakeup(&client->dev, info->flags & I2C_CLIENT_WAKE);
client->flags = info->flags & ~I2C_CLIENT_WAKE;
client->addr = info->addr;
client->irq = info->irq;
strlcpy(client->name, info->type, sizeof(client->name));
/* a new style driver may be bound to this device when we
* return from this function, or any later moment (e.g. maybe
* hotplugging will load the driver module). and the device
* refcount model is the standard driver model one.
*/
status = i2c_attach_client(client);
if (status < 0) {
kfree(client);
client = NULL;
}
return client;
}
我们又遇到了一个新的结构:struct i2c_client,不要被这个结构吓倒了,其实它就是一个嵌入struct device的I2C设备的封装.它和我们之前遇到的struct usb_device结构的作用是一样的.
首先,在clinet里保存该设备的相关消息.特别的, client->adapter指向了它所在的adapter.
特别的,clinet->name为info->name.也是指定好了的.
一切初始化完成之后,便会调用i2c_attach_client( ).看这个函数的字面意思,是将clinet关联起来.到底怎么样关联呢?继续往下看:
int i2c_attach_client(struct i2c_client *client)
{
struct i2c_adapter *adapter = client->adapter;
int res = 0;
//初始化client内嵌的dev结构
//父结点为所在的adapter,所在bus为i2c_bus_type
client->dev.parent = &client->adapter->dev;
client->dev.bus = &i2c_bus_type;
//如果client已经指定了driver,将driver和内嵌的dev关联起来
if (client->driver)
client->dev.driver = &client->driver->driver;
//指定了driver, 但不是newstyle的
if (client->driver && !is_newstyle_driver(client->driver)) {
client->dev.release = i2c_client_release;
client->dev.uevent_suppress = 1;
} else
client->dev.release = i2c_client_dev_release;
//clinet->dev的名称
snprintf(&client->dev.bus_id[0], sizeof(client->dev.bus_id),
"%d-%04x", i2c_adapter_id(adapter), client->addr);
//将内嵌的dev注册
res = device_register(&client->dev);
if (res)
goto out_err;
//将clinet链到adapter->clients中
mutex_lock(&adapter->clist_lock);
list_add_tail(&client->list, &adapter->clients);
mutex_unlock(&adapter->clist_lock);
dev_dbg(&adapter->dev, "client [%s] registered with bus id %s\n",
client->name, client->dev.bus_id);
//如果adapter->cleinet_reqister存在,就调用它
if (adapter->client_register) {
if (adapter->client_register(client)) {
dev_dbg(&adapter->dev, "client_register "
"failed for client [%s] at 0x%02x\n",
client->name, client->addr);
}
}
return 0;
out_err:
dev_err(&adapter->dev, "Failed to attach i2c client %s at 0x%02x "
"(%d)\n", client->name, client->addr, res);
return res;
}
参考上面添加的注释,应该很容易理解这段代码了,就不加详细分析了.这个函数的名字不是i2c_attach_client()么?怎么没看到它的关系过程呢?
这是因为:在代码中设置了client->dev所在的bus为i2c_bus_type .以为只需要有bus为i2c_bus_type的driver注册,就会产生probe了.这个过程呆后面分析i2c driver的时候再来详细分析.
3.2: legacy形式的adapter注册
Legacy形式的adapter注册代码片段如下:
dummy = bus_for_each_drv(&i2c_bus_type, NULL, adap,
i2c_do_add_adapter);
这段代码遍历挂在i2c_bus_type上的驱动,然后对每一个驱动和adapter调用i2c_do_add_adapter().
代码如下:
static int i2c_do_add_adapter(struct device_driver *d, void *data)
{
struct i2c_driver *driver = to_i2c_driver(d);
struct i2c_adapter *adap = data;
if (driver->attach_adapter) {
/* We ignore the return code; if it fails, too bad */
driver->attach_adapter(adap);
}
return 0;
}
该函数很简单,就是调用driver的attach_adapter()接口.
到此为止,adapter的注册已经分析完了.
四:i2c driver注册
在分析i2c driver的时候,有必要先分析一下i2c架构的初始化
代码如下:
static int __init i2c_init(void)
{
int retval;
retval = bus_register(&i2c_bus_type);
if (retval)
return retval;
retval = class_register(&i2c_adapter_class);
if (retval)
goto bus_err;
retval = i2c_add_driver(&dummy_driver);
if (retval)
goto class_err;
return 0;
class_err:
class_unregister(&i2c_adapter_class);
bus_err:
bus_unregister(&i2c_bus_type);
return retval;
}
subsys_initcall(i2c_init);
很明显,i2c_init()会在系统初始化的时候被调用.
在i2c_init中,先注册了i2c_bus_type的bus,i2c_adapter_class的class.然后再调用i2c_add_driver()注册了一个i2c driver.
I2c_bus_type结构如下:
static struct bus_type i2c_bus_type = {
.name = "i2c",
.dev_attrs = i2c_dev_attrs,
.match = i2c_device_match,
.uevent = i2c_device_uevent,
.probe = i2c_device_probe,
.remove = i2c_device_remove,
.shutdown = i2c_device_shutdown,
.suspend = i2c_device_suspend,
.resume = i2c_device_resume,
};
这个结构先放在这里吧,以后还会用到里面的信息的.
从上面的初始化函数里也看到了,注册i2c driver的接口为i2c_add_driver().代码如下:
static inline int i2c_add_driver(struct i2c_driver *driver)
{
return i2c_register_driver(THIS_MODULE, driver);
}
继续跟踪:
int i2c_register_driver(struct module *owner, struct i2c_driver *driver)
{
int res;
/* new style driver methods can't mix with legacy ones */
//如果是一个newstyle的driver.但又定义了attach_adapter/detach_adapter.非法
if (is_newstyle_driver(driver)) {
if (driver->attach_adapter || driver->detach_adapter
|| driver->detach_client) {
printk(KERN_WARNING
"i2c-core: driver [%s] is confused\n",
driver->driver.name);
return -EINVAL;
}
}
/* add the driver to the list of i2c drivers in the driver core */
//关联到i2c_bus_types
driver->driver.owner = owner;
driver->driver.bus = &i2c_bus_type;
/* for new style drivers, when registration returns the driver core
* will have called probe() for all matching-but-unbound devices.
*/
//注册内嵌的driver
res = driver_register(&driver->driver);
if (res)
return res;
mutex_lock(&core_lock);
pr_debug("i2c-core: driver [%s] registered\n", driver->driver.name);
/* legacy drivers scan i2c busses directly */
//遍历所有的adapter,对其都调用driver->attach_adapter
if (driver->attach_adapter) {
struct i2c_adapter *adapter;
down(&i2c_adapter_class.sem);
list_for_each_entry(adapter, &i2c_adapter_class.devices,
dev.node) {
driver->attach_adapter(adapter);
}
up(&i2c_adapter_class.sem);
}
mutex_unlock(&core_lock);
return 0;
}
这里也有两种形式的区分,对于第一种,只需要将内嵌的driver注册就可以了,对于legacy的情况,对每一个adapter都调用driver->attach_adapter().
现在,我们可以将adapter和i2c driver关联起来考虑一下了:
1:如果是news style形式的,在注册adapter的时候,将它上面的i2c 设备转换成了struct client.struct client->dev->bus又指定了和i2c driver同一个bus.因为,它们可以发生probe.
2:如果是legacy形式,就直接找到对应的对象,调用driver->attach_adapter().
五: i2c_bus_type的相关操作
I2c_bus_type的操作主要存在于new-style形式的驱动中.接下来分析一下对应的probe过程:
5.1:match过程分析
Match对应的操作函数为i2c_device_match().代码如下
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;
}
如果该驱动不是一个new-style形式的.或者driver没有定义匹配的id_table.都会匹配失败.
继续跟踪进i2c_match_id():
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;
}
由此可见.如果client的名字和driver->id_table[]中的名称匹配即为成功.
5.2:probe过程分析
Probe对应的函数为: i2c_device_probe()
static int i2c_device_probe(struct device *dev)
{
struct i2c_client *client = to_i2c_client(dev);
struct i2c_driver *driver = to_i2c_driver(dev->driver);
const struct i2c_device_id *id;
int status;
if (!driver->probe)
return -ENODEV;
client->driver = driver;
dev_dbg(dev, "probe\n");
if (driver->id_table)
id = i2c_match_id(driver->id_table, client);
else
id = NULL;
status = driver->probe(client, id);
if (status)
client->driver = NULL;
return status;
}
这个函数也很简单,就是将probe流程回溯到i2c driver的probe()
六:其它的扩展
分析完adapter和i2c driver的注册之后,好像整个架构也差不多了,其它,扩展的东西还有很多.
我们举一个legacy形式的例子,这个例子是在kernel中随便搜索出来的:
在linux-2.6.26.3/drivers/hwmon/ad7418.c中,初始化函数为:
static int __init ad7418_init(void)
{
return i2c_add_driver(&ad7418_driver);
}
i2c_driver ad7418_driver结构如下:
static struct i2c_driver ad7418_driver = {
.driver = {
.name = "ad7418",
},
.attach_adapter = ad7418_attach_adapter,
.detach_client = ad7418_detach_client,
};
该结构中没有probe()函数,可以断定是一个legacy形式的驱动.这类驱动注册的时候,会调用driver的attach_adapter函数.在这里也就是ad7418_attach_adapter.
这个函数代码如下:
static int ad7418_attach_adapter(struct i2c_adapter *adapter)
{
if (!(adapter->class & I2C_CLASS_HWMON))
return 0;
return i2c_probe(adapter, &addr_data, ad7418_detect);
}
在这里我们又遇到了一个i2c-core中的函数,i2c_probe().在分析这个函数之前,先来看下addr_data是什么?
#define I2C_CLIENT_MODULE_PARM(var,desc) \
static unsigned short var[I2C_CLIENT_MAX_OPTS] = I2C_CLIENT_DEFAULTS; \
static unsigned int var##_num; \
module_param_array(var, short, &var##_num, 0); \
MODULE_PARM_DESC(var,desc)
#define I2C_CLIENT_MODULE_PARM_FORCE(name) \
I2C_CLIENT_MODULE_PARM(force_##name, \
"List of adapter,address pairs which are " \
"unquestionably assumed to contain a `" \
# name "' chip")
#define I2C_CLIENT_INSMOD_COMMON \
I2C_CLIENT_MODULE_PARM(probe, "List of adapter,address pairs to scan " \
"additionally"); \
I2C_CLIENT_MODULE_PARM(ignore, "List of adapter,address pairs not to " \
"scan"); \
static const struct i2c_client_address_data addr_data = { \
.normal_i2c = normal_i2c, \
.probe = probe, \
.ignore = ignore, \
.forces = forces, \
}
#define I2C_CLIENT_FORCE_TEXT \
"List of adapter,address pairs to boldly assume to be present"
由此可知道,addr_data中的三个成员都是模块参数.在加载模块的时候可以用参数的方式对其赋值.三个模块参数为别为probe,ignore,force.另外需要指出的是normal_i2c不能以模块参数的方式对其赋值,只能在驱动内部静态指定.
从模块参数的模述看来, probe是指"List of adapter,address pairs to scan additionally"
Ignore是指"List of adapter,address pairs not to scan "
Force是指"List of adapter,address pairs to boldly assume to be present"
事实上,它们里面的数据都是成对出现的.前面一部份表示所在的总线号,ANY_I2C_BUS表示任一总线.后一部份表示设备的地址.
现在可以来跟踪i2c_probe()的代码了.如下:
int i2c_probe(struct i2c_adapter *adapter,
const struct i2c_client_address_data *address_data,
int (*found_proc) (struct i2c_adapter *, int, int))
{
int i, err;
int adap_id = i2c_adapter_id(adapter);
/* Force entries are done first, and are not affected by ignore
entries */
//先扫描force里面的信息,注意它是一个二级指针.ignore里的信息对它是无效的
if (address_data->forces) {
const unsigned short * const *forces = address_data->forces;
int kind;
for (kind = 0; forces[kind]; kind++) {
for (i = 0; forces[kind][i] != I2C_CLIENT_END;
i += 2) {
if (forces[kind][i] == adap_id
|| forces[kind][i] == ANY_I2C_BUS) {
dev_dbg(&adapter->dev, "found force "
"parameter for adapter %d, "
"addr 0x%02x, kind %d\n",
adap_id, forces[kind][i + 1],
kind);
err = i2c_probe_address(adapter,
forces[kind][i + 1],
kind, found_proc);
if (err)
return err;
}
}
}
}
/* Stop here if we can't use SMBUS_QUICK */
//如果adapter不支持quick.不能够遍历这个adapter上面的设备
if (!i2c_check_functionality(adapter, I2C_FUNC_SMBUS_QUICK)) {
if (address_data->probe[0] == I2C_CLIENT_END
&& address_data->normal_i2c[0] == I2C_CLIENT_END)
return 0;
dev_warn(&adapter->dev, "SMBus Quick command not supported, "
"can't probe for chips\n");
return -1;
}
/* Probe entries are done second, and are not affected by ignore
entries either */
//遍历probe上面的信息.ignore上的信息也对它是没有影响的
for (i = 0; address_data->probe[i] != I2C_CLIENT_END; i += 2) {
if (address_data->probe[i] == adap_id
|| address_data->probe[i] == ANY_I2C_BUS) {
dev_dbg(&adapter->dev, "found probe parameter for "
"adapter %d, addr 0x%02x\n", adap_id,
address_data->probe[i + 1]);
err = i2c_probe_address(adapter,
address_data->probe[i + 1],
-1, found_proc);
if (err)
return err;
}
}
/* Normal entries are done last, unless shadowed by an ignore entry */
//最后遍历normal_i2c上面的信息.它上面的信息不能在ignore中.
for (i = 0; address_data->normal_i2c[i] != I2C_CLIENT_END; i += 1) {
int j, ignore;
ignore = 0;
for (j = 0; address_data->ignore[j] != I2C_CLIENT_END;
j += 2) {
if ((address_data->ignore[j] == adap_id ||
address_data->ignore[j] == ANY_I2C_BUS)
&& address_data->ignore[j + 1]
== address_data->normal_i2c[i]) {
dev_dbg(&adapter->dev, "found ignore "
"parameter for adapter %d, "
"addr 0x%02x\n", adap_id,
address_data->ignore[j + 1]);
ignore = 1;
break;
}
}
if (ignore)
continue;
dev_dbg(&adapter->dev, "found normal entry for adapter %d, "
"addr 0x%02x\n", adap_id,
address_data->normal_i2c[i]);
err = i2c_probe_address(adapter, address_data->normal_i2c[i],
-1, found_proc);
if (err)
return err;
}
return 0;
}
这段代码很简单,结合代码上面添加的注释应该很好理解.如果匹配成功,则会调用i2c_probe_address ().这个函数代码如下:
static int i2c_probe_address(struct i2c_adapter *adapter, int addr, int kind,
int (*found_proc) (struct i2c_adapter *, int, int))
{
int err;
/* Make sure the address is valid */
//地址小于0x03或者大于0x77都是不合法的
if (addr < 0x03 || addr > 0x77) {
dev_warn(&adapter->dev, "Invalid probe address 0x%02x\n",
addr);
return -EINVAL;
}
/* Skip if already in use */
//adapter上已经有这个设备了
if (i2c_check_addr(adapter, addr))
return 0;
/* Make sure there is something at this address, unless forced */
//如果kind小于0.检查adapter上是否有这个设备
if (kind < 0) {
if (i2c_smbus_xfer(adapter, addr, 0, 0, 0,
I2C_SMBUS_QUICK, NULL) < 0)
return 0;
/* prevent 24RF08 corruption */
if ((addr & ~0x0f) == 0x50)
i2c_smbus_xfer(adapter, addr, 0, 0, 0,
I2C_SMBUS_QUICK, NULL);
}
/* Finally call the custom detection function */
//调用回调函数
err = found_proc(adapter, addr, kind);
/* -ENODEV can be returned if there is a chip at the given address
but it isn't supported by this chip driver. We catch it here as
this isn't an error. */
if (err == -ENODEV)
err = 0;
if (err)
dev_warn(&adapter->dev, "Client creation failed at 0x%x (%d)\n",
addr, err);
return err;
}
首先,对传入的参数进行一系列的合法性检查.另外,如果该adapter上已经有了这个地址的设备了.也会返回失败.所有adapter下面的设备都是以adapter->dev为父结点的.因此只需要遍历adapter->dev下面的子设备就可以得到当前地址是不是被占用了.
如果kind < 0.还得要adapter检查该总线是否有这个地址的设备.方法是向这个地址发送一个Read的Quick请求.如果该地址有应答,则说明这个地址上有这个设备.另外还有一种情况是在24RF08设备的特例.
如果adapter上确实有这个设备,就会调用驱动调用时的回调函数.
在上面涉及到了IIC的传输方式,有疑问的可以参考intel ICH5手册的有关smbus部份.
跟踪i2c_smbus_xfer().代码如下:
s32 i2c_smbus_xfer(struct i2c_adapter * adapter, u16 addr, unsigned short flags,
char read_write, u8 command, int size,
union i2c_smbus_data * data)
{
s32 res;
flags &= I2C_M_TEN | I2C_CLIENT_PEC;
if (adapter->algo->smbus_xfer) {
mutex_lock(&adapter->bus_lock);
res = adapter->algo->smbus_xfer(adapter,addr,flags,read_write,
command,size,data);
mutex_unlock(&adapter->bus_lock);
} else
res = i2c_smbus_xfer_emulated(adapter,addr,flags,read_write,
command,size,data);
return res;
}
如果adapter有smbus_xfer()函数,则直接调用它发送,否则,也就是在adapter不支持smbus协议的情况下,调用i2c_smbus_xfer_emulated()继续处理.
跟进i2c_smbus_xfer_emulated().代码如下:
static s32 i2c_smbus_xfer_emulated(struct i2c_adapter * adapter, u16 addr,
unsigned short flags,
char read_write, u8 command, int size,
union i2c_smbus_data * data)
{
/* So we need to generate a series of msgs. In the case of writing, we
need to use only one message; when reading, we need two. We initialize
most things with sane defaults, to keep the code below somewhat
simpler. */
//写操作只会进行一次交互,而读操作,有时会有两次操作.
//因为有时候读操作要先写command,再从总线上读数据
//在这里为了代码的简洁.使用了两个缓存区,将两种情况统一起来.
unsigned char msgbuf0[I2C_SMBUS_BLOCK_MAX+3];
unsigned char msgbuf1[I2C_SMBUS_BLOCK_MAX+2];
//一般来说,读操作要交互两次.例外的情况我们在下面会接着分析
int num = read_write == I2C_SMBUS_READ?2:1;
//与设备交互的数据,一般在msg[0]存放写入设备的信息,在msb[1]里存放接收到的
//信息.不过也有例外的
//msg[2]的初始化,默认发送缓存区占一个字节,无接收缓存
struct i2c_msg msg[2] = { { addr, flags, 1, msgbuf0 },
{ addr, flags | I2C_M_RD, 0, msgbuf1 }
};
int i;
u8 partial_pec = 0;
//将要发送的信息copy到发送缓存区的第一字节
msgbuf0[0] = command;
switch(size) {
//quick类型的,其它并不传输有效数据,只是将地址写到总线上,等待应答即可
//所以将发送缓存区长度置为0 .再根据读/写操作,调整msg[0]的标志位
//这类传输只需要一次总线交互
case I2C_SMBUS_QUICK:
msg[0].len = 0;
/* Special case: The read/write field is used as data */
msg[0].flags = flags | (read_write==I2C_SMBUS_READ)?I2C_M_RD:0;
num = 1;
break;
case I2C_SMBUS_BYTE:
//BYTE类型指一次写和读只有一个字节.这种情况下,读和写都只会交互一次
//这种类型的读有例外,它读取出来的数据不是放在msg[1]中的,而是存放在msg[0]
if (read_write == I2C_SMBUS_READ) {
/* Special case: only a read! */
msg[0].flags = I2C_M_RD | flags;
num = 1;
}
break;
case I2C_SMBUS_BYTE_DATA:
//Byte_Data是指命令+数据的传输形式.在这种情况下,写只需要一次交互,读却要两次
//第一次将command写到总线上,第二次要转换方向.要将设备地址和read标志写入总线.
//应回答之后再进行read操作
//写操作占两字节,分别是command+data.读操作的有效数据只有一个字节
//交互次数用初始化值就可以了
if (read_write == I2C_SMBUS_READ)
msg[1].len = 1;
else {
msg[0].len = 2;
msgbuf0[1] = data->byte;
}
break;
case I2C_SMBUS_WORD_DATA:
//Word_Data是指命令+双字节的形式.这种情况跟Byte_Data的情况类似
//两者相比只是交互的数据大小不同
if (read_write == I2C_SMBUS_READ)
msg[1].len = 2;
else {
msg[0].len=3;
msgbuf0[1] = data->word & 0xff;
msgbuf0[2] = data->word >> 8;
}
break;
case I2C_SMBUS_PROC_CALL:
//Proc_Call的方式与write 的Word_Data相似,只不过写完Word_Data之后,要等待它的应答
//应该它需要交互两次,一次写一次读
num = 2; /* Special case */
read_write = I2C_SMBUS_READ;
msg[0].len = 3;
msg[1].len = 2;
msgbuf0[1] = data->word & 0xff;
msgbuf0[2] = data->word >> 8;
break;
case I2C_SMBUS_BLOCK_DATA:
//Block_Data:指command+N段数据的情况.
//如果是读操作,它首先要写command到总线,然后再读N段数据.要写的command已经
//放在msg[0]了.现在只需要将msg[1]的标志置I2C_M_RECV_LEN位,msg[1]有效长度为1字节.因为
//adapter驱动会处理好的.现在现在还不知道要传多少段数据.
//对于写的情况:msg[1]照例不需要.将要写的数据全部都放到msb[0]中.相应的也要更新
//msg[0]中的缓存区长度
if (read_write == I2C_SMBUS_READ) {
msg[1].flags |= I2C_M_RECV_LEN;
msg[1].len = 1; /* block length will be added by
the underlying bus driver */
} else {
//data->block[0]表示后面有多少段数据.总长度要加2是因为command+count+N段数据
msg[0].len = data->block[0] + 2;
if (msg[0].len > I2C_SMBUS_BLOCK_MAX + 2) {
dev_err(&adapter->dev, "smbus_access called with "
"invalid block write size (%d)\n",
data->block[0]);
return -1;
}
for (i = 1; i < msg[0].len; i++)
msgbuf0[i] = data->block[i-1];
}
break;
case I2C_SMBUS_BLOCK_PROC_CALL:
//Proc_Call:表示写完Block_Data之后,要等它的应答消息它和Block_Data相比,只是多了一部份应答而已
num = 2; /* Another special case */
read_write = I2C_SMBUS_READ;
if (data->block[0] > I2C_SMBUS_BLOCK_MAX) {
dev_err(&adapter->dev, "%s called with invalid "
"block proc call size (%d)\n", __func__,
data->block[0]);
return -1;
}
msg[0].len = data->block[0] + 2;
for (i = 1; i < msg[0].len; i++)
msgbuf0[i] = data->block[i-1];
msg[1].flags |= I2C_M_RECV_LEN;
msg[1].len = 1; /* block length will be added by
the underlying bus driver */
break;
case I2C_SMBUS_I2C_BLOCK_DATA:
//I2c Block_Data与Block_Data相似,只不过read的时候,数据长度是预先定义好了的.另外
//与Block_Data相比,中间不需要传输Count字段.(Count表示数据段数目)
if (read_write == I2C_SMBUS_READ) {
msg[1].len = data->block[0];
} else {
msg[0].len = data->block[0] + 1;
if (msg[0].len > I2C_SMBUS_BLOCK_MAX + 1) {
dev_err(&adapter->dev, "i2c_smbus_xfer_emulated called with "
"invalid block write size (%d)\n",
data->block[0]);
return -1;
}
for (i = 1; i <= data->block[0]; i++)
msgbuf0[i] = data->block[i];
}
break;
default:
dev_err(&adapter->dev, "smbus_access called with invalid size (%d)\n",
size);
return -1;
}
//如果启用了PEC.Quick和I2c Block_Data是不支持PEC的
i = ((flags & I2C_CLIENT_PEC) && size != I2C_SMBUS_QUICK
&& size != I2C_SMBUS_I2C_BLOCK_DATA);
if (i) {
/* Compute PEC if first message is a write */
//如果第一个操作是写操作
if (!(msg[0].flags & I2C_M_RD)) {
//如果只是写操作
if (num == 1) /* Write only */
//如果只有写操作,写缓存区要扩充一个字节,用来存放计算出来的PEC
i2c_smbus_add_pec(&msg[0]);
else /* Write followed by read */
//如果后面还有读操作,先计算前面写部份的PEC(注意这种情况下不需要
//扩充写缓存区,因为不需要发送PEC.只会接收到PEC)
partial_pec = i2c_smbus_msg_pec(0, &msg[0]);
}
/* Ask for PEC if last message is a read */
//如果最后一次是读消息.还要接收到来自slave的PEC.所以接收缓存区要扩充一个字节
if (msg[num-1].flags & I2C_M_RD)
msg[num-1].len++;
}
if (i2c_transfer(adapter, msg, num) < 0)
return -1;
/* Check PEC if last message is a read */
//操作完了之后,如果最后一个操作是PEC的读操作.检验后面的PEC是否正确
if (i && (msg[num-1].flags & I2C_M_RD)) {
if (i2c_smbus_check_pec(partial_pec, &msg[num-1]) < 0)
return -1;
}
//操作完了,现在可以将数据放到data部份返回了.
if (read_write == I2C_SMBUS_READ)
switch(size) {
case I2C_SMBUS_BYTE:
data->byte = msgbuf0[0];
break;
case I2C_SMBUS_BYTE_DATA:
data->byte = msgbuf1[0];
break;
case I2C_SMBUS_WORD_DATA:
case I2C_SMBUS_PROC_CALL:
data->word = msgbuf1[0] | (msgbuf1[1] << 8);
break;
case I2C_SMBUS_I2C_BLOCK_DATA:
for (i = 0; i < data->block[0]; i++)
data->block[i+1] = msgbuf1[i];
break;
case I2C_SMBUS_BLOCK_DATA:
case I2C_SMBUS_BLOCK_PROC_CALL:
for (i = 0; i < msgbuf1[0] + 1; i++)
data->block[i] = msgbuf1[i];
break;
}
return 0;
}
在这个函数添上了很详细的注释,配和intel的datasheet,应该很容易看懂.在上面的交互过程中,调用了子函数i2c_transfer().它的代码如下所示:
int i2c_transfer(struct i2c_adapter * adap, struct i2c_msg *msgs, int num)
{
int ret;
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);
}
ret = adap->algo->master_xfer(adap,msgs,num);
mutex_unlock(&adap->bus_lock);
return ret;
} else {
dev_dbg(&adap->dev, "I2C level transfers not supported\n");
return -ENOSYS;
}
}
因为在这里的同步用的是mutex.首先判断判断是否充许睡眠,如果不允许,尝试获锁.如果获锁失败,则返回,这样的操作是避免进入睡眠,我们在后面也可以看到,实际的传输工作交给了adap->algo->master_xfer()完成.
在这里,我们终于把i2c_probe_address()的执行分析完了,经过这个分析,我们也知道了数据是怎么样传输的.我们接着i2c_probe()往下看.如果i2c_probe_address()成功.说明总线上确实有这样的设备.那么就会调用驱动中的回调函数.在ad7148的驱动中,如下所示:
return i2c_probe(adapter, &addr_data, ad7418_detect);
也就是说,要调用的回调函数是ad7418_detect().这个函数中我们只分析和i2c框架相关的部份.代码片段如下所示:
static int ad7418_detect(struct i2c_adapter *adapter, int address, int kind)
{
struct i2c_client *client;
……
……
client->addr = address;
client->adapter = adapter;
client->driver = &ad7418_driver;
i2c_set_clientdata(client, data);
……
……
if ((err = i2c_attach_client(client)))
goto exit_free;
……
……
}
结合上面关于new-style形式的驱动分析.发现这里走的是同一个套路,即初始化了client.然后调用i2c_attach_client().后面的流程就跟上面分析的一样了.只不过,不相同的是,这里clinet已经指定了驱动为ad7418_driver.应该在注册clinet->dev之后,就不会走bus->match和bus->probe的流程了.
七:i2c dev节点操作
现在来分析上面架构图中的i2c-dev.c中的部份.这个部份为用户空间提供了操作adapter的接口.这部份代码其实对应就晃一个模块.它的初始化函数为:
module_init(i2c_dev_init);
i2c_dev_init()代码如下:
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))
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.
