一:前言
I2c是philips提出的外設總線。I2C只有兩條線,一條串行數據線:SDA,一條是時鐘線SCL.正因爲這樣,它方便了工程人員的佈線。另外,I2C是一種多主機控制總線。它和USB總線不同,USB是基於master-slave機制,任何設備的通信必須由主機發起纔可以。而I2C是基於multi master機制。一同總線上可允許多個master.關於I2C協議的知識,這裏不再贅述。可自行下載spec閱讀即可。
二:I2C架構概述
如上圖所示,每一條I2C對應一個adapter.在kernel中,每一個adapter提供了一個描述的結構(struct i2c_adapter),也定義了adapter支持的操作(struct i2c_adapter)。再通過i2c core層將i2c設備與i2c adapter關聯起來。
這個圖只是提供了一個大概的框架。在下面的代碼分析中,從下至上的來分析這個框架圖。以下的代碼分析是基於linux 2.6.26.分析的代碼基本位於: linux-2.6.26.3/drivers/i2c/位置。
三:adapter註冊
在kernel中提供了兩個adapter註冊接口,分別爲i2c_add_adapter()和i2c_add_numbered_adapter()。由於在系統中可能存在多個adapter,因爲將每一條I2C總線對應一個編號,下文中稱爲I2C總線號。這個總線號的PCI中的總線號不同。它和硬件無關,只是軟件上便於區分而已。
對於i2c_add_adapter()而言,它使用的是動態總線號,即由系統給其分析一個總線號,而i2c_add_numbered_adapter()則是自己指定總線號,如果這個總線號非法或者是被佔用,就會註冊失敗。
分別來看一下這兩個函數的代碼:
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);
}
在這裏涉及到一個idr結構。idr結構本來是爲了配合page cache中的radix tree而設計的。在這裏我們只需要知道,它是一種高效的搜索樹,且這個樹預先存放了一些內存。避免在內存不夠的時候出現問題。所在,在往idr中插入結構的時候,首先要調用idr_pre_get()爲它預留足夠的空閒內存,然後再調用idr_get_new_above()將結構插入idr中,該函數以參數的形式返回一個id.以後憑這個id就可以在idr中找到相對應的結構了。對這個數據結構操作不太理解的可以查閱本站《 linux文件系統之文件的讀寫》中有關radix tree的分析。
注意一下idr_get_new_above(&i2c_adapter_idr, adapter,__i2c_first_dynamic_bus_num, &id)的參數的含義,它是將adapter結構插入到i2c_adapter_idr中,存放位置的id必須要大於或者等於__i2c_first_dynamic_bus_num,
然後將對應的id號存放在adapter->nr中。調用i2c_register_adapter(adapter)對這個adapter進行進一步註冊。
看一下另外一人註冊函數: 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;
}
對比一下就知道差別了,在這裏它已經指定好了adapter->nr了。如果分配的id不和指定的相等,便返回錯誤。
過一步跟蹤i2c_register_adapter()。代碼如下:
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;
}
首先對adapter和adapter中內嵌的struct device結構進行必須的初始化。之後將adapter內嵌的struct device註冊。
在這裏注意一下adapter->dev的初始化。它的類別爲i2c_adapter_class,如果沒有父結點,則將其父結點設爲platform_bus.adapter->dev的名字爲i2c + 總線號。
測試一下:
[eric@mochow i2c]$ cd /sys/class/i2c-adapter/
[eric@mochow i2c-adapter]$ ls
i2c-0
可以看到,在我的PC上,有一個I2C adapter,看下詳細信息:
[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,該adapter是一個PCI設備。
繼續往下看:
之後,在註釋中看到,有兩種類型的driver,一種是new-style drivers,另外一種是legacy drivers
New-style drivers是在2.6近版的kernel加入的。它們最主要的區別是在adapter和i2c driver的匹配上。
3.1: new-style 形式的adapter註冊
對於第一種,也就是new-style drivers,將相關代碼再次列出如下:
if (adap->nr < __i2c_first_dynamic_bus_num)
i2c_scan_static_board_info(adap);
如果adap->nr 小於__i2c_first_dynamic_bus_num的話,就會進入到i2c_scan_static_board_info()。
結合我們之前分析的adapter的兩種註冊分式: i2c_add_adapter()所分得的總線號肯會不會小於__i2c_first_dynamic_bus_num.只有i2c_add_numbered_adapter()纔有可能滿足:
(adap->nr < __i2c_first_dynamic_bus_num)
而且必須要調用i2c_register_board_info()將板子上的I2C設備信息預先註冊時纔會更改__i2c_first_dynamic_bus_num的值。在x86上只沒有使用i2c_register_board_info()的。因此,x86平臺上的分析可以忽略掉new-style driver的方式。不過,還是詳細分析這種情況下。
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;
}
這個函數比較簡單, struct i2c_board_info用來表示I2C設備的一些情況,比如所在的總線。名稱,地址,中斷號等。最後,這些信息會被存放到__i2c_board_list鏈表。
跟蹤i2c_scan_static_board_info():代碼如下:
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);
}
該函數遍歷掛在__i2c_board_list鏈表上面的i2c設備的信息,也就是我們在啓動的時候指出的i2c設備的信息。
如果指定設備是位於adapter所在的I2C總線上,那麼,就調用i2c_new_device()。代碼如下:
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;
}
我們又遇到了一個新的結構:struct i2c_client,不要被這個結構嚇倒了,其實它就是一個嵌入struct device的I2C設備的封裝。它和我們之前遇到的struct usb_device結構的作用是一樣的。
首先,在clinet裏保存該設備的相關消息。特別的, client->adapter指向了它所在的adapter.
特別的,clinet->name爲info->name.也是指定好了的。
一切初始化完成之後,便會調用i2c_attach_client( )。看這個函數的字面意思,是將clinet關聯起來。到底怎麼樣關聯呢?繼續往下看:
int i2c_attach_client(struct i2c_client *client)
{
struct i2c_adapter *adapter = client->adapter;
int res = 0;
//初始化client內嵌的dev結構
//父結點爲所在的adapter,所在bus爲i2c_bus_type
client->dev.parent = &client->adapter->dev;
client->dev.bus = &i2c_bus_type;
//如果client已經指定了driver,將driver和內嵌的dev關聯起來
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的名稱
snprintf(&client->dev.bus_id[0], sizeof(client->dev.bus_id),
"%d-%04x", i2c_adapter_id(adapter), client->addr);
//將內嵌的dev註冊
res = device_register(&client->dev);
if (res)
goto out_err;
//將clinet鏈到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,就調用它
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;
}
參考上面添加的註釋,應該很容易理解這段代碼了,就不加詳細分析了。這個函數的名字不是i2c_attach_client()麼?怎麼沒看到它的關係過程呢?
這是因爲:在代碼中設置了client->dev所在的bus爲i2c_bus_type .以爲只需要有bus爲i2c_bus_type的driver註冊,就會產生probe了。這個過程呆後面分析i2c driver的時候再來詳細分析。
3.2: legacy形式的adapter註冊
Legacy形式的adapter註冊代碼片段如下:
dummy = bus_for_each_drv(&i2c_bus_type, NULL, adap,
i2c_do_add_adapter);
這段代碼遍歷掛在i2c_bus_type上的驅動,然後對每一個驅動和adapter調用i2c_do_add_adapter()。
代碼如下:
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;
}
該函數很簡單,就是調用driver的attach_adapter()接口。
到此爲止,adapter的註冊已經分析完了。
四:i2c driver註冊
在分析i2c driver的時候,有必要先分析一下i2c架構的初始化
代碼如下:
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);
很明顯,i2c_init()會在系統初始化的時候被調用。
在i2c_init中,先註冊了i2c_bus_type的bus,i2c_adapter_class的class.然後再調用i2c_add_driver()註冊了一個i2c driver.
I2c_bus_type結構如下:
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,
};
這個結構先放在這裏吧,以後還會用到裏面的信息的。
從上面的初始化函數裏也看到了,註冊i2c driver的接口爲i2c_add_driver()。代碼如下:
static inline int i2c_add_driver(struct i2c_driver *driver)
{
return i2c_register_driver(THIS_MODULE, driver);
}
繼續跟蹤:
int i2c_register_driver(struct module *owner, struct i2c_driver *driver)
{
int res;
/* new style driver methods can't mix with legacy ones */
//如果是一個newstyle的driver.但又定義了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 */
//關聯到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.
*/
//註冊內嵌的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 */
//遍歷所有的adapter,對其都調用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;
}