Linux的I2C驱动架构
1. Linux的I2C驱动架构Linux中I2C总线的驱动分为两个部分，总线驱动（BUS）和设备驱动（DEVICE）。其中总线驱动的职责，是为系统中每个I2C总线增加相应的读写方法。但是总线驱动本身并不会进行任何的通讯，它只是存在在那里，等待设备驱动调用其函数。
设备驱动则是与挂在I2C总线上的具体的设备通讯的驱动。通过I2C总线驱动提供的函数，设备驱动可以忽略不同总线控制器的差异，不考虑其实现细节地与硬件设备通讯。
1.1 总线驱动在系统开机时，首先装载的是I2C总线驱动。一个总线驱动用于支持一条特定的I2C总线的读写。一个总线驱动通常需要两个模块，一个struct i2c_adapter和一个struct i2c_algorithm来描述：
/*
* The following structs are for those who like to implement new bus drivers:
* i2c_algorithm is the interface to a class of hardware solutions which can
* be addressed using the same bus algorithms - i.e. bit-banging or the PCF8584
* to name two of the most common.
*/
struct i2c_algorithm {
/* If an adapter algorithm can't do I2C-level access, set master_xfer
to NULL. If an adapter algorithm can do SMBus access, set
smbus_xfer. If set to NULL, the SMBus protocol is simulated
using common I2C messages */
/* master_xfer should return the number of messages successfully
processed, or a negative value on error */
int (*master_xfer)(struct i2c_adapter *adap,struct i2c_msg *msgs,
int num);
int (*smbus_xfer) (struct i2c_adapter *adap, u16 addr,
unsigned short flags, char read_write,
u8 command, int size, union i2c_smbus_data * data);
/* --- these optional/future use for some adapter types.*/
int (*slave_send)(struct i2c_adapter *,char*,int);
int (*slave_recv)(struct i2c_adapter *,char*,int);
/* --- ioctl like call to set div. parameters. */
int (*algo_control)(struct i2c_adapter *, unsigned int, unsigned long);
/* To determine what the adapter supports */
u32 (*functionality) (struct i2c_adapter *);
};
static struct i2c_adapter pb1550_board_adapter = {
name: "pb1550 adapter",
id: I2C_HW_AU1550_PSC,
algo: NULL,
algo_data: &pb1550_i2c_info,
inc_use: pb1550_inc_use,
dec_use: pb1550_dec_use,
client_register: pb1550_reg,
client_unregister: pb1550_unreg,
client_count: 0,
};
/*
* i2c_adapter is the structure used to identify a physical i2c bus along
* with the access algorithms necessary to access it.
*/
struct i2c_adapter {
struct module *owner;
unsigned int id;
unsigned int class;
struct i2c_algorithm *algo;/* the algorithm to access the bus */
void *algo_data;
/* --- administration stuff. */
int (*client_register)(struct i2c_client *);
int (*client_unregister)(struct i2c_client *);
/* data fields that are valid for all devices */
struct mutex bus_lock;
struct mutex clist_lock;
int timeout;
int retries;
struct device dev; /* the adapter device */
struct class_device class_dev; /* the class device */
int nr;
struct list_head clients;
struct list_head list;
char name[I2C_NAME_SIZE];
struct completion dev_released;
struct completion class_dev_released;
};
这个样例挂接了一个叫做“pb1550 adapter”的驱动。但这个模块并未提供读写函数，具体的读写方法由第二个模块，struct i2c_algorithm提供。
static struct i2c_algorithm au1550_algo = {
.name = "Au1550 algorithm",
.id = I2C_ALGO_AU1550,
.master_xfer = au1550_xfer,
.functionality = au1550_func,
};
i2c_adap->algo = &au1550_algo;
这个样例给上述总线驱动增加了读写“算法”。通常情况下每个I2C总线驱动都定义一个自己的读写算法，但鉴于有些总线使用相同的算法，因而可以共用同一套读写函数。本例中的驱动定义了自己的读写算法模块，起名叫“Au1550 algorithm”。
全部填妥后，通过调用：
i2c_add_adapter(i2c_adap);
将这两个模块注册到操作系统里，总线驱动就算装上了。对于AMD au1550，这部分已经由AMD提供了。
1.2 设备驱动如前所述，总线驱动只是提供了对一条总线的读写机制，本身并不会去做通信。通信是由I2C设备驱动来做的，设备驱动透过I2C总线同具体的设备进行通讯。一个设备驱动有两个模块来描述，struct i2c_driver和struct i2c_client。
当系统开机、I2C总线驱动装入完成后，就可以装入设备驱动了。首先装入如下结构：
static struct i2c_driver driver = {
.name = "i2c TV tuner driver",
.id = I2C_DRIVERID_TUNER,
.flags = I2C_DF_NOTIFY,
.attach_adapter = tuner_probe,
.detach_client = tuner_detach,
.command = tuner_command,
};
i2c_add_driver(&driver);
这个i2c_driver一旦装入完成，其中的attach_adapter函数就会被调用。在其中可以遍历系统中的每个i2c总线驱动，探测想要访问的设备：
static int tuner_probe(struct i2c_adapter *adap)
{
return i2c_probe(adap, &addr_data, tuner_attach);
}
注意探测可能会找到多个设备，因而不仅一个I2C总线可以挂多个不同类型的设备，一个设备驱动也可以同时为挂在多个不同I2C总线上的设备服务。
每当设备驱动探测到了一个它能支持的设备，它就创建一个struct i2c_client来标识这个设备：
new_client->addr = address;
new_client->adapter = adapter;
new_client->driver = &driver;
/* Tell the I2C layer a new client has arrived */
err = i2c_attach_client(new_client);
if (err)
goto error;
/*
* i2c_client identifies a single device (i.e. chip) that is connected to an
* i2c bus. The behaviour is defined by the routines of the driver. This
* function is mainly used for lookup & other admin. functions.
*/
struct i2c_client {
unsigned int flags; /* div., see below */
unsigned short addr; /* chip address - NOTE: 7bit */
/* addresses are stored in the */
/* _LOWER_ 7 bits */
struct i2c_adapter *adapter; /* the adapter we sit on */
struct i2c_driver *driver; /* and our access routines */
int usage_count; /* How many accesses currently */
/* to the client */
struct device dev; /* the device structure */
struct list_head list;
char name[I2C_NAME_SIZE];
struct completion released;
};
可见，一个i2c_client代表着位于adapter总线上，地址为address，使用driver来驱动的一个设备。它将总线驱动与设备驱动，以及设备地址绑定在了一起。一个i2c_client就代表着一个I2C设备。
当得到I2C设备后，就可以直接对此设备进行读写：
/*
* The master routines are the ones normally used to transmit data to devices
* on a bus (or read from them). Apart from two basic transfer functions to
* transmit one message at a time, a more complex version can be used to
* transmit an arbitrary number of messages without interruption.
*/
extern int i2c_master_send(struct i2c_client *,const char* ,int);
extern int i2c_master_recv(struct i2c_client *,char* ,int);
与通常意义上的读写函数一样，这两个函数对i2c_client指针指定的设备，读写int个char。返回值为读写的字节数。对于我们现有的SLIC的驱动，只要将最后要往总线上进行读写的数据引出传输到这两个函数中，移植工作就算完成了，我们将得到一个Linux版的I2C设备驱动。I2C总线使用方法研究(作者LUOFUCHONG)
前言：
记得以前曾研究过Linux内核下i2c子系统，了解了i2c总线上，适配器、设备驱动的注册过程与使用方法，详细请查看：
1、i2c总线上，适配器、设备驱动注册：
2、i2c总线使用：
最近因为项目的需要，又重新看了一遍linux下的i2c子系统，有了新的体会。俗话说，好记性不如烂笔头，还是记下来比较好，一来当作总结，二来希望起到启发后人的效果^_^
正文：
Linux的世界充斥着各种各样的想法，i2c子系统同样，新、旧架构并存。至于旧的架构，网上有非常详细的介绍，本人不打算再去重复，咱们直奔新的架构去。
Linux下的设备驱动，顾名思义，就是由设备和驱动来组成的，i2c总线上的设备驱动也不例外。
一、i2c设备的注册
第一步：
记得以前的i2c设备驱动，设备部分喜欢驱动运行的时候动态创建，新式的驱动倾向于向传统的linux下设备驱动看齐，采用静态定义的方式来注册设备，使用接口为：
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);//申请表示i2c设备的结构体空间
if (!devinfo) {
pr_debug("i2c-core: can't register boardinfo!\n");
status = -ENOMEM;
break;
}
/* 填写i2c设备描述结构 */
devinfo->busnum = busnum;
devinfo->board_info = *info;
list_add_tail(&devinfo->list, &__i2c_board_list);//添加到全局链表__i2c_board_list中
}
mutex_unlock(&__i2c_board_lock);
return status;
}
第二步：
系统初始化的时候，会根据板级i2c设备配置信息，创建i2c客户端设备(i2c_client),添加到i2c子系统中：
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) { //遍历全局链表__i2c_board_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);
}
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);//创建i2c_client设备
if (!client)
return NULL;
client->adapter = adap;
client->dev.platform_data = info->platform_data;
if (info->archdata)
client->dev.archdata = *info->archdata;
client->flags = info->flags;
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);//接入设备到i2c总线上
if (status < 0) {
kfree(client);
client = NULL;
}
return client;
}
int i2c_attach_client(struct i2c_client *client)
{
struct i2c_adapter *adapter = client->adapter;
int res;
/* Check for address business */
res = i2c_check_addr(adapter, client->addr);//地址检测
if (res)
return res;
client->dev.parent = &client->adapter->dev;
client->dev.bus = &i2c_bus_type;
if (client->driver)
client->dev.driver = &client->driver->driver;
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;
snprintf(&client->dev.bus_id[0], sizeof(client->dev.bus_id),
"%d-%04x", i2c_adapter_id(adapter), client->addr);
res = device_register(&client->dev); //注册设备
if (res)
goto out_err;
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);
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;
}
注：
特别要提一下的是这个“i2c_check_addr”，引用<<i2c 源代码情景分析>>里的话：“i2c 设备的7 位地址是就当前i2c 总线而言的，是“相对地址”。不同的i2c 总线上的设备可以使用相同的7 位地址，但是它们所在的i2c 总线不同。所以在系统中一个i2c 设备的“绝对地址”由二元组（i2c 适配器的ID 和设备在该总线上的7 位地址）表示。”，所以这个函数的作用主要是排除同一i2c总线上出现多个地址相同的设备。
二、驱动的注册：
第一步：
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;
/* new style driver methods can't mix with legacy ones */
if (is_newstyle_driver(driver)) { //呵呵，新旧架构的驱动可以从这里看出来
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 */
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.
*/
res = driver_register(&driver->driver); //注册驱动
if (res)
return res;
mutex_lock(&core_lock);
pr_debug("i2c-core: driver [%s] registered\n", driver->driver.name);
INIT_LIST_HEAD(&driver->clients);
/* Walk the adapters that are already present */
class_for_each_device(&i2c_adapter_class, NULL, driver,
__attach_adapter);
mutex_unlock(&core_lock);
return 0;
}
第二步：
还记得在第一部分，我们注册了一i2c设备，其总线类型为i2c_bus_type，现在我们又在这总线上注册一驱动，那不得了，满足了设备+驱动的条件，i2c_bus_type总线上探测函数要被执行了。
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,
};
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);
int status;
if (!driver->probe || !driver->id_table)
return -ENODEV;
client->driver = driver;
if (!device_can_wakeup(&client->dev))
device_init_wakeup(&client->dev,
client->flags & I2C_CLIENT_WAKE);
dev_dbg(dev, "probe\n");
status = driver->probe(client, i2c_match_id(driver->id_table, client));//调用具体i2c设备驱动的探测函数
if (status)
client->driver = NULL;
return status;
}