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.
架构如下：
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 “, 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 “, 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]$ lsi2c-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 `– uevent3 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 driversNew-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 __initi2c_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! “); 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 “, 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 “, 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 “, client->name, client->addr); } } return 0;out_err: dev_err(&adapter->dev, “Failed to attach i2c client %s at 0x%02x ” “(%d) “, 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.
想想我的影子，我会在你身后给你一个拥抱;
