Linux设备驱动程序架构分析之I2C架构（基于3.10.1内核）

作者：刘昊昱

博客：/liuhaoyutz

内核版本：3.10.1

I2C体系架构的硬件实体包括两部分：

硬件I2C Adapter：硬件I2C Adapter表示一个硬件I2C适配器，也就是I2C控制器。一般是SOC中的一个接口，也可以用GPIO模拟。硬件I2C Adapter主要用来在I2C接口上产生I2C时钟信号和数据信号。硬件I2C Device：表示一个I2C从设备，I2C从设备并行的挂接在I2C总线上，通过I2C接口与I2C主控制器通信。

Linux的I2C体系架构包括6个部分：

I2C Core，定义了一些函数和数据结构，用于支持I2C控制器驱动（I2C Adapter drvier）和I2C设备驱动（I2C client driver）。I2C Core的存在使I2C控制器驱动和I2C设备驱动独立开来，具有更好的可移置性，同时简化了驱动开发的工作量。但是，I2C Core的存在也使Linux I2C体系结构理解起来更有难度。I2C Adapter，代表一个I2C控制器，用struct i2c_adapter来表示。Algorithm，访问I2C控制器的接口，Algorithm直接操作I2C控制器的硬件寄存器来执行数据的发送和接收。它提供操作I2C控制器的最底层操作函数。每个I2C Adapter都有自己的Algorithm，每个I2C Adapter通过自己的Algorithm与挂接在该控制器上的I2C从设备进行通信。I2C Client，代表一个挂接在I2C总线上的I2C设备。I2C设备驱动（I2C client driver）I2C-dev，I2C控制器的设备文件，通常命名为i2c-0、i2c-1等等,是I2C控制器的应用层访问接口。

一、I2C相关数据结构

I2C控制器（例如S3C2440 I2C控制器）对应的数据结构是i2c_adapter，i2c_adapter结构定义在include/linux/i2c.h文件中，从注释可以看出，i2c_adapter代表一条物理的I2C总线，同时还包括访问该I2C总线的方法。其内容如下：

409/*410 * i2c_adapter is the structure used to identify a physical i2c bus along411 * with the access algorithms necessary to access it.412 */413struct i2c_adapter {414 struct module *owner;415 unsigned int class; /* classes to allow probing for */416 const struct i2c_algorithm *algo; /* the algorithm to access the bus */417 void *algo_data;418419 /* data fields that are valid for all devices */420 struct rt_mutex bus_lock;421422 int timeout; /* in jiffies */423 int retries;424 struct device dev;/* the adapter device */425426 int nr;427 char name[48];428 struct completion dev_released;429430 struct mutex userspace_clients_lock;431 struct list_head userspace_clients;432433 struct i2c_bus_recovery_info *bus_recovery_info;434};

i2c_algorithm定义在include/linux/i2c.h文件中，其内容如下：

347/*348 * The following structs are for those who like to implement new bus drivers:349 * i2c_algorithm is the interface to a class of hardware solutions which can350 * be addressed using the same bus algorithms - i.e. bit-banging or the PCF8584351 * to name two of the most common.352 */353struct i2c_algorithm {354 /* If an adapter algorithm can't do I2C-level access, set master_xfer355 to NULL. If an adapter algorithm can do SMBus access, set356 smbus_xfer. If set to NULL, the SMBus protocol is simulated357 using common I2C messages */358 /* master_xfer should return the number of messages successfully359 processed, or a negative value on error */360 int (*master_xfer)(struct i2c_adapter *adap, struct i2c_msg *msgs,361int num);362 int (*smbus_xfer) (struct i2c_adapter *adap, u16 addr,363unsigned short flags, char read_write,364u8 command, int size, union i2c_smbus_data *data);365366 /* To determine what the adapter supports */367 u32 (*functionality) (struct i2c_adapter *);368};

struct i2c_client代表一个I2C设备，该结构体定义在include/linux/i2c.h文件中，其内容如下：

200/**201 * struct i2c_client - represent an I2C slave device202 * @flags: I2C_CLIENT_TEN indicates the device uses a ten bit chip address;203 * I2C_CLIENT_PEC indicates it uses SMBus Packet Error Checking204 * @addr: Address used on the I2C bus connected to the parent adapter.205 * @name: Indicates the type of the device, usually a chip name that's206 * generic enough to hide second-sourcing and compatible revisions.207 * @adapter: manages the bus segment hosting this I2C device208 * @driver: device's driver, hence pointer to access routines209 * @dev: Driver model device node for the slave.210 * @irq: indicates the IRQ generated by this device (if any)211 * @detected: member of an i2c_driver.clients list or i2c-core's212 * userspace_devices list213 *214 * An i2c_client identifies a single device (i.e. chip) connected to an215 * i2c bus. The behaviour exposed to Linux is defined by the driver216 * managing the device.217 */218struct i2c_client {219 unsigned short flags; /* div., see below*/220 unsigned short addr; /* chip address - NOTE: 7bit */221/* addresses are stored in the */222/* _LOWER_ 7 bits */223 char name[I2C_NAME_SIZE];224 struct i2c_adapter *adapter; /* the adapter we sit on */225 struct i2c_driver *driver; /* and our access routines */226 struct device dev;/* the device structure*/227 int irq; /* irq issued by device*/228 struct list_head detected;229};

struct i2c_driver代表一个I2C设备驱动程序，该结构体定义在include/linux/i2c.h文件中，其内容如下：

124/**125 * struct i2c_driver - represent an I2C device driver126 * @class: What kind of i2c device we instantiate (for detect)127 * @attach_adapter: Callback for bus addition (deprecated)128 * @probe: Callback for device binding129 * @remove: Callback for device unbinding130 * @shutdown: Callback for device shutdown131 * @suspend: Callback for device suspend132 * @resume: Callback for device resume133 * @alert: Alert callback, for example for the SMBus alert protocol134 * @command: Callback for bus-wide signaling (optional)135 * @driver: Device driver model driver136 * @id_table: List of I2C devices supported by this driver137 * @detect: Callback for device detection138 * @address_list: The I2C addresses to probe (for detect)139 * @clients: List of detected clients we created (for i2c-core use only)140 *141 * The driver.owner field should be set to the module owner of this driver.142 * The driver.name field should be set to the name of this driver.143 *144 * For automatic device detection, both @detect and @address_list must145 * be defined. @class should also be set, otherwise only devices forced146 * with module parameters will be created. The detect function must147 * fill at least the name field of the i2c_board_info structure it is148 * handed upon successful detection, and possibly also the flags field.149 *150 * If @detect is missing, the driver will still work fine for enumerated151 * devices. Detected devices simply won't be supported. This is expected152 * for the many I2C/SMBus devices which can't be detected reliably, and153 * the ones which can always be enumerated in practice.154 *155 * The i2c_client structure which is handed to the @detect callback is156 * not a real i2c_client. It is initialized just enough so that you can157 * call i2c_smbus_read_byte_data and friends on it. Don't do anything158 * else with it. In particular, calling dev_dbg and friends on it is159 * not allowed.160 */161struct i2c_driver {162 unsigned int class;163164 /* Notifies the driver that a new bus has appeared. You should avoid165* using this, it will be removed in a near future.166*/167 int (*attach_adapter)(struct i2c_adapter *) __deprecated;168169 /* Standard driver model interfaces */170 int (*probe)(struct i2c_client *, const struct i2c_device_id *);171 int (*remove)(struct i2c_client *);172173 /* driver model interfaces that don't relate to enumeration */174 void (*shutdown)(struct i2c_client *);175 int (*suspend)(struct i2c_client *, pm_message_t mesg);176 int (*resume)(struct i2c_client *);177178 /* Alert callback, for example for the SMBus alert protocol.179* The format and meaning of the data value depends on the protocol.180* For the SMBus alert protocol, there is a single bit of data passed181* as the alert response's low bit ("event flag").182*/183 void (*alert)(struct i2c_client *, unsigned int data);184185 /* a ioctl like command that can be used to perform specific functions186* with the device.187*/188 int (*command)(struct i2c_client *client, unsigned int cmd, void *arg);189190 struct device_driver driver;191 const struct i2c_device_id *id_table;192193 /* Device detection callback for automatic device creation */194 int (*detect)(struct i2c_client *, struct i2c_board_info *);195 const unsigned short *address_list;196 struct list_head clients;197};

二、i2c_adapter的注册

i2c_add_adapter和i2c_add_numbered_adapter函数都可以注册I2C adapter，这两个函数的区别是，i2c_add_adapter动态分配一个bus number，而i2c_add_numbered_adapter使用i2c_adapter.nr指定的bus number。

下面是i2c_add_adapter函数的定义，它定义在drivers/i2c/i2c-core.c文件中，其内容如下：

1099/**1100 * i2c_add_adapter - declare i2c adapter, use dynamic bus number1101 * @adapter: the adapter to add1102 * Context: can sleep1103 *1104 * This routine is used to declare an I2C adapter when its bus number1105 * doesn't matter or when its bus number is specified by an dt alias.1106 * Examples of bases when the bus number doesn't matter: I2C adapters1107 * dynamically added by USB links or PCI plugin cards.1108 *1109 * When this returns zero, a new bus number was allocated and stored1110 * in adap->nr, and the specified adapter became available for clients.1111 * Otherwise, a negative errno value is returned.1112 */1113int i2c_add_adapter(struct i2c_adapter *adapter)1114{1115 struct device *dev = &adapter->dev;1116 int id;11171118 if (dev->of_node) {1119 id = of_alias_get_id(dev->of_node, "i2c");1120 if (id >= 0) {1121 adapter->nr = id;1122 return __i2c_add_numbered_adapter(adapter);1123 }1124 }11251126 mutex_lock(&core_lock);1127 id = idr_alloc(&i2c_adapter_idr, adapter,1128__i2c_first_dynamic_bus_num, 0, GFP_KERNEL);1129 mutex_unlock(&core_lock);1130 if (id < 0)1131 return id;11321133 adapter->nr = id;11341135 return i2c_register_adapter(adapter);1136}

1118-1124行，跳过。

1127-1128行，调用idr_alloc动态分配一个idr entry，并用返回的id作为bus number。

1135行，调用i2c_register_adapter函数。这个函数我们后面再分析。

下面是i2c_add_numbered_adapter函数的定义，它同样定义在drivers/i2c/i2c-core.c文件中，其内容如下：

1139/**1140 * i2c_add_numbered_adapter - declare i2c adapter, use static bus number1141 * @adap: the adapter to register (with adap->nr initialized)1142 * Context: can sleep1143 *1144 * This routine is used to declare an I2C adapter when its bus number1145 * matters. For example, use it for I2C adapters from system-on-chip CPUs,1146 * or otherwise built in to the system's mainboard, and where i2c_board_info1147 * is used to properly configure I2C devices.1148 *1149 * If the requested bus number is set to -1, then this function will behave1150 * identically to i2c_add_adapter, and will dynamically assign a bus number.1151 *1152 * If no devices have pre-been declared for this bus, then be sure to1153 * register the adapter before any dynamically allocated ones. Otherwise1154 * the required bus ID may not be available.1155 *1156 * When this returns zero, the specified adapter became available for1157 * clients using the bus number provided in adap->nr. Also, the table1158 * of I2C devices pre-declared using i2c_register_board_info() is scanned,1159 * and the appropriate driver model device nodes are created. Otherwise, a1160 * negative errno value is returned.1161 */1162int i2c_add_numbered_adapter(struct i2c_adapter *adap)1163{1164 if (adap->nr == -1) /* -1 means dynamically assign bus id */1165 return i2c_add_adapter(adap);11661167 return __i2c_add_numbered_adapter(adap);1168}

可以看到如果adap->nr的值为-1，则调用i2c_add_adapter动态分配一个bus number并注册。否则，调用__i2c_add_numbered_adapter函数，该函数定义在drivers/i2c/i2c-core.c文件中，其内容如下：

1078/**1079 * __i2c_add_numbered_adapter - i2c_add_numbered_adapter where nr is never -11080 * @adap: the adapter to register (with adap->nr initialized)1081 * Context: can sleep1082 *1083 * See i2c_add_numbered_adapter() for details.1084 */1085static int __i2c_add_numbered_adapter(struct i2c_adapter *adap)1086{1087 int id;10881089 mutex_lock(&core_lock);1090 id = idr_alloc(&i2c_adapter_idr, adap, adap->nr, adap->nr + 1,1091GFP_KERNEL);1092 mutex_unlock(&core_lock);1093 if (id < 0)1094 return id == -ENOSPC ? -EBUSY : id;10951096 return i2c_register_adapter(adap);1097}

1090行，调用idr_alloc函数用指定的daap->nr作为bus number分配一个idr entry。

1096行，调用i2c_register_adapter函数。

所以，不论是用i2c_add_adapter还是用i2c_add_numbered_adapter函数注册i2c_adapter，最终都会调用i2c_register_adapter函数完成注册。

i2c_register_adapter函数定义在drivers/i2c/i2c-core.c文件中，其内容如下：

980static int i2c_register_adapter(struct i2c_adapter *adap)981{982 int res = 0;983984 /* Can't register until after driver model init */985 if (unlikely(WARN_ON(!i2c_bus_type.p))) {986 res = -EAGAIN;987 goto out_list;988 }989990 /* Sanity checks */991 if (unlikely(adap->name[0] == '\0')) {992 pr_err("i2c-core: Attempt to register an adapter with "993"no name!\n");994 return -EINVAL;995 }996 if (unlikely(!adap->algo)) {997 pr_err("i2c-core: Attempt to register adapter '%s' with "998"no algo!\n", adap->name);999 return -EINVAL;1000 }10011002 rt_mutex_init(&adap->bus_lock);1003 mutex_init(&adap->userspace_clients_lock);1004 INIT_LIST_HEAD(&adap->userspace_clients);10051006 /* Set default timeout to 1 second if not already set */1007 if (adap->timeout == 0)1008 adap->timeout = HZ;10091010 dev_set_name(&adap->dev, "i2c-%d", adap->nr);1011 adap->dev.bus = &i2c_bus_type;1012 adap->dev.type = &i2c_adapter_type;1013 res = device_register(&adap->dev);1014 if (res)1015 goto out_list;10161017 dev_dbg(&adap->dev, "adapter [%s] registered\n", adap->name);10181019#ifdef CONFIG_I2C_COMPAT1020 res = class_compat_create_link(i2c_adapter_compat_class, &adap->dev,1021 adap->dev.parent);1022 if (res)1023 dev_warn(&adap->dev,1024 "Failed to create compatibility class link\n");1025#endif10261027 /* bus recovery specific initialization */1028 if (adap->bus_recovery_info) {1029 struct i2c_bus_recovery_info *bri = adap->bus_recovery_info;10301031 if (!bri->recover_bus) {1032 dev_err(&adap->dev, "No recover_bus() found, not using recovery\n");1033 adap->bus_recovery_info = NULL;1034 goto exit_recovery;1035 }10361037 /* Generic GPIO recovery */1038 if (bri->recover_bus == i2c_generic_gpio_recovery) {1039 if (!gpio_is_valid(bri->scl_gpio)) {1040dev_err(&adap->dev, "Invalid SCL gpio, not using recovery\n");1041adap->bus_recovery_info = NULL;1042goto exit_recovery;1043 }10441045 if (gpio_is_valid(bri->sda_gpio))1046bri->get_sda = get_sda_gpio_value;1047 else1048bri->get_sda = NULL;10491050 bri->get_scl = get_scl_gpio_value;1051 bri->set_scl = set_scl_gpio_value;1052 } else if (!bri->set_scl || !bri->get_scl) {1053 /* Generic SCL recovery */1054 dev_err(&adap->dev, "No {get|set}_gpio() found, not using recovery\n");1055 adap->bus_recovery_info = NULL;1056 }1057 }10581059exit_recovery:1060 /* create pre-declared device nodes */1061 if (adap->nr < __i2c_first_dynamic_bus_num)1062 i2c_scan_static_board_info(adap);10631064 /* Notify drivers */1065 mutex_lock(&core_lock);1066 bus_for_each_drv(&i2c_bus_type, NULL, adap, __process_new_adapter);1067 mutex_unlock(&core_lock);10681069 return 0;10701071out_list:1072 mutex_lock(&core_lock);1073 idr_remove(&i2c_adapter_idr, adap->nr);1074 mutex_unlock(&core_lock);1075 return res;1076}

984-1013行，完成基本的检查和初始化工作后，注册i2c_adapter.dev。

1027-1059行，如果有必要，为bus recovery作一些准备工作。

1060-1062行，如果有预先声明的i2c设备，则注册对应的i2c_client。

先看1061行，判断adap->nr是否小于__i2c_first_dynamic_bus_num，只有是使用i2c_add_numbered_adapter注册i2c_adapter时，该判断才会成立。用i2c_add_adapter函数注册i2c_adapter时，动态分配的bus number一定是大于或等于__i2c_first_dynamic_bus_num的。

如果1061行的判断成立，说明是使用预先定义的bus number，因此，就可能会有预置的I2C设备信息。所以1062行，调用i2c_scan_static_board_info函数，遍历预置I2C设备信息列表，创建对应的i2c_client。

i2c_scan_static_board_info函数定义在drivers/i2c/i2c-core.c文件中，其内容如下：

941static void i2c_scan_static_board_info(struct i2c_adapter *adapter)942{943 struct i2c_devinfo *devinfo;944945 down_read(&__i2c_board_lock);946 list_for_each_entry(devinfo, &__i2c_board_list, list) {947 if (devinfo->busnum == adapter->nr948&& !i2c_new_device(adapter,949 &devinfo->board_info))950 dev_err(&adapter->dev,951"Can't create device at 0x%02x\n",952devinfo->board_info.addr);953 }954 up_read(&__i2c_board_lock);955}

该函数遍历__i2c_board_list链表，如果某个节点的devinfo->busnum等于adapter->nr，即该I2C设备属于这个注册的I2C adapter，则调用i2c_new_device创建并注册该I2C设备对应的i2c_client。

i2c_new_device定义在drivers/i2c/i2c-core.c文件中，其内容如下：

612/**613 * i2c_new_device - instantiate an i2c device614 * @adap: the adapter managing the device615 * @info: describes one I2C device; bus_num is ignored616 * Context: can sleep617 *618 * Create an i2c device. Binding is handled through driver model619 * probe()/remove() methods. A driver may be bound to this device when we620 * return from this function, or any later moment (e.g. maybe hotplugging will621 * load the driver module). This call is not appropriate for use by mainboard622 * initialization logic, which usually runs during an arch_initcall() long623 * before any i2c_adapter could exist.624 *625 * This returns the new i2c client, which may be saved for later use with626 * i2c_unregister_device(); or NULL to indicate an error.627 */628struct i2c_client *629i2c_new_device(struct i2c_adapter *adap, struct i2c_board_info const *info)630{631 struct i2c_client *client;632 int status;633634 client = kzalloc(sizeof *client, GFP_KERNEL);635 if (!client)636 return NULL;637638 client->adapter = adap;639640 client->dev.platform_data = info->platform_data;641642 if (info->archdata)643 client->dev.archdata = *info->archdata;644645 client->flags = info->flags;646 client->addr = info->addr;647 client->irq = info->irq;648649 strlcpy(client->name, info->type, sizeof(client->name));650651 /* Check for address validity */652 status = i2c_check_client_addr_validity(client);653 if (status) {654 dev_err(&adap->dev, "Invalid %d-bit I2C address 0x%02hx\n",655 client->flags & I2C_CLIENT_TEN ? 10 : 7, client->addr);656 goto out_err_silent;657 }658659 /* Check for address business */660 status = i2c_check_addr_busy(adap, client->addr);661 if (status)662 goto out_err;663664 client->dev.parent = &client->adapter->dev;665 client->dev.bus = &i2c_bus_type;666 client->dev.type = &i2c_client_type;667 client->dev.of_node = info->of_node;668 ACPI_HANDLE_SET(&client->dev, info->acpi_node.handle);669670 /* For 10-bit clients, add an arbitrary offset to avoid collisions */671 dev_set_name(&client->dev, "%d-%04x", i2c_adapter_id(adap),672 client->addr | ((client->flags & I2C_CLIENT_TEN)673 ? 0xa000 : 0));674 status = device_register(&client->dev);675 if (status)676 goto out_err;677678 dev_dbg(&adap->dev, "client [%s] registered with bus id %s\n",679 client->name, dev_name(&client->dev));680681 return client;682683out_err:684 dev_err(&adap->dev, "Failed to register i2c client %s at 0x%02x "685 "(%d)\n", client->name, client->addr, status);686out_err_silent:687 kfree(client);688 return NULL;689}

这个函数很直接，就是初始化i2c_client的各个成员，并最终注册client->dev，从而完成对i2c设备的注册。

这里要说明一下，预置的i2c设备信息是通过调用i2c_register_board_info函数完成的，该函数定义在drivers/i2c/i2c-boardinfo.c文件中，其内容如下：

42/**43 * i2c_register_board_info - statically declare I2C devices44 * @busnum: identifies the bus to which these devices belong45 * @info: vector of i2c device descriptors46 * @len: how many descriptors in the vector; may be zero to reserve47 * the specified bus number.48 *49 * Systems using the Linux I2C driver stack can declare tables of board info50 * while they initialize. This should be done in board-specific init code51 * near arch_initcall() time, or equivalent, before any I2C adapter driver is52 * registered. For example, mainboard init code could define several devices,53 * as could the init code for each daughtercard in a board stack.54 *55 * The I2C devices will be created later, after the adapter for the relevant56 * bus has been registered. After that moment, standard driver model tools57 * are used to bind "new style" I2C drivers to the devices. The bus number58 * for any device declared using this routine is not available for dynamic59 * allocation.60 *61 * The board info passed can safely be __initdata, but be careful of embedded62 * pointers (for platform_data, functions, etc) since that won't be copied.63 */64int __init65i2c_register_board_info(int busnum,66 struct i2c_board_info const *info, unsigned len)67{68 int status;6970 down_write(&__i2c_board_lock);7172 /* dynamic bus numbers will be assigned after the last static one */73 if (busnum >= __i2c_first_dynamic_bus_num)74 __i2c_first_dynamic_bus_num = busnum + 1;7576 for (status = 0; len; len--, info++) {77 struct i2c_devinfo *devinfo;7879 devinfo = kzalloc(sizeof(*devinfo), GFP_KERNEL);80 if (!devinfo) {81 pr_debug("i2c-core: can't register boardinfo!\n");82 status = -ENOMEM;83 break;84 }8586 devinfo->busnum = busnum;87 devinfo->board_info = *info;88 list_add_tail(&devinfo->list, &__i2c_board_list);89 }9091 up_write(&__i2c_board_lock);9293 return status;94}

可以看到，该函数将预置的i2c设备信息保存在__i2c_board_list链表中。

回到i2c_register_adapter函数：

1066行，调用bus_for_each_drv函数，该函数定义在drivers/base/bus.c文件中，其内容如下：

417/**418 * bus_for_each_drv - driver iterator419 * @bus: bus we're dealing with.420 * @start: driver to start iterating on.421 * @data: data to pass to the callback.422 * @fn: function to call for each driver.423 *424 * This is nearly identical to the device iterator above.425 * We iterate over each driver that belongs to @bus, and call426 * @fn for each. If @fn returns anything but 0, we break out427 * and return it. If @start is not NULL, we use it as the head428 * of the list.429 *430 * NOTE: we don't return the driver that returns a non-zero431 * value, nor do we leave the reference count incremented for that432 * driver. If the caller needs to know that info, it must set it433 * in the callback. It must also be sure to increment the refcount434 * so it doesn't disappear before returning to the caller.435 */436int bus_for_each_drv(struct bus_type *bus, struct device_driver *start,437 void *data, int (*fn)(struct device_driver *, void *))438{439 struct klist_iter i;440 struct device_driver *drv;441 int error = 0;442443 if (!bus)444 return -EINVAL;445446 klist_iter_init_node(&bus->p->klist_drivers, &i,447 start ? &start->p->knode_bus : NULL);448 while ((drv = next_driver(&i)) && !error)449 error = fn(drv, data);450 klist_iter_exit(&i);451 return error;452}

这个函数我们在《Linux设备模型分析之device（基于3.10.1内核）》一文中已经分析过了。448-449行，这个while循环依次遍历bus->p->klist_drivers中的所有device_driver，对于每个device_driver，调用fn(drv，data)函数。这里，传递过来的fn参数是__process_new_adapter，data参数是adap。这个循环是一个关键点，注册一个新的i2c_adapter后，要为该i2c_adapter上的i2c设备匹配驱动程序，这个匹配过程就是通过这个循环调用__process_new_adapter函数完成的。

__process_new_adapter函数定义在drivers/i2c/i2c-core.c文件中，其内容如下：

975static int __process_new_adapter(struct device_driver *d, void *data)976{977 return i2c_do_add_adapter(to_i2c_driver(d), data);978}

注意i2c_do_add_adapter函数的第一个参数，从device_driver转换为i2c_driver。

i2c_do_add_adapter函数定义在drivers/i2c/i2c-core.c文件中，其内容如下：

957static int i2c_do_add_adapter(struct i2c_driver *driver,958 struct i2c_adapter *adap)959{960 /* Detect supported devices on that bus, and instantiate them */961 i2c_detect(adap, driver);962963 /* Let legacy drivers scan this bus for matching devices */964 if (driver->attach_adapter) {965 dev_warn(&adap->dev, "%s: attach_adapter method is deprecated\n",966 driver->driver.name);967 dev_warn(&adap->dev, "Please use another way to instantiate "968 "your i2c_client\n");969 /* We ignore the return code; if it fails, too bad */970 driver->attach_adapter(adap);971 }972 return 0;973}

961行，调用i2c_detect函数，探测并初始化该i2c总线上的i2c设备。该函数定义在drivers/i2c/i2c-core.c文件中，其内容如下：

1730static int i2c_detect(struct i2c_adapter *adapter, struct i2c_driver *driver)1731{1732 const unsigned short *address_list;1733 struct i2c_client *temp_client;1734 int i, err = 0;1735 int adap_id = i2c_adapter_id(adapter);17361737 address_list = driver->address_list;1738 if (!driver->detect || !address_list)1739 return 0;17401741 /* Stop here if the classes do not match */1742 if (!(adapter->class & driver->class))1743 return 0;17441745 /* Set up a temporary client to help detect callback */1746 temp_client = kzalloc(sizeof(struct i2c_client), GFP_KERNEL);1747 if (!temp_client)1748 return -ENOMEM;1749 temp_client->adapter = adapter;17501751 for (i = 0; address_list[i] != I2C_CLIENT_END; i += 1) {1752 dev_dbg(&adapter->dev, "found normal entry for adapter %d, "1753 "addr 0x%02x\n", adap_id, address_list[i]);1754 temp_client->addr = address_list[i];1755 err = i2c_detect_address(temp_client, driver);1756 if (unlikely(err))1757 break;1758 }17591760 kfree(temp_client);1761 return err;1762}

1755行，调用i2c_detect_address，探测指定的地址上的I2C设备是否存在，如果存在，注册该i2c设备。i2c_detect_address函数定义在drivers/i2c/i2c-core.c文件中，其内容如下：

1675static int i2c_detect_address(struct i2c_client *temp_client,1676 struct i2c_driver *driver)1677{1678 struct i2c_board_info info;1679 struct i2c_adapter *adapter = temp_client->adapter;1680 int addr = temp_client->addr;1681 int err;16821683 /* Make sure the address is valid */1684 err = i2c_check_addr_validity(addr);1685 if (err) {1686 dev_warn(&adapter->dev, "Invalid probe address 0x%02x\n",1687 addr);1688 return err;1689 }16901691 /* Skip if already in use */1692 if (i2c_check_addr_busy(adapter, addr))1693 return 0;16941695 /* Make sure there is something at this address */1696 if (!i2c_default_probe(adapter, addr))1697 return 0;16981699 /* Finally call the custom detection function */1700 memset(&info, 0, sizeof(struct i2c_board_info));1701 info.addr = addr;1702 err = driver->detect(temp_client, &info);1703 if (err) {1704 /* -ENODEV is returned if the detection fails. We catch it1705 here as this isn't an error. */1706 return err == -ENODEV ? 0 : err;1707 }17081709 /* Consistency check */1710 if (info.type[0] == '\0') {1711 dev_err(&adapter->dev, "%s detection function provided "1712 "no name for 0x%x\n", driver->driver.name,1713 addr);1714 } else {1715 struct i2c_client *client;17161717 /* Detection succeeded, instantiate the device */1718 dev_dbg(&adapter->dev, "Creating %s at 0x%02x\n",1719 info.type, info.addr);1720 client = i2c_new_device(adapter, &info);1721 if (client)1722 list_add_tail(&client->detected, &driver->clients);1723 else1724 dev_err(&adapter->dev, "Failed creating %s at 0x%02x\n",1725info.type, info.addr);1726 }1727 return 0;1728}

1702行，调用driver->detect。

1720行，如果探测到i2c设备确实存在，调用i2c_new_device函数初始化对应的i2c_client结构体并注册。i2c_new_device函数我们在前面已经分析过。

至此，i2c_adapter的注册过程我们就清楚了。

三、i2c_driver的注册

i2c_driver的注册是通过调用i2c_add_driver宏完成的，该宏定义在include/linux/i2c.h文件中：

497/* use a define to avoid include chaining to get THIS_MODULE */498#define i2c_add_driver(driver) \499 i2c_register_driver(THIS_MODULE, driver)

i2c_register_driver函数定义在drivers/i2c/i2c-core.c文件中，其内容如下：

1307/*1308 * An i2c_driver is used with one or more i2c_client (device) nodes to access1309 * i2c slave chips, on a bus instance associated with some i2c_adapter.1310 */13111312int i2c_register_driver(struct module *owner, struct i2c_driver *driver)1313{1314 int res;13151316 /* Can't register until after driver model init */1317 if (unlikely(WARN_ON(!i2c_bus_type.p)))1318 return -EAGAIN;13191320 /* add the driver to the list of i2c drivers in the driver core */1321 driver->driver.owner = owner;1322 driver->driver.bus = &i2c_bus_type;13231324 /* When registration returns, the driver core1325* will have called probe() for all matching-but-unbound devices.1326*/1327 res = driver_register(&driver->driver);1328 if (res)1329 return res;13301331 /* Drivers should switch to dev_pm_ops instead. */1332 if (driver->suspend)1333 pr_warn("i2c-core: driver [%s] using legacy suspend method\n",1334 driver->driver.name);1335 if (driver->resume)1336 pr_warn("i2c-core: driver [%s] using legacy resume method\n",1337 driver->driver.name);13381339 pr_debug("i2c-core: driver [%s] registered\n", driver->driver.name);13401341 INIT_LIST_HEAD(&driver->clients);1342 /* Walk the adapters that are already present */1343 i2c_for_each_dev(driver, __process_new_driver);13441345 return 0;1346}

1327行，调用driver_register注册i2c_driver.driver。参考《Linux设备模型分析之device_driver（基于3.10.1内核）》对Linux设备模型的分析，在driver_register执行过程中，如果I2C总线上找到了与该驱动匹配的I2C设备，则i2c_driver.probe函数会被调用执行。

1343行，调用i2c_for_each_dev遍历所有已存在的i2c_adapter。该函数定义在drivers/i2c/i2c-core.c文件中，其内容如下：

1288int i2c_for_each_dev(void *data, int (*fn)(struct device *, void *))1289{1290 int res;12911292 mutex_lock(&core_lock);1293 res = bus_for_each_dev(&i2c_bus_type, NULL, data, fn);1294 mutex_unlock(&core_lock);12951296 return res;1297}

1293行，调用bus_for_each_dev，这个函数定义在drivers/base/bus.c文件中，其内容如下：

286intbus_for_each_dev(struct bus_type *bus, struct device *start,287 void *data, int (*fn)(struct device *, void *))288{289 struct klist_iter i;290 struct device *dev;291 int error = 0;292293 if (!bus || !bus->p)294 return -EINVAL;295296 klist_iter_init_node(&bus->p->klist_devices, &i,297 (start ?&start->p->knode_bus : NULL));298 while ((dev = next_device(&i)) && !error)299 error = fn(dev, data);300 klist_iter_exit(&i);301 return error;302}

我们在《Linux设备模型分析之device_driver（基于3.10.1内核）》一文中已经分析过这个函数。这里，传递过来的data参数是要注册的i2c_driver，fn参数是__process_new_driver函数，所以我们来看__process_new_driver函数，该函数定义在drivers/i2c/i2c-core.c文件中，其内容如下：

1300static int __process_new_driver(structdevice *dev, void *data)1301{1302 if (dev->type != &i2c_adapter_type)1303 return 0;1304 return i2c_do_add_adapter(data, to_i2c_adapter(dev));1305}

i2c_do_add_adapter函数定义在drivers/i2c/i2c-core.c文件中，其内容如下：

957static int i2c_do_add_adapter(structi2c_driver *driver,958 struct i2c_adapter *adap)959{960 /* Detect supported devices on that bus, and instantiate them */961 i2c_detect(adap, driver);962963 /* Let legacy drivers scan this bus for matching devices */964 if (driver->attach_adapter) {965 dev_warn(&adap->dev, "%s: attach_adapter method isdeprecated\n",966 driver->driver.name);967 dev_warn(&adap->dev, "Please use another way to instantiate"968 "your i2c_client\n");969 /* We ignore the return code; if it fails, too bad */970 driver->attach_adapter(adap);971 }972 return 0;973}

这个函数我们在分析i2c_adapter的注册过程时已经分析过了，它主要完成i2c_driver与i2c_adapter上的i2c设备的匹配工作，如果匹配成功，初始化并注册对应的i2c_client。

至此，i2c_driver的注册过程我们就清楚了。

四、i2c_bus_type分析

i2c_init 函数完成Linux i2c框架的初始化工作，该函数定义在drivers/i2c/i2c-core.c文件中，其内容如下：

1429static int __init i2c_init(void)1430{1431 int retval;14321433 retval = bus_register(&i2c_bus_type);1434 if (retval)1435 return retval;1436#ifdef CONFIG_I2C_COMPAT1437 i2c_adapter_compat_class =class_compat_register("i2c-adapter");1438 if (!i2c_adapter_compat_class) {1439 retval = -ENOMEM;1440 goto bus_err;1441 }1442#endif1443 retval = i2c_add_driver(&dummy_driver);1444 if (retval)1445 goto class_err;1446 return 0;14471448class_err:1449#ifdef CONFIG_I2C_COMPAT1450 class_compat_unregister(i2c_adapter_compat_class);1451bus_err:1452#endif1453 bus_unregister(&i2c_bus_type);1454 return retval;1455}

1433行，调用bus_register注册了i2c_bus_type。i2c_bus_tpye定义在drivers/i2c/i2c-core.c文件中，其内容如下：

442structbus_type i2c_bus_type = {443 .name = "i2c",444 .match= i2c_device_match,445 .probe= i2c_device_probe,446 .remove= i2c_device_remove,447 .shutdown = i2c_device_shutdown,448 .pm= &i2c_device_pm_ops,449};

其类型是bus_type，所以它代表i2c总线。我们来关注一下i2c_device_match和i2c_device_probe函数。

i2c_device_match函数定义在drivers/i2c/i2c-core.c文件中，其内容如下：

72static int i2c_device_match(struct device *dev, struct device_driver*drv)73{74 struct i2c_client *client = i2c_verify_client(dev);75 struct i2c_driver *driver;7677 if (!client)78 return 0;7980 /* Attempt an OF style match*/81 if(of_driver_match_device(dev, drv))82 return 1;8384 /* Then ACPI style match */85 if(acpi_driver_match_device(dev, drv))86 return 1;8788 driver = to_i2c_driver(drv);89 /* match on an id table ifthere is one */90 if (driver->id_table)91 returni2c_match_id(driver->id_table, client) != NULL;9293 return 0;94}

88行，将device_driver转换为i2c_driver。

90-91行，如果driver->id_table不为空，则调用i2c_match_id函数，该函数定义在drivers/i2c/i2c-core.c文件中，其内容如下：

61static const struct i2c_device_id *i2c_match_id(const structi2c_device_id *id,62 const struct i2c_client *client)63{64 while (id->name[0]) {65 if(strcmp(client->name, id->name) == 0)66 return id;67 id++;68 }69 return NULL;70}

可以看到，如果client->name和id->name相同，则匹配成功，返回id。如果返回NULL，则表示匹配失败。

分析到这里，我们要回顾一下分析Linux设备模型时涉及到的一个函数driver_match_device，该函数定义在drivers/base/base.h文件中，其内容如下：

116static inline intdriver_match_device(struct device_driver *drv,117 struct device *dev)118{119 return drv->bus->match ? drv->bus->match(dev, drv) : 1;120}

当进行device和device_driver的匹配时，会调用这个函数，该函数返回值为0，表示match失败。只有match成功时，才会进一步进行probe。

可以看到，如果drv->bus->match存在，会调用drv->bus->match(dev,drv)。所以，当i2c设备(i2c_client)和i2c驱动(i2c_driver)进行匹配操作时，就会调用i2c_device_match函数。

在分析i2c_device_probe函数之前，我们要回顾一下分析Linux设备模型时涉及到的一个函数driver_probe_device，device_driver探测支持的device时，会调用到这个函数，它又会进一步调用really_probe函数，really_probe函数定义在drivers/base/dd.c文件中，其内容如下：

265static int really_probe(struct device*dev, struct device_driver *drv)266{267 int ret = 0;268269 atomic_inc(&probe_count);270 pr_debug("bus: '%s': %s: probing driver %s with device %s\n",271 drv->bus->name, __func__, drv->name, dev_name(dev));272 WARN_ON(!list_empty(&dev->devres_head));273274 dev->driver = drv;275276 /* If using pinctrl, bind pins now before probing */277 ret = pinctrl_bind_pins(dev);278 if (ret)279 goto probe_failed;280281 if (driver_sysfs_add(dev)) {282 printk(KERN_ERR "%s: driver_sysfs_add(%s) failed\n",283 __func__, dev_name(dev));284 goto probe_failed;285 }286287 if (dev->bus->probe) {288 ret = dev->bus->probe(dev);289 if (ret)290 goto probe_failed;291 } else if (drv->probe) {292 ret = drv->probe(dev);293 if (ret)294 goto probe_failed;295 }296297 driver_bound(dev);298 ret = 1;299 pr_debug("bus: '%s': %s: bound device %s to driver %s\n",300 drv->bus->name, __func__, dev_name(dev), drv->name);301 goto done;302303probe_failed:304 devres_release_all(dev);305 driver_sysfs_remove(dev);306 dev->driver = NULL;307 dev_set_drvdata(dev, NULL);308309 if (ret == -EPROBE_DEFER) {310 /* Driver requested deferred probing */311 dev_info(dev, "Driver %s requests probe deferral\n",drv->name);312 driver_deferred_probe_add(dev);313 } else if (ret != -ENODEV && ret != -ENXIO) {314 /* driver matched but the probe failed */315 printk(KERN_WARNING316"%s: probe of %s failedwith error %d\n",317drv->name, dev_name(dev),ret);318 } else {319 pr_debug("%s: probe of %s rejects match %d\n",320drv->name, dev_name(dev),ret);321 }322 /*323 * Ignore errors returned by ->probe so that the next driver can try324 * its luck.325 */326 ret = 0;327done:328 atomic_dec(&probe_count);329 wake_up(&probe_waitqueue);330 return ret;331}

这个函数我们现在需要关注的是287-295行，如果dev->bus->probe存在，则调用dev->bus->probe(dev)，如果dev->bus->probe不存在，并且drv->probe存在，才会调用drv->probe(dev)。所以，对于i2c设备和i2c驱动，探测设备时会优先调用i2c_bus_type.probe函数。而i2c_bus_type.probe即i2c_device_probe会转而调用i2c_driver.probe函数。

现在我们可以来看i2c_device_probe函数了，它定义在drivers/i2c/i2c-core.c文件中，其内容如下：

233staticint i2c_device_probe(struct device *dev)234{235 struct i2c_client *client =i2c_verify_client(dev);236 struct i2c_driver *driver;237 int status;238239 if (!client)240 return 0;241242 driver = to_i2c_driver(dev->driver);243 if (!driver->probe || !driver->id_table)244 return -ENODEV;245 client->driver = driver;246 if (!device_can_wakeup(&client->dev))247 device_init_wakeup(&client->dev,248client->flags &I2C_CLIENT_WAKE);249 dev_dbg(dev, "probe\n");250251 status = driver->probe(client, i2c_match_id(driver->id_table,client));252 if (status) {253 client->driver = NULL;254 i2c_set_clientdata(client, NULL);255 }256 return status;257}

242行，取得i2c_driver。

243行，如果i2c_driver没有定义probe或者i2c_driver没有定义id_table，则直接退出。所以我们在写i2c驱动时，必须定义i2c_driver.probe和i2c_driver.id_table。

251行，调用i2c_driver.probe。这时，我们的i2c驱动程序定义的probe函数就会执行。