作者：劉昊昱 

核心版本：3.10.1

I2C體系架構的硬體實體包括兩部分：

• 硬體I2C Adapter：硬體I2C Adapter表示一個硬體I2C介面卡，也就是I2C控制器。一般是SOC中的一個介面，也可以用GPIO模擬。硬體I2C Adapter主要用來在I2C介面上產生I2C時鐘訊號和資料訊號。
• 硬體I2C Device：表示一個I2C從裝置，I2C從裝置並行的掛接在I2C總線上，通過I2C介面與I2C主控制器通訊。

Linux的I2C體系架構包括6個部分：

• I2C Core，定義了一些函式和資料結構，用於支援I2C控制器驅動（I2C Adapter drvier）和I2C裝置驅動（I2C client driver）。I2C Core的存在使I2C控制器驅動和I2C裝置驅動獨立開來，具有更好的可移置性，同時簡化了驅動開發的工作量。但是，I2C Core的存在也使Linux I2C體系結構理解起來更有難度。
• I2C Adapter，代表一個I2C控制器，用struct i2c_adapter來表示。
• Algorithm，訪問I2C控制器的介面，Algorithm直接操作I2C控制器的硬體暫存器來執行資料的傳送和接收。它提供操作I2C控制器的最底層操作函式。每個I2C Adapter都有自己的Algorithm，每個I2C Adapter通過自己的Algorithm與掛接在該控制器上的I2C從裝置進行通訊。
• I2C Client，代表一個掛接在I2C總線上的I2C裝置。
• I2C裝置驅動（I2C client driver）
• I2C-dev，I2C控制器的裝置檔案，通常命名為i2c-0、i2c-1等等,是I2C控制器的應用層訪問介面。

一、I2C相關資料結構

I2C控制器（例如S3C2440 I2C控制器）對應的資料結構是i2c_adapter，i2c_adapter結構定義在include/linux/i2c.h檔案中，從註釋可以看出，i2c_adapter代表一條物理的I2C匯流排，同時還包括訪問該I2C匯流排的方法。其內容如下：

409/*
410 * i2c_adapter is the structure used to identify a physical i2c bus along
411 * 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;
418
419    /* data fields that are valid for all devices   */
420    struct rt_mutex bus_lock;
421
422    int timeout;            /* in jiffies */
423    int retries;
424    struct device dev;      /* the adapter device */
425
426    int nr;
427    char name[48];
428    struct completion dev_released;
429
430    struct mutex userspace_clients_lock;
431    struct list_head userspace_clients;
432
433    struct i2c_bus_recovery_info *bus_recovery_info;
434};
 

i2c_algorithm定義在include/linux/i2c.h檔案中，其內容如下：

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 can
350 * be addressed using the same bus algorithms - i.e. bit-banging or the PCF8584
351 * to name two of the most common.
352 */
353struct i2c_algorithm {
354    /* If an adapter algorithm can't do I2C-level access, set master_xfer
355       to NULL. If an adapter algorithm can do SMBus access, set
356       smbus_xfer. If set to NULL, the SMBus protocol is simulated
357       using common I2C messages */
358    /* master_xfer should return the number of messages successfully
359       processed, or a negative value on error */
360    int (*master_xfer)(struct i2c_adapter *adap, struct i2c_msg *msgs,
361               int num);
362    int (*smbus_xfer) (struct i2c_adapter *adap, u16 addr,
363               unsigned short flags, char read_write,
364               u8 command, int size, union i2c_smbus_data *data);
365
366    /* To determine what the adapter supports */
367    u32 (*functionality) (struct i2c_adapter *);
368};

struct i2c_client代表一個I2C裝置，該結構體定義在include/linux/i2c.h檔案中，其內容如下：

200/**
201 * struct i2c_client - represent an I2C slave device
202 * @flags: I2C_CLIENT_TEN indicates the device uses a ten bit chip address;
203 *  I2C_CLIENT_PEC indicates it uses SMBus Packet Error Checking
204 * @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's
206 *  generic enough to hide second-sourcing and compatible revisions.
207 * @adapter: manages the bus segment hosting this I2C device
208 * @driver: device's driver, hence pointer to access routines
209 * @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's
212 *  userspace_devices list
213 *
214 * An i2c_client identifies a single device (i.e. chip) connected to an
215 * i2c bus. The behaviour exposed to Linux is defined by the driver
216 * 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代表一個I2C裝置驅動程式，該結構體定義在include/linux/i2c.h檔案中，其內容如下：

124/**
125 * struct i2c_driver - represent an I2C device driver
126 * @class: What kind of i2c device we instantiate (for detect)
127 * @attach_adapter: Callback for bus addition (deprecated)
128 * @probe: Callback for device binding
129 * @remove: Callback for device unbinding
130 * @shutdown: Callback for device shutdown
131 * @suspend: Callback for device suspend
132 * @resume: Callback for device resume
133 * @alert: Alert callback, for example for the SMBus alert protocol
134 * @command: Callback for bus-wide signaling (optional)
135 * @driver: Device driver model driver
136 * @id_table: List of I2C devices supported by this driver
137 * @detect: Callback for device detection
138 * @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 must
145 * be defined. @class should also be set, otherwise only devices forced
146 * with module parameters will be created. The detect function must
147 * fill at least the name field of the i2c_board_info structure it is
148 * handed upon successful detection, and possibly also the flags field.
149 *
150 * If @detect is missing, the driver will still work fine for enumerated
151 * devices. Detected devices simply won't be supported. This is expected
152 * for the many I2C/SMBus devices which can't be detected reliably, and
153 * the ones which can always be enumerated in practice.
154 *
155 * The i2c_client structure which is handed to the @detect callback is
156 * not a real i2c_client. It is initialized just enough so that you can
157 * call i2c_smbus_read_byte_data and friends on it. Don't do anything
158 * else with it. In particular, calling dev_dbg and friends on it is
159 * not allowed.
160 */
161struct i2c_driver {
162    unsigned int class;
163
164    /* Notifies the driver that a new bus has appeared. You should avoid
165     * using this, it will be removed in a near future.
166     */
167    int (*attach_adapter)(struct i2c_adapter *) __deprecated;
168
169    /* Standard driver model interfaces */
170    int (*probe)(struct i2c_client *, const struct i2c_device_id *);
171    int (*remove)(struct i2c_client *);
172
173    /* 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 *);
177
178    /* 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 passed
181     * as the alert response's low bit ("event flag").
182     */
183    void (*alert)(struct i2c_client *, unsigned int data);
184
185    /* a ioctl like command that can be used to perform specific functions
186     * with the device.
187     */
188    int (*command)(struct i2c_client *client, unsigned int cmd, void *arg);
189
190    struct device_driver driver;
191    const struct i2c_device_id *id_table;
192
193    /* 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的註冊

i2c_add_adapter和i2c_add_numbered_adapter函式都可以註冊I2C adapter，這兩個函式的區別是，i2c_add_adapter動態分配一個bus number，而i2c_add_numbered_adapter使用i2c_adapter.nr指定的bus number。

下面是i2c_add_adapter函式的定義，它定義在drivers/i2c/i2c-core.c檔案中，其內容如下：

1099/**
1100 * i2c_add_adapter - declare i2c adapter, use dynamic bus number
1101 * @adapter: the adapter to add
1102 * Context: can sleep
1103 *
1104 * This routine is used to declare an I2C adapter when its bus number
1105 * 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 adapters
1107 * dynamically added by USB links or PCI plugin cards.
1108 *
1109 * When this returns zero, a new bus number was allocated and stored
1110 * 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;
1117
1118    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    }
1125
1126    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;
1132
1133    adapter->nr = id;
1134
1135    return i2c_register_adapter(adapter);
1136}

1118-1124行，跳過。

1127-1128行，呼叫idr_alloc動態分配一個idr entry，並用返回的id作為bus number。

1135行，呼叫i2c_register_adapter函式。這個函式我們後面再分析。

下面是i2c_add_numbered_adapter函式的定義，它同樣定義在drivers/i2c/i2c-core.c檔案中，其內容如下：

1139/**
1140 * i2c_add_numbered_adapter - declare i2c adapter, use static bus number
1141 * @adap: the adapter to register (with adap->nr initialized)
1142 * Context: can sleep
1143 *
1144 * This routine is used to declare an I2C adapter when its bus number
1145 * 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_info
1147 * is used to properly configure I2C devices.
1148 *
1149 * If the requested bus number is set to -1, then this function will behave
1150 * 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 to
1153 * register the adapter before any dynamically allocated ones.  Otherwise
1154 * the required bus ID may not be available.
1155 *
1156 * When this returns zero, the specified adapter became available for
1157 * clients using the bus number provided in adap->nr.  Also, the table
1158 * of I2C devices pre-declared using i2c_register_board_info() is scanned,
1159 * and the appropriate driver model device nodes are created.  Otherwise, a
1160 * 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);
1166
1167    return __i2c_add_numbered_adapter(adap);
1168}

可以看到如果adap->nr的值為-1，則呼叫i2c_add_adapter動態分配一個bus number並註冊。否則，呼叫__i2c_add_numbered_adapter函式，該函式定義在drivers/i2c/i2c-core.c檔案中，其內容如下：

1078/**
1079 * __i2c_add_numbered_adapter - i2c_add_numbered_adapter where nr is never -1
1080 * @adap: the adapter to register (with adap->nr initialized)
1081 * Context: can sleep
1082 *
1083 * See i2c_add_numbered_adapter() for details.
1084 */
1085static int __i2c_add_numbered_adapter(struct i2c_adapter *adap)
1086{
1087    int id;
1088
1089    mutex_lock(&core_lock);
1090    id = idr_alloc(&i2c_adapter_idr, adap, adap->nr, adap->nr + 1,
1091               GFP_KERNEL);
1092    mutex_unlock(&core_lock);
1093    if (id < 0)
1094        return id == -ENOSPC ? -EBUSY : id;
1095
1096    return i2c_register_adapter(adap);
1097}

1090行，呼叫idr_alloc函式用指定的daap->nr作為bus number分配一個idr entry。

1096行，呼叫i2c_register_adapter函式。

所以，不論是用i2c_add_adapter還是用i2c_add_numbered_adapter函式註冊i2c_adapter，最終都會呼叫i2c_register_adapter函式完成註冊。

i2c_register_adapter函式定義在drivers/i2c/i2c-core.c檔案中，其內容如下：

 980static int i2c_register_adapter(struct i2c_adapter *adap)
 981{
 982    int res = 0;
 983
 984    /* 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    }
 989
 990    /* 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    }
1001
1002    rt_mutex_init(&adap->bus_lock);
1003    mutex_init(&adap->userspace_clients_lock);
1004    INIT_LIST_HEAD(&adap->userspace_clients);
1005
1006    /* Set default timeout to 1 second if not already set */
1007    if (adap->timeout == 0)
1008        adap->timeout = HZ;
1009
1010    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;
1016
1017    dev_dbg(&adap->dev, "adapter [%s] registered\n", adap->name);
1018
1019#ifdef CONFIG_I2C_COMPAT
1020    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#endif
1026
1027    /* bus recovery specific initialization */
1028    if (adap->bus_recovery_info) {
1029        struct i2c_bus_recovery_info *bri = adap->bus_recovery_info;
1030
1031        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        }
1036
1037        /* Generic GPIO recovery */
1038        if (bri->recover_bus == i2c_generic_gpio_recovery) {
1039            if (!gpio_is_valid(bri->scl_gpio)) {
1040                dev_err(&adap->dev, "Invalid SCL gpio, not using recovery\n");
1041                adap->bus_recovery_info = NULL;
1042                goto exit_recovery;
1043            }
1044
1045            if (gpio_is_valid(bri->sda_gpio))
1046                bri->get_sda = get_sda_gpio_value;
1047            else
1048                bri->get_sda = NULL;
1049
1050            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    }
1058
1059exit_recovery:
1060    /* create pre-declared device nodes */
1061    if (adap->nr < __i2c_first_dynamic_bus_num)
1062        i2c_scan_static_board_info(adap);
1063
1064    /* 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);
1068
1069    return 0;
1070
1071out_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，完成基本的檢查和初始化工作後，註冊i2c_adapter.dev。

1027-1059行，如果有必要，為bus recovery作一些準備工作。

1060-1062行，如果有預先宣告的i2c裝置，則註冊對應的i2c_client。

先看1061行，判斷adap->nr是否小於__i2c_first_dynamic_bus_num，只有是使用i2c_add_numbered_adapter註冊i2c_adapter時，該判斷才會成立。用i2c_add_adapter函式註冊i2c_adapter時，動態分配的bus number一定是大於或等於__i2c_first_dynamic_bus_num的。

如果1061行的判斷成立，說明是使用預先定義的bus number，因此，就可能會有預置的I2C裝置資訊。所以1062行，呼叫i2c_scan_static_board_info函式，遍歷預置I2C裝置資訊列表，建立對應的i2c_client。

i2c_scan_static_board_info函式定義在drivers/i2c/i2c-core.c檔案中，其內容如下：

 941static void i2c_scan_static_board_info(struct i2c_adapter *adapter)
 942{
 943    struct i2c_devinfo  *devinfo;
 944
 945    down_read(&__i2c_board_lock);
 946    list_for_each_entry(devinfo, &__i2c_board_list, list) {
 947        if (devinfo->busnum == adapter->nr
 948                && !i2c_new_device(adapter,
 949                        &devinfo->board_info))
 950            dev_err(&adapter->dev,
 951                "Can't create device at 0x%02x\n",
 952                devinfo->board_info.addr);
 953    }
 954    up_read(&__i2c_board_lock);
 955}

該函式遍歷__i2c_board_list連結串列，如果某個節點的devinfo->busnum等於adapter->nr，即該I2C裝置屬於這個註冊的I2C adapter，則呼叫i2c_new_device建立並註冊該I2C裝置對應的i2c_client。

i2c_new_device定義在drivers/i2c/i2c-core.c檔案中，其內容如下：

 612/**
 613 * i2c_new_device - instantiate an i2c device
 614 * @adap: the adapter managing the device
 615 * @info: describes one I2C device; bus_num is ignored
 616 * Context: can sleep
 617 *
 618 * Create an i2c device. Binding is handled through driver model
 619 * probe()/remove() methods.  A driver may be bound to this device when we
 620 * return from this function, or any later moment (e.g. maybe hotplugging will
 621 * load the driver module).  This call is not appropriate for use by mainboard
 622 * initialization logic, which usually runs during an arch_initcall() long
 623 * before any i2c_adapter could exist.
 624 *
 625 * This returns the new i2c client, which may be saved for later use with
 626 * 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;
 633
 634    client = kzalloc(sizeof *client, GFP_KERNEL);
 635    if (!client)
 636        return NULL;
 637
 638    client->adapter = adap;
 639
 640    client->dev.platform_data = info->platform_data;
 641
 642    if (info->archdata)
 643        client->dev.archdata = *info->archdata;
 644
 645    client->flags = info->flags;
 646    client->addr = info->addr;
 647    client->irq = info->irq;
 648
 649    strlcpy(client->name, info->type, sizeof(client->name));
 650
 651    /* 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    }
 658
 659    /* Check for address business */
 660    status = i2c_check_addr_busy(adap, client->addr);
 661    if (status)
 662        goto out_err;
 663
 664    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);
 669
 670    /* 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;
 677
 678    dev_dbg(&adap->dev, "client [%s] registered with bus id %s\n",
 679        client->name, dev_name(&client->dev));
 680
 681    return client;
 682
 683out_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}

這個函式很直接，就是初始化i2c_client的各個成員，並最終註冊client->dev，從而完成對i2c裝置的註冊。

這裡要說明一下，預置的i2c裝置資訊是通過呼叫i2c_register_board_info函式完成的，該函式定義在drivers/i2c/i2c-boardinfo.c檔案中，其內容如下：

42/**
43 * i2c_register_board_info - statically declare I2C devices
44 * @busnum: identifies the bus to which these devices belong
45 * @info: vector of i2c device descriptors
46 * @len: how many descriptors in the vector; may be zero to reserve
47 *  the specified bus number.
48 *
49 * Systems using the Linux I2C driver stack can declare tables of board info
50 * while they initialize.  This should be done in board-specific init code
51 * near arch_initcall() time, or equivalent, before any I2C adapter driver is
52 * 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 relevant
56 * bus has been registered.  After that moment, standard driver model tools
57 * are used to bind "new style" I2C drivers to the devices.  The bus number
58 * for any device declared using this routine is not available for dynamic
59 * allocation.
60 *
61 * The board info passed can safely be __initdata, but be careful of embedded
62 * pointers (for platform_data, functions, etc) since that won't be copied.
63 */
64int __init
65i2c_register_board_info(int busnum,
66    struct i2c_board_info const *info, unsigned len)
67{
68    int status;
69
70    down_write(&__i2c_board_lock);
71
72    /* 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;
75
76    for (status = 0; len; len--, info++) {
77        struct i2c_devinfo  *devinfo;
78
79        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        }
85
86        devinfo->busnum = busnum;
87        devinfo->board_info = *info;
88        list_add_tail(&devinfo->list, &__i2c_board_list);
89    }
90
91    up_write(&__i2c_board_lock);
92
93    return status;
94}

可以看到，該函式將預置的i2c裝置資訊儲存在__i2c_board_list連結串列中。

回到i2c_register_adapter函式：

1066行，呼叫bus_for_each_drv函式，該函式定義在drivers/base/bus.c檔案中，其內容如下：

 417/**
 418 * bus_for_each_drv - driver iterator
 419 * @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 call
 426 * @fn for each. If @fn returns anything but 0, we break out
 427 * and return it. If @start is not NULL, we use it as the head
 428 * of the list.
 429 *
 430 * NOTE: we don't return the driver that returns a non-zero
 431 * value, nor do we leave the reference count incremented for that
 432 * driver. If the caller needs to know that info, it must set it
 433 * in the callback. It must also be sure to increment the refcount
 434 * 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;
 442
 443    if (!bus)
 444        return -EINVAL;
 445
 446    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}

這個函式我們在《Linux裝置模型分析之device（基於3.10.1核心）》一文中已經分析過了。448-449行，這個while迴圈依次遍歷bus->p->klist_drivers中的所有device_driver，對於每個device_driver，呼叫fn(drv，data)函式。這裡，傳遞過來的fn引數是__process_new_adapter，data引數是adap。這個迴圈是一個關鍵點，註冊一個新的i2c_adapter後，要為該i2c_adapter上的i2c裝置匹配驅動程式，這個匹配過程就是通過這個迴圈呼叫__process_new_adapter函式完成的。

__process_new_adapter函式定義在drivers/i2c/i2c-core.c檔案中，其內容如下：

 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函式的第一個引數，從device_driver轉換為i2c_driver。

i2c_do_add_adapter函式定義在drivers/i2c/i2c-core.c檔案中，其內容如下：

 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);
 962
 963    /* 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，呼叫i2c_detect函式，探測並初始化該i2c總線上的i2c裝置。該函式定義在drivers/i2c/i2c-core.c檔案中，其內容如下：

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);
1736
1737    address_list = driver->address_list;
1738    if (!driver->detect || !address_list)
1739        return 0;
1740
1741    /* Stop here if the classes do not match */
1742    if (!(adapter->class & driver->class))
1743        return 0;
1744
1745    /* 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;
1750
1751    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    }
1759
1760    kfree(temp_client);
1761    return err;
1762}

1755行，呼叫i2c_detect_address，探測指定的地址上的I2C裝置是否存在，如果存在，註冊該i2c裝置。i2c_detect_address函式定義在drivers/i2c/i2c-core.c檔案中，其內容如下：

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;
1682
1683    /* 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    }
1690
1691    /* Skip if already in use */
1692    if (i2c_check_addr_busy(adapter, addr))
1693        return 0;
1694
1695    /* Make sure there is something at this address */
1696    if (!i2c_default_probe(adapter, addr))
1697        return 0;
1698
1699    /* 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 it
1705           here as this isn't an error. */
1706        return err == -ENODEV ? 0 : err;
1707    }
1708
1709    /* 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;
1716
1717        /* 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        else
1724            dev_err(&adapter->dev, "Failed creating %s at 0x%02x\n",
1725                info.type, info.addr);
1726    }
1727    return 0;
1728}

1702行，呼叫driver->detect。

1720行，如果探測到i2c裝置確實存在，呼叫i2c_new_device函式初始化對應的i2c_client結構體並註冊。i2c_new_device函式我們在前面已經分析過。

至此，i2c_adapter的註冊過程我們就清楚了。

三、i2c_driver的註冊

i2c_driver的註冊是通過呼叫i2c_add_driver巨集完成的，該巨集定義在include/linux/i2c.h檔案中：

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函式定義在drivers/i2c/i2c-core.c檔案中，其內容如下：

1307/*
1308 * An i2c_driver is used with one or more i2c_client (device) nodes to access
1309 * i2c slave chips, on a bus instance associated with some i2c_adapter.
1310 */
1311
1312int i2c_register_driver(struct module *owner, struct i2c_driver *driver)
1313{
1314    int res;
1315
1316    /* Can't register until after driver model init */
1317    if (unlikely(WARN_ON(!i2c_bus_type.p)))
1318        return -EAGAIN;
1319
1320    /* 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;
1323
1324    /* When registration returns, the driver core
1325     * will have called probe() for all matching-but-unbound devices.
1326     */
1327    res = driver_register(&driver->driver);
1328    if (res)
1329        return res;
1330
1331    /* 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);
1338
1339    pr_debug("i2c-core: driver [%s] registered\n", driver->driver.name);
1340
1341    INIT_LIST_HEAD(&driver->clients);
1342    /* Walk the adapters that are already present */
1343    i2c_for_each_dev(driver, __process_new_driver);
1344
1345    return 0;
1346}

1327行，呼叫driver_register註冊i2c_driver.driver。參考《 》對Linux裝置模型的分析，在driver_register執行過程中，如果I2C總線上找到了與該驅動匹配的I2C裝置，則i2c_driver.probe函式會被呼叫執行。

1343行，呼叫i2c_for_each_dev遍歷所有已存在的i2c_adapter。該函式定義在drivers/i2c/i2c-core.c檔案中，其內容如下：

1288int i2c_for_each_dev(void *data, int (*fn)(struct device *, void *))
1289{
1290    int res;
1291
1292    mutex_lock(&core_lock);
1293    res = bus_for_each_dev(&i2c_bus_type, NULL, data, fn);
1294    mutex_unlock(&core_lock);
1295
1296    return res;
1297}

1293行，呼叫bus_for_each_dev，這個函式定義在drivers/base/bus.c檔案中，其內容如下：

 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;
 292
 293   if (!bus || !bus->p)
 294       return -EINVAL;
 295
 296   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}

我們在《 Linux裝置模型分析之device_driver（基於3.10.1核心）》一文中已經分析過這個函式。這裡，傳遞過來的data引數是要註冊的i2c_driver，fn引數是__process_new_driver函式，所以我們來看__process_new_driver函式，該函式定義在drivers/i2c/i2c-core.c檔案中，其內容如下：

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函式定義在drivers/i2c/i2c-core.c檔案中，其內容如下：

 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);
 962
 963   /* 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}

這個函式我們在分析i2c_adapter的註冊過程時已經分析過了，它主要完成i2c_driver與i2c_adapter上的i2c裝置的匹配工作，如果匹配成功，初始化並註冊對應的i2c_client。

至此，i2c_driver的註冊過程我們就清楚了。

四、i2c_bus_type分析

i2c_init 函式完成Linux i2c框架的初始化工作，該函式定義在drivers/i2c/i2c-core.c檔案中，其內容如下：

1429static int __init i2c_init(void)
1430{
1431   int retval;
1432
1433   retval = bus_register(&i2c_bus_type);
1434   if (retval)
1435       return retval;
1436#ifdef CONFIG_I2C_COMPAT
1437   i2c_adapter_compat_class =class_compat_register("i2c-adapter");
1438   if (!i2c_adapter_compat_class) {
1439       retval = -ENOMEM;
1440       goto bus_err;
1441   }
1442#endif
1443   retval = i2c_add_driver(&dummy_driver);
1444   if (retval)
1445       goto class_err;
1446   return 0;
1447
1448class_err:
1449#ifdef CONFIG_I2C_COMPAT
1450   class_compat_unregister(i2c_adapter_compat_class);
1451bus_err:
1452#endif
1453   bus_unregister(&i2c_bus_type);
1454   return retval;
1455}

1433行，呼叫bus_register註冊了i2c_bus_type。i2c_bus_tpye定義在drivers/i2c/i2c-core.c檔案中，其內容如下：

 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};

其型別是bus_type，所以它代表i2c匯流排。我們來關注一下i2c_device_match和i2c_device_probe函式。

i2c_device_match函式定義在drivers/i2c/i2c-core.c檔案中，其內容如下：

 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;
  76
 77    if (!client)
 78        return 0;
  79
 80    /* Attempt an OF style match*/
 81    if(of_driver_match_device(dev, drv))
 82        return 1;
  83
 84    /* Then ACPI style match */
 85    if(acpi_driver_match_device(dev, drv))
 86        return 1;
  87
 88    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;
  92
 93    return 0;
  94}

88行，將device_driver轉換為i2c_driver。

90-91行，如果driver->id_table不為空，則呼叫i2c_match_id函式，該函式定義在drivers/i2c/i2c-core.c檔案中，其內容如下：

 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，則匹配成功，返回id。如果返回NULL，則表示匹配失敗。

分析到這裡，我們要回顧一下分析Linux裝置模型時涉及到的一個函式driver_match_device，該函式定義在drivers/base/base.h檔案中，其內容如下：

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}

當進行device和device_driver的匹配時，會呼叫這個函式，該函式返回值為0，表示match失敗。只有match成功時，才會進一步進行probe。

可以看到，如果drv->bus->match存在，會呼叫drv->bus->match(dev,drv)。所以，當i2c裝置(i2c_client)和i2c驅動(i2c_driver)進行匹配操作時，就會呼叫i2c_device_match函式。

在分析i2c_device_probe函式之前，我們要回顧一下分析Linux裝置模型時涉及到的一個函式driver_probe_device，device_driver探測支援的device時，會呼叫到這個函式，它又會進一步呼叫really_probe函式，really_probe函式定義在drivers/base/dd.c檔案中，其內容如下：

265static int really_probe(struct device*dev, struct device_driver *drv)
266{
267   int ret = 0;
268
269   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));
273
274   dev->driver = drv;
275
276   /* If using pinctrl, bind pins now before probing */
277   ret = pinctrl_bind_pins(dev);
278   if (ret)
279       goto probe_failed;
280
281   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   }
286
287   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   }
296
297   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;
302
303probe_failed:
304   devres_release_all(dev);
305   driver_sysfs_remove(dev);
306   dev->driver = NULL;
307   dev_set_drvdata(dev, NULL);
308
309   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_WARNING
316               "%s: probe of %s failedwith error %d\n",
317               drv->name, dev_name(dev),ret);
318   } else {
319       pr_debug("%s: probe of %s rejects match %d\n",
320               drv->name, dev_name(dev),ret);
321   }
322   /*
323    * Ignore errors returned by ->probe so that the next driver can try
324    * its luck.
325    */
326   ret = 0;
327done:
328   atomic_dec(&probe_count);
329   wake_up(&probe_waitqueue);
330   return ret;
331}

這個函式我們現在需要關注的是287-295行，如果dev->bus->probe存在，則呼叫dev->bus->probe(dev)，如果dev->bus->probe不存在，並且drv->probe存在，才會呼叫drv->probe(dev)。所以，對於i2c裝置和i2c驅動，探測裝置時會優先呼叫i2c_bus_type.probe函式。而i2c_bus_type.probe即i2c_device_probe會轉而呼叫i2c_driver.probe函式。

現在我們可以來看i2c_device_probe函數了，它定義在drivers/i2c/i2c-core.c檔案中，其內容如下：

 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;
 238
 239   if (!client)
 240       return 0;
 241
 242   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,
 248                    client->flags &I2C_CLIENT_WAKE);
 249   dev_dbg(dev, "probe\n");
 250
 251   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沒有定義probe或者i2c_driver沒有定義id_table，則直接退出。所以我們在寫i2c驅動時，必須定義i2c_driver.probe和i2c_driver.id_table。

251行，呼叫i2c_driver.probe。這時，我們的i2c驅動程式定義的probe函式就會執行。