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：http://blog.csdn.net/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,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);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 + 1,1091               GFP_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; 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    }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)) {1040                dev_err(&adap->dev, "Invalid SCL gpio, not using recovery\n");1041                adap->bus_recovery_info = NULL;1042                goto exit_recovery;1043            }10441045            if (gpio_is_valid(bri->sda_gpio))1046                bri->get_sda = get_sda_gpio_value;1047            else1048                bri->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; 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}

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 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}

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 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}

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); 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，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",1725                info.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; 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); 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}

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;  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，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",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 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; 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。