《linux设备驱动开发详解》笔记——15 linux i2c驱动

　　结合实际代码和书中描述，可能跟书上有一定出入。本文后续芯片相关代码参考ZYNQ。

15.1 总体结构

　　如下图，i2c驱动分为如下几个重要模块

• 核心层core，完成i2c总线、设备、驱动模型，对用户提供sys文件系统访问支持；为i2c内部adpter等提供注册接口。
• adpter，适配器，实际就是CPU集成的IIC控制器，有cpu控制，完成i2c物理总线操作，busses文件夹里，有各个cpu adapter的具体实现；algorithm集成到这个模块里，而且是adapter的一部分，主要实现总  线传输的标准化。
• i2c-dev，把adapter设备化，采用标准的file_operations字符设备的形式，便于用户层直接读写adapter设备文件，不是主流，一般也就是调试时用用。
• muxes，i2c切换芯片，不重点讨论。

　　

自己理解的结构图，与书中有出入，简化了。

i2c总线设备驱动模型，与platform类似，软件架构都一样。通过这种模型，最终在sysfs中创建文件，且创建文件时要填充类似file_operations类似的结构体中的读写函数，从而完成用户层到底层驱动的传递。

15.2 adapter和algorithm——最终对外提供硬件无关接口

　　busses/i2c-cadence.c文件是ZYNQ i2c控制器的驱动，实现i2c的adapter和algorithm。adapter采用platform机制。

　　 先看看关键数据结构：

15.2.1 关键数据接结构

/**
 * struct cdns_i2c - I2C device private data structure
 * @membase:        Base address of the I2C device
 * @adap:        I2C adapter instance
 * @p_msg:        Message pointer
 * @err_status:        Error status in Interrupt Status Register
 * @xfer_done:        Transfer complete status
 * @p_send_buf:        Pointer to transmit buffer
 * @p_recv_buf:        Pointer to receive buffer
 * @suspended:        Flag holding the device's PM status
 * @send_count:        Number of bytes still expected to send
 * @recv_count:        Number of bytes still expected to receive
 * @curr_recv_count:    Number of bytes to be received in current transfer
 * @irq:        IRQ number
 * @input_clk:        Input clock to I2C controller
 * @i2c_clk:        Maximum I2C clock speed
 * @bus_hold_flag:    Flag used in repeated start for clearing HOLD bit
 * @clk:        Pointer to struct clk
 * @clk_rate_change_nb:    Notifier block for clock rate changes
 */
struct cdns_i2c {
    void __iomem *membase;
    struct i2c_adapter adap;
    struct i2c_msg *p_msg;
    int err_status;
    struct completion xfer_done;
    unsigned char *p_send_buf;
    unsigned char *p_recv_buf;
    u8 suspended;
    unsigned int send_count;
    unsigned int recv_count;
    unsigned int curr_recv_count;
    int irq;
    unsigned long input_clk;
    unsigned int i2c_clk;
    unsigned int bus_hold_flag;
    struct clk *clk;
    struct notifier_block clk_rate_change_nb;
};

/*
 * i2c_adapter is the structure used to identify a physical i2c bus along
 * with the access algorithms necessary to access it.
 */
struct i2c_adapter {
    struct module *owner;
    unsigned int class;          /* classes to allow probing for */
    const struct i2c_algorithm *algo; /* the algorithm to access the bus，实体也在此文件中定义，并在probe函数里指向此实体 */
    void *algo_data;

    /* data fields that are valid for all devices    */
    struct rt_mutex bus_lock;

    int timeout;            /* in jiffies */
    int retries;
    struct device dev;        /* the adapter device */

    int nr;
    char name[];
    struct completion dev_released;

    struct mutex userspace_clients_lock;
    struct list_head userspace_clients;

    struct i2c_bus_recovery_info *bus_recovery_info;
};

/**
 * struct i2c_algorithm - represent I2C transfer method
 * @master_xfer: Issue a set of i2c transactions to the given I2C adapter
 *   defined by the msgs array, with num messages available to transfer via
 *   the adapter specified by adap.
 * @smbus_xfer: Issue smbus transactions to the given I2C adapter. If this
 *   is not present, then the bus layer will try and convert the SMBus calls
 *   into I2C transfers instead.
 * @functionality: Return the flags that this algorithm/adapter pair supports
 *   from the I2C_FUNC_* flags.
 *
 * The following structs are for those who like to implement new bus drivers:
 * i2c_algorithm is the interface to a class of hardware solutions which can
 * be addressed using the same bus algorithms - i.e. bit-banging or the PCF8584
 * to name two of the most common.
 *
 * The return codes from the @master_xfer field should indicate the type of
 * error code that occured during the transfer, as documented in the kernel
 * Documentation file Documentation/i2c/fault-codes.
 */
struct i2c_algorithm {
    /* If an adapter algorithm can't do I2C-level access, set master_xfer
       to NULL. If an adapter algorithm can do SMBus access, set
       smbus_xfer. If set to NULL, the SMBus protocol is simulated
       using common I2C messages */
    /* master_xfer should return the number of messages successfully
       processed, or a negative value on error */
    int (*master_xfer)(struct i2c_adapter *adap, struct i2c_msg *msgs,
               int num);
    int (*smbus_xfer) (struct i2c_adapter *adap, u16 addr,
               unsigned short flags, char read_write,
               u8 command, int size, union i2c_smbus_data *data);

    /* To determine what the adapter supports */
    u32 (*functionality) (struct i2c_adapter *);
};

15.2.2 adapter的注册与注销

1. adapter被组织成platform驱动形式
2. 顺便理解platform的匹配机制:

• dts里有若干device的描述，linux在初始化时会把这些设备展开，形成设备列表。
• platform driver中有匹配字段of_match_table，估计注册platform驱动时（module_platform_driver），platform bus负责匹配此字段和已有的dts设备列表。
• platform driver和dev列表匹配上以后，driver中的probe就会执行，同时dev列表中的信息以probe形参struct platform_device *pdev的形式传递给probe()函数。

　　一般linux的iic、spi、usb等外设都是这个思路，适配器为别人提供总线，但是其本身是挂到platform总线上的。

　　  3. adapter通过iic-core.c核心层提供的接口，注册或注销到iic总线上

• i2c_add_adapter()：添加adapter数据结构，核心层里详述
• i2c_del_adapter()：删除adapter设数据结构

static int cdns_i2c_probe(struct platform_device *pdev)　　// dts里的设备信息传递进来了
{
    struct resource *r_mem;
    struct cdns_i2c *id;
    int ret;

    id = devm_kzalloc(&pdev->dev, sizeof(*id), GFP_KERNEL);
    if (!id)
        return -ENOMEM;
    platform_set_drvdata(pdev, id);

　　 xxx_adapter_hw_init();　　　　//通常初始化iic适配器使用的硬件资源，如申请IO地址、中断号、时钟等
    id->adap.dev.of_node = pdev->dev.of_node;
    id->adap.algo = &cdns_i2c_algo;　　// 把altorithm连进来
    id->adap.timeout = CDNS_I2C_TIMEOUT;
    id->adap.retries = ;        /* Default retry value. */
    id->adap.algo_data = id;
    id->adap.dev.parent = &pdev->dev;

    ret = i2c_add_adapter(&id->adap);
    ...
}

static int cdns_i2c_remove(struct platform_device *pdev)
{
    struct cdns_i2c *id = platform_get_drvdata(pdev);

    i2c_del_adapter(&id->adap);
    xxx_adapter_hw_free();            // 硬件相关资源的free

    return ;
}

static const struct of_device_id cdns_i2c_of_match[] = {
    { .compatible = "cdns,i2c-r1p10", },　　　　　　　　　　
    { /* end of table */ }
};
MODULE_DEVICE_TABLE(of, cdns_i2c_of_match);

static struct platform_driver cdns_i2c_drv = {
    .driver = {
        .name  = DRIVER_NAME,
        .owner = THIS_MODULE,
        .of_match_table = cdns_i2c_of_match,　　　　// dts匹配的依据
        .pm = &cdns_i2c_dev_pm_ops,
    },
    .probe  = cdns_i2c_probe,
    .remove = cdns_i2c_remove,
};

module_platform_driver(cdns_i2c_drv);

iic相关的dts信息如下

iic相关dts
        ps7_i2c_1: ps7-i2c@e0005000 {
            clock-frequency = <>;
            clocks = <&clkc >;
            compatible = "cdns,i2c-r1p10"; 
            interrupt-parent = <&ps7_scugic_0>;
            interrupts = <  >;
            reg = <0xe0005000 0x1000>;
            xlnx,has-interrupt = <0x0>;
            #address-cells = <>;
            #size-cells = <>;
            eeprom@ {
                compatible = "at,24c512";
                reg = <0x52>;
            };
        } ;

15.2.3 i2c总线的通信方法 —— algorithm

　　主要需要实现i2c_algorithm结构体中的master_xfer()和functionality()函数，i2c_algorithm的作用是高度总结iic总线通信机制，把具体的适配器（不同型号）与其他通用驱动隔离开。

　　functionality()函数比较简单，返回支持的通信协议。

　　master_xfer()函数在适配器上完成i2c_msg的数据传输。

static const struct i2c_algorithm cdns_i2c_algo = {
    .master_xfer    = cdns_i2c_master_xfer,
    .functionality    = cdns_i2c_func,
};

/**
 * cdns_i2c_func - Returns the supported features of the I2C driver
 * @adap:    pointer to the i2c adapter structure
 *
 * Return: 32 bit value, each bit corresponding to a feature
 */
static u32 cdns_i2c_func(struct i2c_adapter *adap)
{
    return I2C_FUNC_I2C | I2C_FUNC_10BIT_ADDR |
        (I2C_FUNC_SMBUS_EMUL & ~I2C_FUNC_SMBUS_QUICK) |
        I2C_FUNC_SMBUS_BLOCK_DATA;
}

/**
 * cdns_i2c_master_xfer - The main i2c transfer function
 * @adap:    pointer to the i2c adapter driver instance
 * @msgs:    pointer to the i2c message structure
 * @num:    the number of messages to transfer
 *
 * Initiates the send/recv activity based on the transfer message received.
 *
 * Return: number of msgs processed on success, negative error otherwise
 */
static int cdns_i2c_master_xfer(struct i2c_adapter *adap, struct i2c_msg *msgs,
                int num)
{
    int ret, count;
    u32 reg;
    struct cdns_i2c *id = adap->algo_data;

    /* Check if the bus is free */
    if (cdns_i2c_readreg(CDNS_I2C_SR_OFFSET) & CDNS_I2C_SR_BA)
        return -EAGAIN;

    /*
     * Set the flag to one when multiple messages are to be
     * processed with a repeated start.
     */
    if (num > ) {
        id->bus_hold_flag = ;
        reg = cdns_i2c_readreg(CDNS_I2C_CR_OFFSET);
        reg |= CDNS_I2C_CR_HOLD;
        cdns_i2c_writereg(reg, CDNS_I2C_CR_OFFSET);
    } else {
        id->bus_hold_flag = ;
    }

    /* Process the msg one by one */
    for (count = ; count < num; count++, msgs++) {
        if (count == (num - ))
            id->bus_hold_flag = ;

        ret = cdns_i2c_process_msg(id, msgs, adap);
        if (ret)
            return ret;

        /* Report the other error interrupts to application */
        if (id->err_status) {
            cdns_i2c_master_reset(adap);

            if (id->err_status & CDNS_I2C_IXR_NACK)
                return -ENXIO;

            return -EIO;
        }
    }

    return num;
}

15.3 core

15.3.1 i2c总线、设备、驱动模型建立和维护

创建i2c总线、设备、驱动大框架。

static int __init i2c_init(void)　　// linux初始化时执行
{
    int retval;

    retval = bus_register(&i2c_bus_type);　　// 在sys文件夹中创建i2c总线，完成将i2c总线注册到系统，与platform总线平级，创建 /sys/i2c目录
    if (retval)
        return retval;
#ifdef CONFIG_I2C_COMPAT
    i2c_adapter_compat_class = class_compat_register("i2c-adapter");　　//  创建类，/sys/class/i2c-adapter
    if (!i2c_adapter_compat_class) {
        retval = -ENOMEM;
        goto bus_err;
    }
#endif
    retval = i2c_add_driver(&dummy_driver);　　　　// 注册一个dummy驱动，不知道为了啥？ 在 /sys/bus/i2c/drivers目录下创建了dummy驱动
    if (retval)
        goto class_err;
    return ;

class_err:
#ifdef CONFIG_I2C_COMPAT
    class_compat_unregister(i2c_adapter_compat_class);
bus_err:
#endif
    bus_unregister(&i2c_bus_type);
    return retval;
}

static void __exit i2c_exit(void)
{
    i2c_del_driver(&dummy_driver);
#ifdef CONFIG_I2C_COMPAT
    class_compat_unregister(i2c_adapter_compat_class);
#endif
    bus_unregister(&i2c_bus_type);
}

/* We must initialize early, because some subsystems register i2c drivers
 * in subsys_initcall() code, but are linked (and initialized) before i2c.
 */
postcore_initcall(i2c_init);　　　　// linux初始化时调用
module_exit(i2c_exit);

引出知识：driver中的probe是如何执行的？！总线match以后，总线结构体中的probe会被执行（内核代码实现的），总线probe函数会调用driver中的probe。所有总线、驱动、模型，包括platform都是这种机制。

struct bus_type i2c_bus_type = {
    .name        = "i2c",
    .match        = i2c_device_match,
    .probe        = i2c_device_probe,
    .remove        = i2c_device_remove,
    .shutdown    = i2c_device_shutdown,
    .pm        = &i2c_device_pm_ops,
};

static int i2c_device_probe(struct device *dev)
{
    struct i2c_client    *client = i2c_verify_client(dev);
    struct i2c_driver    *driver;
    int status;

    if (!client)
        return ;

    driver = to_i2c_driver(dev->driver);
    if (!driver->probe || !driver->id_table)
        return -ENODEV;

    if (!device_can_wakeup(&client->dev))
        device_init_wakeup(&client->dev,
                    client->flags & I2C_CLIENT_WAKE);
    dev_dbg(dev, "probe\n");

    acpi_dev_pm_attach(&client->dev, true);
    status = driver->probe(client, i2c_match_id(driver->id_table, client));
    if (status)
        acpi_dev_pm_detach(&client->dev, true);

    return status;
}

15.3.2 添加/删除adapter接口

// i2c/buses/i2c_cadence.c
static int cdns_i2c_probe(struct platform_device *pdev)
{
    ...
    ret = i2c_add_adapter(&id->adap);        // 添加adapter
    ...
}

/**
 * i2c_add_adapter - declare i2c adapter, use dynamic bus number
 * @adapter: the adapter to add
 * Context: can sleep
 *
 * This routine is used to declare an I2C adapter when its bus number
 * doesn't matter or when its bus number is specified by an dt alias.
 * Examples of bases when the bus number doesn't matter: I2C adapters
 * dynamically added by USB links or PCI plugin cards.
 *
 * When this returns zero, a new bus number was allocated and stored
 * in adap->nr, and the specified adapter became available for clients.
 * Otherwise, a negative errno value is returned.
 */
int i2c_add_adapter(struct i2c_adapter *adapter)
{
    struct device *dev = &adapter->dev;
    int id;

    if (dev->of_node) {
        id = of_alias_get_id(dev->of_node, "i2c");
        if (id >= ) {
            adapter->nr = id;                // 从dts中自动获取i2c的adapter的个数
            return __i2c_add_numbered_adapter(adapter);    // 注册各adapter
        }
    }

    mutex_lock(&core_lock);
    id = idr_alloc(&i2c_adapter_idr, adapter,
               __i2c_first_dynamic_bus_num, , GFP_KERNEL);
    mutex_unlock(&core_lock);
    if (id < )
        return id;

    adapter->nr = id;

    return i2c_register_adapter(adapter);
}

/**
 * __i2c_add_numbered_adapter - i2c_add_numbered_adapter where nr is never -1
 * @adap: the adapter to register (with adap->nr initialized)
 * Context: can sleep
 *
 * See i2c_add_numbered_adapter() for details.
 */
static int __i2c_add_numbered_adapter(struct i2c_adapter *adap)
{
    int    id;

    mutex_lock(&core_lock);
    id = idr_alloc(&i2c_adapter_idr, adap, adap->nr, adap->nr + ,
               GFP_KERNEL);
    mutex_unlock(&core_lock);
    if (id < )
        return id == -ENOSPC ? -EBUSY : id;

    return i2c_register_adapter(adap);
}

static int i2c_register_adapter(struct i2c_adapter *adap)
{
    int res = ;

    /* Can't register until after driver model init */
    if (unlikely(WARN_ON(!i2c_bus_type.p))) {
        res = -EAGAIN;
        goto out_list;
    }

    /* Sanity checks */
    if (unlikely(adap->name[] == '\0')) {
        pr_err("i2c-core: Attempt to register an adapter with "
               "no name!\n");
        return -EINVAL;
    }
    if (unlikely(!adap->algo)) {
        pr_err("i2c-core: Attempt to register adapter '%s' with "
               "no algo!\n", adap->name);
        return -EINVAL;
    }

    rt_mutex_init(&adap->bus_lock);
    mutex_init(&adap->userspace_clients_lock);
    INIT_LIST_HEAD(&adap->userspace_clients);

    /* Set default timeout to 1 second if not already set */
    if (adap->timeout == )
        adap->timeout = HZ;

    dev_set_name(&adap->dev, "i2c-%d", adap->nr);
    adap->dev.bus = &i2c_bus_type;
    adap->dev.type = &i2c_adapter_type;
    res = device_register(&adap->dev);
    if (res)
        goto out_list;

    dev_dbg(&adap->dev, "adapter [%s] registered\n", adap->name);

#ifdef CONFIG_I2C_COMPAT
    res = class_compat_create_link(i2c_adapter_compat_class, &adap->dev,
                       adap->dev.parent);
    if (res)
        dev_warn(&adap->dev,
             "Failed to create compatibility class link\n");
#endif

    /* bus recovery specific initialization */
    if (adap->bus_recovery_info) {
        struct i2c_bus_recovery_info *bri = adap->bus_recovery_info;

        if (!bri->recover_bus) {
            dev_err(&adap->dev, "No recover_bus() found, not using recovery\n");
            adap->bus_recovery_info = NULL;
            goto exit_recovery;
        }

        /* Generic GPIO recovery */
        if (bri->recover_bus == i2c_generic_gpio_recovery) {
            if (!gpio_is_valid(bri->scl_gpio)) {
                dev_err(&adap->dev, "Invalid SCL gpio, not using recovery\n");
                adap->bus_recovery_info = NULL;
                goto exit_recovery;
            }

            if (gpio_is_valid(bri->sda_gpio))
                bri->get_sda = get_sda_gpio_value;
            else
                bri->get_sda = NULL;

            bri->get_scl = get_scl_gpio_value;
            bri->set_scl = set_scl_gpio_value;
        } else if (!bri->set_scl || !bri->get_scl) {
            /* Generic SCL recovery */
            dev_err(&adap->dev, "No {get|set}_gpio() found, not using recovery\n");
            adap->bus_recovery_info = NULL;
        }
    }

exit_recovery:
    /* create pre-declared device nodes */
    of_i2c_register_devices(adap);
    acpi_i2c_register_devices(adap);

    if (adap->nr < __i2c_first_dynamic_bus_num)
        i2c_scan_static_board_info(adap);

    /* Notify drivers */
    mutex_lock(&core_lock);
    bus_for_each_drv(&i2c_bus_type, NULL, adap, __process_new_adapter);
    mutex_unlock(&core_lock);

    return ;

out_list:
    mutex_lock(&core_lock);
    idr_remove(&i2c_adapter_idr, adap->nr);
    mutex_unlock(&core_lock);
    return res;
}

15.3.3 添加/删除i2c_driver接口

向i2c_bus注册驱动，可用于匹配i2c总线上的设备，即i2c_client，例如EEPROM等

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

/*
 * An i2c_driver is used with one or more i2c_client (device) nodes to access
 * i2c slave chips, on a bus instance associated with some i2c_adapter.
 */

int i2c_register_driver(struct module *owner, struct i2c_driver *driver)
{
    int res;

    /* Can't register until after driver model init */
    if (unlikely(WARN_ON(!i2c_bus_type.p)))
        return -EAGAIN;

    /* add the driver to the list of i2c drivers in the driver core */
    driver->driver.owner = owner;
    driver->driver.bus = &i2c_bus_type;

    /* When registration returns, the driver core
     * will have called probe() for all matching-but-unbound devices.
     */
    res = driver_register(&driver->driver);
    if (res)
        return res;

    /* Drivers should switch to dev_pm_ops instead. */
    if (driver->suspend)
        pr_warn("i2c-core: driver [%s] using legacy suspend method\n",
            driver->driver.name);
    if (driver->resume)
        pr_warn("i2c-core: driver [%s] using legacy resume method\n",
            driver->driver.name);

    pr_debug("i2c-core: driver [%s] registered\n", driver->driver.name);

    INIT_LIST_HEAD(&driver->clients);
    /* Walk the adapters that are already present */
    i2c_for_each_dev(driver, __process_new_driver);

    return ;
}
EXPORT_SYMBOL(i2c_register_driver);

/**
 * i2c_del_driver - unregister I2C driver
 * @driver: the driver being unregistered
 * Context: can sleep
 */
void i2c_del_driver(struct i2c_driver *driver)
{
    i2c_for_each_dev(driver, __process_removed_driver);

    driver_unregister(&driver->driver);
    pr_debug("i2c-core: driver [%s] unregistered\n", driver->driver.name);
}
EXPORT_SYMBOL(i2c_del_driver);

15.3.4 i2c传输接口

外部设备使用这些标准传输接口编写驱动。

/**
 * i2c_transfer - execute a single or combined I2C message
 * @adap: Handle to I2C bus
 * @msgs: One or more messages to execute before STOP is issued to
 *    terminate the operation; each message begins with a START.
 * @num: Number of messages to be executed.
 *
 * Returns negative errno, else the number of messages executed.
 *
 * Note that there is no requirement that each message be sent to
 * the same slave address, although that is the most common model.
 */
int i2c_transfer(struct i2c_adapter *adap, struct i2c_msg *msgs, int num)
{
    int ret;

    /* REVISIT the fault reporting model here is weak:
     *
     *  - When we get an error after receiving N bytes from a slave,
     *    there is no way to report "N".
     *
     *  - When we get a NAK after transmitting N bytes to a slave,
     *    there is no way to report "N" ... or to let the master
     *    continue executing the rest of this combined message, if
     *    that's the appropriate response.
     *
     *  - When for example "num" is two and we successfully complete
     *    the first message but get an error part way through the
     *    second, it's unclear whether that should be reported as
     *    one (discarding status on the second message) or errno
     *    (discarding status on the first one).
     */

    if (adap->algo->master_xfer) {
#ifdef DEBUG
        for (ret = ; ret < num; ret++) {
            dev_dbg(&adap->dev, "master_xfer[%d] %c, addr=0x%02x, "
                "len=%d%s\n", ret, (msgs[ret].flags & I2C_M_RD)
                ? 'R' : 'W', msgs[ret].addr, msgs[ret].len,
                (msgs[ret].flags & I2C_M_RECV_LEN) ? "+" : "");
        }
#endif

        if (in_atomic() || irqs_disabled()) {
            ret = i2c_trylock_adapter(adap);
            if (!ret)
                /* I2C activity is ongoing. */
                return -EAGAIN;
        } else {
            i2c_lock_adapter(adap);
        }

        ret = __i2c_transfer(adap, msgs, num);
        i2c_unlock_adapter(adap);

        return ret;
    } else {
        dev_dbg(&adap->dev, "I2C level transfers not supported\n");
        return -EOPNOTSUPP;
    }
}

/**
 * i2c_master_send - issue a single I2C message in master transmit mode
 * @client: Handle to slave device
 * @buf: Data that will be written to the slave
 * @count: How many bytes to write, must be less than 64k since msg.len is u16
 *
 * Returns negative errno, or else the number of bytes written.
 */
int i2c_master_send(const struct i2c_client *client, const char *buf, int count)
{
    int ret;
    struct i2c_adapter *adap = client->adapter;
    struct i2c_msg msg;

    msg.addr = client->addr;
    msg.flags = client->flags & I2C_M_TEN;
    msg.len = count;
    msg.buf = (char *)buf;

    ret = i2c_transfer(adap, &msg, );

    /*
     * If everything went ok (i.e. 1 msg transmitted), return #bytes
     * transmitted, else error code.
     */
    return (ret == ) ? count : ret;
}

/**
 * i2c_master_recv - issue a single I2C message in master receive mode
 * @client: Handle to slave device
 * @buf: Where to store data read from slave
 * @count: How many bytes to read, must be less than 64k since msg.len is u16
 *
 * Returns negative errno, or else the number of bytes read.
 */
int i2c_master_recv(const struct i2c_client *client, char *buf, int count)
{
    struct i2c_adapter *adap = client->adapter;
    struct i2c_msg msg;
    int ret;

    msg.addr = client->addr;
    msg.flags = client->flags & I2C_M_TEN;
    msg.flags |= I2C_M_RD;
    msg.len = count;
    msg.buf = buf;

    ret = i2c_transfer(adap, &msg, );

    /*
     * If everything went ok (i.e. 1 msg received), return #bytes received,
     * else error code.
     */
    return (ret == ) ? count : ret;
}

15.4 设备驱动i2c_driver和i2c_client

i2c_dirver就是i2c标准总线设备驱动模型中的驱动部分，i2c_client可理解为i2c总线上挂的外设。

1. 驱动的注册和注销

drivers/misc/eeprom/at24.c

// 初始化和驱动模型
static struct i2c_driver at24_driver = {
    .driver = {
        .name = "at24",
        .owner = THIS_MODULE,
    },
    .probe = at24_probe,
    .remove = at24_remove,
    .id_table = at24_ids,
};

static int __init at24_init(void)
{
    if (!io_limit) {
        pr_err("at24: io_limit must not be 0!\n");
        return -EINVAL;
    }

    io_limit = rounddown_pow_of_two(io_limit);
    return i2c_add_driver(&at24_driver);　　// 匹配后，driver中的probe就能执行
}
module_init(at24_init);

static void __exit at24_exit(void)
{
    i2c_del_driver(&at24_driver);
}
module_exit(at24_exit);

// id列表
static const struct i2c_device_id at24_ids[] = {
    /* needs 8 addresses as A0-A2 are ignored */
    { "24c00", AT24_DEVICE_MAGIC( / , AT24_FLAG_TAKE8ADDR) },
    /* old variants can't be handled with this generic entry! */
    { "24c01", AT24_DEVICE_MAGIC( / , ) },
    { "24c02", AT24_DEVICE_MAGIC( / , ) },
    /* spd is a 24c02 in memory DIMMs */
    { "spd", AT24_DEVICE_MAGIC( / ,
        AT24_FLAG_READONLY | AT24_FLAG_IRUGO) },
    { "24c04", AT24_DEVICE_MAGIC( / , ) },
    /* 24rf08 quirk is handled at i2c-core */
    { "24c08", AT24_DEVICE_MAGIC( / , ) },
    { "24c16", AT24_DEVICE_MAGIC( / , ) },
    { "24c32", AT24_DEVICE_MAGIC( / , AT24_FLAG_ADDR16) },
    { "24c64", AT24_DEVICE_MAGIC( / , AT24_FLAG_ADDR16) },
    { "24c128", AT24_DEVICE_MAGIC( / , AT24_FLAG_ADDR16) },
    { "24c256", AT24_DEVICE_MAGIC( / , AT24_FLAG_ADDR16) },
    { "24c512", AT24_DEVICE_MAGIC( / , AT24_FLAG_ADDR16) },
    { "24c1024", AT24_DEVICE_MAGIC( / , AT24_FLAG_ADDR16) },
    { "at24",  },
    { /* END OF LIST */ }
};

//dts:
        ps7_i2c_1: ps7-i2c@e0005000 {
            clock-frequency = <>;
            clocks = <&clkc >;
            compatible = "cdns,i2c-r1p10";
            interrupt-parent = <&ps7_scugic_0>;
            interrupts = <  >;
            reg = <0xe0005000 0x1000>;
            xlnx,has-interrupt = <0x0>;
            #address-cells = <>;
            #size-cells = <>;
            eeprom@ {
                compatible = "at,24c512";
                reg = <0x50>;
            };
            rtc@ {
                 compatible = "ds,ds1338";
                 reg = <0x68>;
            };

        } ;

2. driver中的probe和remove分析

static int at24_probe(struct i2c_client *client, const struct i2c_device_id *id)
{
    struct at24_platform_data chip;
    bool writable;
    int use_smbus = ;
    struct at24_data *at24;
    int err;
    unsigned i, num_addresses;
    kernel_ulong_t magic;

    if (client->dev.platform_data) {
        chip = *(struct at24_platform_data *)client->dev.platform_data;
    } else {
        if (!id->driver_data)
            return -ENODEV;

        magic = id->driver_data;
        chip.byte_len = BIT(magic & AT24_BITMASK(AT24_SIZE_BYTELEN));
        magic >>= AT24_SIZE_BYTELEN;
        chip.flags = magic & AT24_BITMASK(AT24_SIZE_FLAGS);
        /*
         * This is slow, but we can't know all eeproms, so we better
         * play safe. Specifying custom eeprom-types via platform_data
         * is recommended anyhow.
         */
        chip.page_size = ;

        /* update chipdata if OF is present */
        at24_get_ofdata(client, &chip);

        chip.setup = NULL;
        chip.context = NULL;
    }

    if (!is_power_of_2(chip.byte_len))
        dev_warn(&client->dev,
            "byte_len looks suspicious (no power of 2)!\n");
    if (!chip.page_size) {
        dev_err(&client->dev, "page_size must not be 0!\n");
        return -EINVAL;
    }
    if (!is_power_of_2(chip.page_size))
        dev_warn(&client->dev,
            "page_size looks suspicious (no power of 2)!\n");

    /* Use I2C operations unless we're stuck with SMBus extensions. */
    if (!i2c_check_functionality(client->adapter, I2C_FUNC_I2C)) {
        if (chip.flags & AT24_FLAG_ADDR16)
            return -EPFNOSUPPORT;

        if (i2c_check_functionality(client->adapter,
                I2C_FUNC_SMBUS_READ_I2C_BLOCK)) {
            use_smbus = I2C_SMBUS_I2C_BLOCK_DATA;
        } else if (i2c_check_functionality(client->adapter,
                I2C_FUNC_SMBUS_READ_WORD_DATA)) {
            use_smbus = I2C_SMBUS_WORD_DATA;
        } else if (i2c_check_functionality(client->adapter,
                I2C_FUNC_SMBUS_READ_BYTE_DATA)) {
            use_smbus = I2C_SMBUS_BYTE_DATA;
        } else {
            return -EPFNOSUPPORT;
        }
    }

    if (chip.flags & AT24_FLAG_TAKE8ADDR)
        num_addresses = ;
    else
        num_addresses =    DIV_ROUND_UP(chip.byte_len,
            (chip.flags & AT24_FLAG_ADDR16) ?  : );

    at24 = devm_kzalloc(&client->dev, sizeof(struct at24_data) +
        num_addresses * sizeof(struct i2c_client *), GFP_KERNEL);
    if (!at24)
        return -ENOMEM;

    mutex_init(&at24->lock);
    at24->use_smbus = use_smbus;
    at24->chip = chip;
    at24->num_addresses = num_addresses;

    /*
     * Export the EEPROM bytes through sysfs, since that's convenient.
     * By default, only root should see the data (maybe passwords etc)
     */
    sysfs_bin_attr_init(&at24->bin);
    at24->bin.attr.name = "eeprom";
    at24->bin.attr.mode = chip.flags & AT24_FLAG_IRUGO ? S_IRUGO : S_IRUSR;
    at24->bin.read = at24_bin_read;
    at24->bin.size = chip.byte_len;　　　　// 读写函数使用i2c-core.c提供的标准transfer接口

    at24->macc.read = at24_macc_read;

    writable = !(chip.flags & AT24_FLAG_READONLY);
    if (writable) {
        if (!use_smbus || i2c_check_functionality(client->adapter,
                I2C_FUNC_SMBUS_WRITE_I2C_BLOCK)) {

            unsigned write_max = chip.page_size;

            at24->macc.write = at24_macc_write;

            at24->bin.write = at24_bin_write;
            at24->bin.attr.mode |= S_IWUSR;

            if (write_max > io_limit)
                write_max = io_limit;
            if (use_smbus && write_max > I2C_SMBUS_BLOCK_MAX)
                write_max = I2C_SMBUS_BLOCK_MAX;
            at24->write_max = write_max;

            /* buffer (data + address at the beginning) */
            at24->writebuf = devm_kzalloc(&client->dev,
                write_max + , GFP_KERNEL);
            if (!at24->writebuf)
                return -ENOMEM;
        } else {
            dev_warn(&client->dev,
                "cannot write due to controller restrictions.");
        }
    }

    at24->client[] = client;

    /* use dummy devices for multiple-address chips */
    for (i = ; i < num_addresses; i++) {
        at24->client[i] = i2c_new_dummy(client->adapter,
                    client->addr + i);
        if (!at24->client[i]) {
            dev_err(&client->dev, "address 0x%02x unavailable\n",
                    client->addr + i);
            err = -EADDRINUSE;
            goto err_clients;
        }
    }

    err = sysfs_create_bin_file(&client->dev.kobj, &at24->bin);　　// ！！！ 在sysfs中创建文件，读此文件的读写，就是对EEPROM的读写，at24-bin的读写函数与i2c底层操作挂钩
    if (err)
        goto err_clients;

    i2c_set_clientdata(client, at24);

    dev_info(&client->dev, "%zu byte %s EEPROM, %s, %u bytes/write\n",
        at24->bin.size, client->name,
        writable ? "writable" : "read-only", at24->write_max);
    if (use_smbus == I2C_SMBUS_WORD_DATA ||
        use_smbus == I2C_SMBUS_BYTE_DATA) {
        dev_notice(&client->dev, "Falling back to %s reads, "
               "performance will suffer\n", use_smbus ==
               I2C_SMBUS_WORD_DATA ? "word" : "byte");
    }

    /* export data to kernel code */
    if (chip.setup)
        chip.setup(&at24->macc, chip.context);

    return ;

err_clients:
    for (i = ; i < num_addresses; i++)
        if (at24->client[i])
            i2c_unregister_device(at24->client[i]);

    return err;
}

static int at24_remove(struct i2c_client *client)
{
    struct at24_data *at24;
    int i;

    at24 = i2c_get_clientdata(client);
    sysfs_remove_bin_file(&client->dev.kobj, &at24->bin);

    for (i = ; i < at24->num_addresses; i++)
        i2c_unregister_device(at24->client[i]);

    return ;
}

15.5 i2c-dev.c文件分析

采用file_oprations方式，组织标准字符设备驱动，对adapter进行设备化，应用层可以通过read、write函数对adapter进行直接操作。

15.6 总结

研究过i2c驱动以后，再看spi、usb等驱动框架，几乎是一样的，如下表。