I2C设备与驱动的关联

在Linux操作系统中，驱动程序的加载分为两种：内核启动时自动加载和用户手动加载；硬件设备也可以采用两种方式添加到系统中：在系统启动前及系统运行时的热插拨。下面，我们以arm体系结构下的at91处理器中的I2C控制器为例，介绍一下硬件设备及相关的驱动程序是如何绑定及松绑的。
1.        平台驱动注册过程
1.1 at91_i2c_init()函数
在文件drivers/i2c/busses/i2c-at91.c中，定义了结构体struct platform_driver并进行了初始化，通过使用module_init()宏进行声明，当模块被加载到内核时会调用 at91_i2c_init()函数。在此函数中，调用了platform_driver_register()函数来完成注册。

static struct platform_driver at91_i2c_driver = {
        .probe                = at91_i2c_probe,
        .remove                = __devexit_p(at91_i2c_remove),
        .suspend        = at91_i2c_suspend,
        .resume                = at91_i2c_resume,
        .driver                = {
                .name        = "at91_i2c",
                .owner        = THIS_MODULE,
        },
};

static int __init at91_i2c_init(void)
{
        return platform_driver_register(&at91_i2c_driver);
}
1.2 platform_driver_register()函数
在文件drivers/base/platform.c中，实现并导出了platform_driver_register()函数，以便使其他模块中的函数可以调用此函数。它在完成简单的包装后，调用了driver_register()函数，完成了从平台实现到Linux内核实现的过渡。
    在此，我们需要关注一下platform_match()和platform_drv_probe()函数。platform_match() 函数确定驱动与设备的关联，而platform_drv_probe()函数会在随后介绍的函数中被调用。
//比较驱动信息中的name与设备信息中的name两者是否一致
static int platform_match(struct device * dev, struct device_driver * drv)
{
struct platform_device *pdev = container_of(dev, struct platform_device, dev);

        return (strncmp(pdev->name, drv->name, BUS_ID_SIZE) == 0);
}

struct bus_type platform_bus_type = {
        .name                = "platform",
        .dev_attrs        = platform_dev_attrs,
        .match                = platform_match,
        .uevent                = platform_uevent,
        .suspend        = platform_suspend,
        .suspend_late        = platform_suspend_late,
        .resume_early        = platform_resume_early,
        .resume                = platform_resume,
};
EXPORT_SYMBOL_GPL(platform_bus_type);

/**
*        platform_driver_register
*        @drv: platform driver structure
*/
int platform_driver_register(struct platform_driver *drv)
{
        drv->driver.bus = &platform_bus_type;
        //在really_probe函数中，回调了platform_drv_probe函数
        if (drv->probe)
                drv->driver.probe = platform_drv_probe;
        if (drv->remove)
                drv->driver.remove = platform_drv_remove;
        if (drv->shutdown)
                drv->driver.shutdown = platform_drv_shutdown;
        if (drv->suspend)
                drv->driver.suspend = platform_drv_suspend;
        if (drv->resume)
                drv->driver.resume = platform_drv_resume;
        return driver_register(&drv->driver);
}
EXPORT_SYMBOL_GPL(platform_driver_register);

1.3 driver_register()函数
在文件drivers/base/driver.c中，实现了driver_register()函数。在此函数中，初始化结构体struct device_driver中的klist_device和unloaded字段，通过klist_device字段，可以保存此驱动支持的设备链表，通过“完成”接口机制，完成线程间的同步。链表和“完成”接口的详细信息可以参考文献［1］。返回bus_add_driver()函数的运行结果。

/**
*        driver_register - register driver with bus
*        @drv:        driver to register
*
*        We pass off most of the work to the bus_add_driver() call,
*        since most of the things we have to do deal with the bus
*        structures.
*
*        The one interesting aspect is that we setup @drv->unloaded
*        as a completion that gets complete when the driver reference
*        count reaches 0.
*/
int driver_register(struct device_driver * drv)
{
        if ((drv->bus->probe && drv->probe) ||
            (drv->bus->remove && drv->remove) ||
            (drv->bus->shutdown && drv->shutdown)) {
                printk(KERN_WARNING "Driver '%s' needs updating - please use bus_type methods\n", drv->name);
        }
        klist_init(&drv->klist_devices, NULL, NULL);
        init_completion(&drv->unloaded);
        return bus_add_driver(drv);
}

1.4 bus_add_driver()函数
在文件drivers/base/bus.c中实现了bus_add_driver()函数，它通过语句klist_add_tail(&drv->knode_bus, &bus->klist_drivers); 将驱动信息保存到总线结构中，在设备注册过程中，我们就可以明白此语句的作用了。在此语句之前，调用了driver_attach()函数。
/**
*        bus_add_driver - Add a driver to the bus.
*        @drv:        driver.
*
*/
int bus_add_driver(struct device_driver *drv)
{
        struct bus_type * bus = get_bus(drv->bus);
        int error = 0;

        if (!bus)
                return 0;

        pr_debug("bus %s: add driver %s\n", bus->name, drv->name);
        error = kobject_set_name(&drv->kobj, "%s", drv->name);
        if (error)
                goto out_put_bus;
        drv->kobj.kset = &bus->drivers;
        if ((error = kobject_register(&drv->kobj)))
                goto out_put_bus;

        error = driver_attach(drv);
        if (error)
                goto out_unregister;
        klist_add_tail(&drv->knode_bus, &bus->klist_drivers);
        module_add_driver(drv->owner, drv);

        error = driver_add_attrs(bus, drv);
        if (error) {
                /* How the hell do we get out of this pickle? Give up */
                printk(KERN_ERR "%s: driver_add_attrs(%s) failed\n",
                        __FUNCTION__, drv->name);
        }
        error = add_bind_files(drv);
        if (error) {
                /* Ditto */
                printk(KERN_ERR "%s: add_bind_files(%s) failed\n",
                        __FUNCTION__, drv->name);
        }

        return error;
out_unregister:
        kobject_unregister(&drv->kobj);
out_put_bus:
        put_bus(bus);
        return error;
}

1.5 dd.c文件
在文件drivers/base/dd.c中，实现了设备与驱动交互的核心函数。
1.5.1 driver_attach()函数
函数driver_attach()返回bus_for_each_dev()函数的运行结果。bus_for_each_dev()函数的原型如下：
int bus_for_each_dev(struct bus_type *bus, struct device *start, void *data,
        int (*fn) (struct device *, void *));
该函数迭代了在总线上的每个设备，将相关的device结构传递给fn，同时传递data值。如果start是NULL，将从总线上的第一个设备开始迭代；否则将从start后的第一个设备开始迭代。如果fn返回一个非零值，将停止迭代，而这个值也会从该函数返回（摘自<>第三版）。
该函数是如何知道总线上的每个设备的呢？在设备注册过程中，我会详细介绍。
/*
*        drivers/base/dd.c - The core device/driver interactions.
*
*         This file contains the (sometimes tricky) code that controls the
*        interactions between devices and drivers, which primarily includes
*        driver binding and unbinding.
*/

/**
*        driver_attach - try to bind driver to devices.
*        @drv:        driver.
*
*        Walk the list of devices that the bus has on it and try to
*        match the driver with each one.  If driver_probe_device()
*        returns 0 and the @dev->driver is set, we've found a
*        compatible pair.
*/
int driver_attach(struct device_driver * drv)
{
        return bus_for_each_dev(drv->bus, NULL, drv, __driver_attach);
}
1.5.2 __driver_attach()函数
函数__driver_attach()在调用driver_probe_device()函数前，需要进行线程间的互斥处理。
static int __driver_attach(struct device * dev, void * data)
{
        struct device_driver * drv = data;

        /*
         * Lock device and try to bind to it. We drop the error
         * here and always return 0, because we need to keep trying
         * to bind to devices and some drivers will return an error
         * simply if it didn't support the device.
         *
         * driver_probe_device() will spit a warning if there
         * is an error.
         */

        if (dev->parent)        /* Needed for USB */
                down(&dev->parent->sem);
        down(&dev->sem);
        if (!dev->driver)
                driver_probe_device(drv, dev);
        up(&dev->sem);
        if (dev->parent)
                up(&dev->parent->sem);

        return 0;
}
1.5.3 driver_probe_device()函数
在driver_probe_device()函数中，调用了match函数platform_match()，如果它返回0，表示驱动与设备不一致，函数返回；否则，调用really_probe()函数。
/**
* driver_probe_device - attempt to bind device & driver together
* @drv: driver to bind a device to
* @dev: device to try to bind to the driver
*
* First, we call the bus's match function, if one present, which should
* compare the device IDs the driver supports with the device IDs of the
* device. Note we don't do this ourselves because we don't know the
* format of the ID structures, nor what is to be considered a match and
* what is not.
*
* This function returns 1 if a match is found, an error if one occurs
* (that is not -ENODEV or -ENXIO), and 0 otherwise.
*
* This function must be called with @dev->sem held.  When called for a
* USB interface, @dev->parent->sem must be held as well.
*/
int driver_probe_device(struct device_driver * drv, struct device * dev)
{
        struct stupid_thread_structure *data;
        struct task_struct *probe_task;
        int ret = 0;

        if (!device_is_registered(dev))
                return -ENODEV;
        if (drv->bus->match && !drv->bus->match(dev, drv))
                goto done;

        pr_debug("%s: Matched Device %s with Driver %s\n",
                 drv->bus->name, dev->bus_id, drv->name);

        data = kmalloc(sizeof(*data), GFP_KERNEL);
        if (!data)
                return -ENOMEM;
        data->drv = drv;
        data->dev = dev;

        if (drv->multithread_probe) {
                probe_task = kthread_run(really_probe, data,
                                         "probe-%s", dev->bus_id);
                if (IS_ERR(probe_task))
                        ret = really_probe(data);
        } else
                ret = really_probe(data);

done:
        return ret;
}

struct stupid_thread_structure {
        struct device_driver *drv;
        struct device *dev;
};

1.5.4 really_probe()函数
在really_probe()函数中，实现了设备与驱动的绑定。语句如下：dev->driver = drv;和
ret = drv->probe(dev); probe()函数的实现如下：
include/linux/platform_device.h

#define to_platform_device(x) container_of((x), struct platform_device, dev)

drivers/base/platform.c

#define to_platform_driver(drv)        (container_of((drv), struct platform_driver, driver))


static int platform_drv_probe(struct device *_dev)
{
        struct platform_driver *drv = to_platform_driver(_dev->driver);
        struct platform_device *dev = to_platform_device(_dev);

        return drv->probe(dev);
}
在此函数中，回调了我们在i2c-at91.c文件中实现的探测函数at91_i2c_probe()，至此，平台驱动的注册过程结束。

static atomic_t probe_count = ATOMIC_INIT(0);
static DECLARE_WAIT_QUEUE_HEAD(probe_waitqueue);

static int really_probe(void *void_data)
{
        struct stupid_thread_structure *data = void_data;
        struct device_driver *drv = data->drv;
        struct device *dev = data->dev;
        int ret = 0;

        atomic_inc(&probe_count);
        pr_debug("%s: Probing driver %s with device %s\n",
                 drv->bus->name, drv->name, dev->bus_id);
        WARN_ON(!list_empty(&dev->devres_head));

        dev->driver = drv;
        if (driver_sysfs_add(dev)) {
                printk(KERN_ERR "%s: driver_sysfs_add(%s) failed\n",
                        __FUNCTION__, dev->bus_id);
                goto probe_failed;
        }

        if (dev->bus->probe) {
                ret = dev->bus->probe(dev);
                if (ret)
                        goto probe_failed;
        } else if (drv->probe) {
                ret = drv->probe(dev);
                if (ret)
                        goto probe_failed;
        }
        //设备与驱动绑定后，对系统中已注册的组件进行事件通知。
        driver_bound(dev);
        ret = 1;
        pr_debug("%s: Bound Device %s to Driver %s\n",
                 drv->bus->name, dev->bus_id, drv->name);
        goto done;

probe_failed:
        devres_release_all(dev);
        driver_sysfs_remove(dev);
        dev->driver = NULL;

        if (ret != -ENODEV && ret != -ENXIO) {
                /* driver matched but the probe failed */
                printk(KERN_WARNING
                       "%s: probe of %s failed with error %d\n",
                       drv->name, dev->bus_id, ret);
        }
        /*
         * Ignore errors returned by ->probe so that the next driver can try
         * its luck.
         */
        ret = 0;
done:
        kfree(data);
        atomic_dec(&probe_count);
        wake_up(&probe_waitqueue);
        return ret;
}


















2.        平台驱动卸载过程
平台驱动卸载过程是注册的逆过程，详细信息可参考注册过程进行分析。
2.1 at91_i2c_exit()函数
static void __exit at91_i2c_exit(void)
{
        platform_driver_unregister(&at91_i2c_driver);
}

2.2 platform_driver_unregister()函数

/**
*        platform_driver_unregister
*        @drv: platform driver structure
*/
void platform_driver_unregister(struct platform_driver *drv)
{
        driver_unregister(&drv->driver);
}
EXPORT_SYMBOL_GPL(platform_driver_unregister);

2.3 driver_unregister()函数

/**
*        driver_unregister - remove driver from system.
*        @drv:        driver.
*
*        Again, we pass off most of the work to the bus-level call.
*
*        Though, once that is done, we wait until @drv->unloaded is completed.
*        This will block until the driver refcount reaches 0, and it is
*        released. Only modular drivers will call this function, and we
*        have to guarantee that it won't complete, letting the driver
*        unload until all references are gone.
*/

void driver_unregister(struct device_driver * drv)
{
        bus_remove_driver(drv);
        /*
         * If the driver is a module, we are probably in
         * the module unload path, and we want to wait
         * for everything to unload before we can actually
         * finish the unload.
         */
        if (drv->owner)
                wait_for_completion(&drv->unloaded);
}

2.4 bus_remove_driver()函数
/**
*        bus_remove_driver - delete driver from bus's knowledge.
*        @drv:        driver.
*
*        Detach the driver from the devices it controls, and remove
*        it from its bus's list of drivers. Finally, we drop the reference
*        to the bus we took in bus_add_driver().
*/

void bus_remove_driver(struct device_driver * drv)
{
        if (!drv->bus)
                return;

        remove_bind_files(drv);
        driver_remove_attrs(drv->bus, drv);
        klist_remove(&drv->knode_bus);
        pr_debug("bus %s: remove driver %s\n", drv->bus->name, drv->name);
        driver_detach(drv);
        module_remove_driver(drv);
        kobject_unregister(&drv->kobj);
        put_bus(drv->bus);
}

2.5 driver_detach()函数
/**
* driver_detach - detach driver from all devices it controls.
* @drv: driver.
*/
void driver_detach(struct device_driver * drv)
{
        struct device * dev;

        for (;;) {
                spin_lock(&drv->klist_devices.k_lock);
                if (list_empty(&drv->klist_devices.k_list)) {
                        spin_unlock(&drv->klist_devices.k_lock);
                        break;
                }
                dev = list_entry(drv->klist_devices.k_list.prev,
                                struct device, knode_driver.n_node);
                get_device(dev);
                spin_unlock(&drv->klist_devices.k_lock);

                if (dev->parent)        /* Needed for USB */
                        down(&dev->parent->sem);
                down(&dev->sem);
                if (dev->driver == drv)
                        __device_release_driver(dev);
                up(&dev->sem);
                if (dev->parent)
                        up(&dev->parent->sem);
                put_device(dev);
        }
}

2.6 __device_release_driver()函数
/**
*        device_release_driver - manually detach device from driver.
*        @dev:        device.
*
*        Manually detach device from driver.
*
*        __device_release_driver() must be called with @dev->sem held.
*        When called for a USB interface, @dev->parent->sem must be held
*        as well.
*/

static void __device_release_driver(struct device * dev)
{
        struct device_driver * drv;

        drv = dev->driver;
        if (drv) {
                get_driver(drv);
                driver_sysfs_remove(dev);
                sysfs_remove_link(&dev->kobj, "driver");
                klist_remove(&dev->knode_driver);

                if (dev->bus)
                        blocking_notifier_call_chain(&dev->bus->bus_notifier,
                                                     BUS_NOTIFY_UNBIND_DRIVER,
                                                     dev);

                if (dev->bus && dev->bus->remove)
                        dev->bus->remove(dev);
                else if (drv->remove)
                        drv->remove(dev);
                devres_release_all(dev);
                dev->driver = NULL;
                put_driver(drv);
        }
}

2.7 platform_drv_remove()函数
static int platform_drv_remove(struct device *_dev)
{
        struct platform_driver *drv = to_platform_driver(_dev->driver);
        struct platform_device *dev = to_platform_device(_dev);

        return drv->remove(dev);
}

在此函数中，回调了我们在i2c-at91.c文件中实现的移除函数at91_i2c_remove()，至此，平台驱动的卸载过程结束。

在注册和卸载过程中，Linux采用了一些变量来保存相关的信息，比如引用计数、通知链等，感兴趣的人员，可以详细的阅读此部分内容。