接下来，我们看mmc_block.c，还是从初始化函数mmc_blk_init开始分析：
static int __init mmc_blk_init(void)
{
int res = -ENOMEM;
res = register_blkdev(major, "mmc");
if (res < 0) {
printk(KERN_WARNING "Unable to get major %d for MMC media: %d\n",
major, res);
goto out;
}
if (major == 0)
major = res;
return mmc_register_driver(&mmc_driver);
out:
return res;
}
在mmc_blk_init中, register_blkdev(major, "mmc")的作用是注册一个块设备。如果传递的major为0，这内核将分派一个新的主设备号给设备。
register_blkdev的功能比较少，一是动态分配设备号，二是在/proc/devices中创建一个入口项 。故通过它之后，系统还是不能使用块设备的。
接着我们看
return mmc_register_driver(&mmc_driver);
mmc_register_driver在mmc_sysfs.c中定义:
static struct mmc_driver mmc_driver = {
.drv = {
.name = "mmcblk",
},
.probe = mmc_blk_probe,
.remove = mmc_blk_remove,
.suspend = mmc_blk_suspend,
.resume = mmc_blk_resume,
};
/**
* mmc_register_driver - register a media driver
* @drv: MMC media driver
*/
int mmc_register_driver(struct mmc_driver *drv)
{
drv->drv.bus = &mmc_bus_type;
return driver_register(&drv->drv); //在mmc_bus总线上面注册驱动
}
这两句代码比较好理解吧。在注册一个struct driver之前，都要先设置它的bus，这个bus正是我们刚才core里面注册的那个mmc_bus_type，类似的，在platform_driver_register中我们也可所以看到:
drv->driver.bus=&platform_bus_type
driver_register将到相应总线mmc_bus_type上去搜索相应设备。找到设备后就设置dev->driver=drv,并调用mmc_bus_type总线的probe函数被调用，即mmmc_bus_probe函数.我们跟踪driver_register(&drv->drv)，它会调应bus_add_driver。
/**
* 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, klist_devices_get, klist_devices_put);
init_completion(&drv->unloaded);
return bus_add_driver(drv);
}
/**
* 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) {
pr_debug("bus %s: add driver %s\n", bus->name, drv->name);
error = kobject_set_name(&drv->kobj, "%s", drv->name);
if (error) {
put_bus(bus);
return error;
}
drv->kobj.kset = &bus->drivers;
if ((error = kobject_register(&drv->kobj))) {
put_bus(bus);
return error;
}
driver_attach(drv);
klist_add_tail(&drv->knode_bus, &bus->klist_drivers);
module_add_driver(drv->owner, drv);
driver_add_attrs(bus, drv);
add_bind_files(drv);
}
return error;
}
/**
* 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.
*/
void driver_attach(struct device_driver * drv)
{
bus_for_each_dev(drv->bus, NULL, drv, __driver_attach);
}
/**
* bus_for_each_dev - device iterator.
* @bus: bus type.
* @start: device to start iterating from.
* @data: data for the callback.
* @fn: function to be called for each device.
*
* Iterate over @bus's list of devices, and call @fn for each,
* passing it @data. If @start is not NULL, we use that device to
* begin iterating from.
*
* We check the return of @fn each time. If it returns anything
* other than 0, we break out and return that value.
*
* NOTE: The device that returns a non-zero value is not retained
* in any way, nor is its refcount incremented. If the caller needs
* to retain this data, it should do, and increment the reference
* count in the supplied callback.
*/
int bus_for_each_dev(struct bus_type * bus, struct device * start,
void * data, int (*fn)(struct device *, void *))
{
struct klist_iter i;
struct device * dev;
int error = 0;
if (!bus)
return -EINVAL;
klist_iter_init_node(&bus->klist_devices, &i,
(start ? &start->knode_bus : NULL));
while ((dev = next_device(&i)) && !error)//从总线上面找到每一个device，然后调用
//回调函数fn，现在去看看回调函数
error = fn(dev, data);
klist_iter_exit(&i);
return error;
}
回调函数如下：
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);//dev是刚从bus中取出的device
up(&dev->sem);
if (dev->parent)
up(&dev->parent->sem);
return 0;
}
/**
* driver_probe_device - attempt to bind device & driver.
* @drv: driver.
* @dev: device.
*
* 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)
{
int ret = 0;
//drv是我们当前插入的驱动数据
if (drv->bus->match && !drv->bus->match(dev, drv))//判断我们的bus中是否有match函数,并调用它
goto Done;
pr_debug("%s: Matched Device %s with Driver %s\n",
drv->bus->name, dev->bus_id, drv->name);
dev->driver = drv;
if (dev->bus->probe) {
ret = dev->bus->probe(dev);
if (ret) {
dev->driver = NULL;
goto ProbeFailed;
}
} else if (drv->probe) {
ret = drv->probe(dev);
if (ret) {
dev->driver = NULL;
goto ProbeFailed;
}
}
device_bind_driver(dev); //如果匹配成功，驱动就加载好 了，但是我们现在还没有插入sd卡，干函数不会执行；
ret = 1;
pr_debug("%s: Bound Device %s to Driver %s\n",
drv->bus->name, dev->bus_id, drv->name);
goto Done;
ProbeFailed:
if (ret == -ENODEV || ret == -ENXIO) {
/* Driver matched, but didn't support device
* or device not found.
* Not an error; keep going.
*/
ret = 0;
} else {
/* driver matched but the probe failed */
printk(KERN_WARNING
"%s: probe of %s failed with error %d\n",
drv->name, dev->bus_id, ret);
}
Done:
return ret;
}
在调用kobject_register之后，我们可以在/sys/bus/mmc/driver目录下看到mmcblk文件driver_attach函数会遍历相应总线(mmc_bus_type)上的dev，对这些dev执行总线的match函数(mmc_bus_match)。如果match成功，它会调用mmc_bus_type总线的probe函数mmc_bus_probe(如果总线的probe存在的话).我们在以上走过的流程中可以看到，我们并没有向总线添加任何设备，故mmc_bus_probe是不会调用的。但相应的driver已经注册到系统了。
执行玩block模块在drivers目录你将看到：
# insmod mmc_block.ko
# ls /sys/bus/mmc/drivers/
mmcblk
#