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);
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。
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。
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) {
printk(KERN_ERR "%s: driver_add_attrs(%s) failed\n",
__FUNCTION__, drv->name);
}
error = add_bind_files(drv);
if (error) {
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<<Linux设备驱动程序>>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。
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;
if (dev->parent)
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。
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) {
printk(KERN_WARNING
"%s: probe of %s failed with error %d\n",
drv->name, dev->bus_id, ret);
}
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
void platform_driver_unregister(struct platform_driver *drv)
{
driver_unregister(&drv->driver);
}
EXPORT_SYMBOL_GPL(platform_driver_unregister);
2.3 driver_unregister()函数
void driver_unregister(struct device_driver * drv)
{
bus_remove_driver(drv);
if (drv->owner)
wait_for_completion(&drv->unloaded);
}
2.4 bus_remove_driver()函数
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
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)
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
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。
3. 设备注册过程
3.1 at91_add_device_i2c()函数
在文件arch/arm/mach-at91/at91sam9263_devices.c中，对I2C设备进行了定义。
#if defined(CONFIG_I2C_AT91) || defined(CONFIG_I2C_AT91_MODULE)
static struct resource twi_resources[] = {
[0] = {
.start = AT91SAM9263_BASE_TWI,
.end = AT91SAM9263_BASE_TWI + SZ_16K - 1,
.flags = IORESOURCE_MEM,
},
[1] = {
.start = AT91SAM9263_ID_TWI,
.end = AT91SAM9263_ID_TWI,
.flags = IORESOURCE_IRQ,
},
};
static struct platform_device at91sam9263_twi_device = {
//此名字与驱动信息中的名字"at91_i2c"是一致的，在match()函数调用中，会返回1。
.name = "at91_i2c",
.id = -1,
.resource = twi_resources,
.num_resources = ARRAY_SIZE(twi_resources),
};
void __init at91_add_device_i2c(void)
{
at91_set_A_periph(AT91_PIN_PB4, 0);
at91_set_multi_drive(AT91_PIN_PB4, 1);
at91_set_A_periph(AT91_PIN_PB5, 0);
at91_set_multi_drive(AT91_PIN_PB5, 1);
//对设备进行注册
platform_device_register(&at91sam9263_twi_device);
}
#else
void __init at91_add_device_i2c(void) {}
#endif
3.2 platform.c文件
在文件drivers/base/platform.c中，实现了下面的函数。
3.2.1 platform_device_register()函数
int platform_device_register(struct platform_device * pdev)
{
device_initialize(&pdev->dev);
return platform_device_add(pdev);
}
EXPORT_SYMBOL_GPL(platform_device_register);
3.2.2 platform_device_add()函数
int platform_device_add(struct platform_device *pdev)
{
int i, ret = 0;
if (!pdev)
return -EINVAL;
if (!pdev->dev.parent)
pdev->dev.parent = &platform_bus;
pdev->dev.bus = &platform_bus_type;
if (pdev->id != -1)
snprintf(pdev->dev.bus_id, BUS_ID_SIZE, "%s.%u", pdev->name, pdev->id);
else
strlcpy(pdev->dev.bus_id, pdev->name, BUS_ID_SIZE);
for (i = 0; i < pdev->num_resources; i++) {
struct resource *p, *r = &pdev->resource[i];
if (r->name == NULL)
r->name = pdev->dev.bus_id;
p = r->parent;
if (!p) {
if (r->flags & IORESOURCE_MEM)
p = &iomem_resource;
else if (r->flags & IORESOURCE_IO)
p = &ioport_resource;
}
if (p && insert_resource(p, r)) {
printk(KERN_ERR
"%s: failed to claim resource %d\n",
pdev->dev.bus_id, i);
ret = -EBUSY;
goto failed;
}
}
pr_debug("Registering platform device '%s'. Parent at %s\n",
pdev->dev.bus_id, pdev->dev.parent->bus_id);
ret = device_add(&pdev->dev);
if (ret == 0)
return ret;
failed:
while (--i >= 0)
if (pdev->resource[i].flags & (IORESOURCE_MEM|IORESOURCE_IO))
release_resource(&pdev->resource[i]);
return ret;
}
EXPORT_SYMBOL_GPL(platform_device_add);
3.3 device_add()函数
在文件drivers/base/core.c中实现了device_add()函数。
int device_add(struct device *dev)
{
struct device *parent = NULL;
char *class_name = NULL;
struct class_interface *class_intf;
int error = -EINVAL;
dev = get_device(dev);
if (!dev || !strlen(dev->bus_id))
goto Error;
pr_debug("DEV: registering device: ID = '%s'\n", dev->bus_id);
parent = get_device(dev->parent);
error = setup_parent(dev, parent);
if (error)
goto Error;
kobject_set_name(&dev->kobj, "%s", dev->bus_id);
error = kobject_add(&dev->kobj);
if (error)
goto Error;
if (platform_notify)
platform_notify(dev);
if (dev->bus)
blocking_notifier_call_chain(&dev->bus->bus_notifier,
BUS_NOTIFY_ADD_DEVICE, dev);
dev->uevent_attr.attr.name = "uevent";
dev->uevent_attr.attr.mode = S_IWUSR;
if (dev->driver)
dev->uevent_attr.attr.owner = dev->driver->owner;
dev->uevent_attr.store = store_uevent;
error = device_create_file(dev, &dev->uevent_attr);
if (error)
goto attrError;
if (MAJOR(dev->devt)) {
struct device_attribute *attr;
attr = kzalloc(sizeof(*attr), GFP_KERNEL);
if (!attr) {
error = -ENOMEM;
goto ueventattrError;
}
attr->attr.name = "dev";
attr->attr.mode = S_IRUGO;
if (dev->driver)
attr->attr.owner = dev->driver->owner;
attr->show = show_dev;
error = device_create_file(dev, attr);
if (error) {
kfree(attr);
goto ueventattrError;
}
dev->devt_attr = attr;
}
if (dev->class) {
sysfs_create_link(&dev->kobj, &dev->class->subsys.kset.kobj,
"subsystem");
if (dev->kobj.parent != &dev->class->subsys.kset.kobj)
sysfs_create_link(&dev->class->subsys.kset.kobj,
&dev->kobj, dev->bus_id);
if (parent) {
sysfs_create_link(&dev->kobj, &dev->parent->kobj,
"device");
#ifdef CONFIG_SYSFS_DEPRECATED
class_name = make_class_name(dev->class->name,
&dev->kobj);
if (class_name)
sysfs_create_link(&dev->parent->kobj,
&dev->kobj, class_name);
#endif
}
}
if ((error = device_add_attrs(dev)))
goto AttrsError;
if ((error = device_add_groups(dev)))
goto GroupError;
if ((error = device_pm_add(dev)))
goto PMError;
if ((error = bus_add_device(dev)))
goto BusError;
if (!dev->uevent_suppress)
kobject_uevent(&dev->kobj, KOBJ_ADD);
if ((error = bus_attach_device(dev)))
goto AttachError;
if (parent)
klist_add_tail(&dev->knode_parent, &parent->klist_children);
if (dev->class) {
down(&dev->class->sem);
list_add_tail(&dev->node, &dev->class->devices);
list_for_each_entry(class_intf, &dev->class->interfaces, node)
if (class_intf->add_dev)
class_intf->add_dev(dev, class_intf);
up(&dev->class->sem);
}
Done:
kfree(class_name);
put_device(dev);
return error;
AttachError:
bus_remove_device(dev);
BusError:
device_pm_remove(dev);
PMError:
if (dev->bus)
blocking_notifier_call_chain(&dev->bus->bus_notifier,
BUS_NOTIFY_DEL_DEVICE, dev);
device_remove_groups(dev);
GroupError:
device_remove_attrs(dev);
AttrsError:
if (dev->devt_attr) {
device_remove_file(dev, dev->devt_attr);
kfree(dev->devt_attr);
}
if (dev->class) {
sysfs_remove_link(&dev->kobj, "subsystem");
if (dev->kobj.parent != &dev->class->subsys.kset.kobj)
sysfs_remove_link(&dev->class->subsys.kset.kobj,
dev->bus_id);
if (parent) {
#ifdef CONFIG_SYSFS_DEPRECATED
char *class_name = make_class_name(dev->class->name,
&dev->kobj);
if (class_name)
sysfs_remove_link(&dev->parent->kobj,
class_name);
kfree(class_name);
#endif
sysfs_remove_link(&dev->kobj, "device");
}
down(&dev->class->sem);
list_for_each_entry(class_intf, &dev->class->interfaces, node)
if (class_intf->remove_dev)
class_intf->remove_dev(dev, class_intf);
list_del_init(&dev->node);
up(&dev->class->sem);
}
ueventattrError:
device_remove_file(dev, &dev->uevent_attr);
attrError:
kobject_uevent(&dev->kobj, KOBJ_REMOVE);
kobject_del(&dev->kobj);
Error:
if (parent)
put_device(parent);
goto Done;
}
3.4 bus_attach_device()函数
在文件drivers/base/bus.c中实现了bus_attach_device()函数。
int bus_attach_device(struct device * dev)
{
struct bus_type *bus = dev->bus;
int ret = 0;
if (bus) {
dev->is_registered = 1;
ret = device_attach(dev);
if (ret >= 0) {
//将设备信息存储在bus结构中。在驱动注册后，利用bus_for_each_dev()函数可以得到所有设备的
//信息，完成驱动与设备之间的绑定。
klist_add_tail(&dev->knode_bus, &bus->klist_devices);
ret = 0;
} else
dev->is_registered = 0;
}
return ret;
}
3.5 dd.c文件
在文件drivers/base/dd.c中实现了device_attach()函数。
3.5.1 device_attach()函数
int device_attach(struct device * dev)
{
int ret = 0;
down(&dev->sem);
if (dev->driver) {
ret = device_bind_driver(dev);
if (ret == 0)
ret = 1;
} else
ret = bus_for_each_drv(dev->bus, NULL, dev, __device_attach);
up(&dev->sem);
return ret;
}
static int __device_attach(struct device_driver * drv, void * data)
{
struct device * dev = data;
//在此之后，利用驱动信息来探测设备，参考第一部分的介绍
return driver_probe_device(drv, dev);
}
3.5.2 device_bind_driver()函数
int device_bind_driver(struct device *dev)
{
int ret;
ret = driver_sysfs_add(dev);
if (!ret)
driver_bound(dev); //通知链
return ret;
}
到此，设备与驱动的两种绑定方式：在设备注册时进行绑定及在驱动注册时进行绑定就介绍完了