目录
前言：
一、源码分析
1、device_create()
2、device_register
3、device_add
二、device_create()、device_register()、device_add()使用中的区别
1、device_create()
2、device_register()
3、device_add()
前言：
Linux设备驱动开发中，经常会用到device_create()函数创建设备节点，除了device_create()内核源码中也经常用device_register()和device_add()创建设备节点。
本文主要讲解device_create()、device_register()、deivce_add()的区别。
一、源码分析
1、device_create()
device_create函数是我们写设备驱动经常用到的函数，它的功能是创建一个设备，并注册它到sysfs中，源码如下：
/* 源码路径：drivers/base/core.c */
struct device *device_create(struct class *class, struct device *parent,dev_t devt, void *drvdata, const char *fmt, ...)
{va_list vargs;struct device *dev;va_start(vargs, fmt);dev = device_create_vargs(class, parent, devt, drvdata, fmt, vargs);va_end(vargs);return dev;
}
device_create() 最终调用 device_create_groups_vargs()函数，device_create_groups_vargs功能是创建一个device并且将它注册到sysfs中
device_create()
--->device_create_vargs()
--->--->device_create_groups_vargs()device_create_groups_vargs()函数源码：
static __printf(6, 0) struct device *
device_create_groups_vargs(struct class *class, struct device *parent,dev_t devt, void *drvdata,const struct attribute_group **groups,const char *fmt, va_list args)
{struct device *dev = NULL;int retval = -ENODEV;if (class == NULL || IS_ERR(class))goto error;dev = kzalloc(sizeof(*dev), GFP_KERNEL);if (!dev) {retval = -ENOMEM;goto error;}device_initialize(dev); //初始化devicedev->devt = devt;dev->class = class;dev->parent = parent;dev->groups = groups;dev->release = device_create_release;dev_set_drvdata(dev, drvdata);retval = kobject_set_name_vargs(&dev->kobj, fmt, args); //设置device节点名字if (retval)goto error;retval = device_add(dev);if (retval)goto error;return dev;error:put_device(dev);return ERR_PTR(retval);
}可以看到，device_create()是device_add()的封装，最终还是调用 device_add()
2、device_register
/* drivers/base/core.c */
int device_register(struct device *dev)
{device_initialize(dev); //初始化devicereturn device_add(dev);
}device_register()也是device_add()的封装。
3、device_add
device_add()会在/sys目录对应设备目录下创建uevent属性节点，应用层的udev则会根据uevent来创建/dev目录下的设备节点，这里关于udev的部分不再赘述
/* drivers/base/core.c */
int device_add(struct device *dev)
{struct device *parent;struct kobject *kobj;struct class_interface *class_intf;int error = -EINVAL;struct kobject *glue_dir = NULL;dev = get_device(dev);if (!dev)goto done;if (!dev->p) {error = device_private_init(dev);if (error)goto done;}/** for statically allocated devices, which should all be converted* some day, we need to initialize the name. We prevent reading back* the name, and force the use of dev_name()*/if (dev->init_name) {dev_set_name(dev, "%s", dev->init_name);dev->init_name = NULL;}/* subsystems can specify simple device enumeration */if (!dev_name(dev) && dev->bus && dev->bus->dev_name)dev_set_name(dev, "%s%u", dev->bus->dev_name, dev->id);if (!dev_name(dev)) {error = -EINVAL;goto name_error;}pr_debug("device: '%s': %s\n", dev_name(dev), __func__);parent = get_device(dev->parent);kobj = get_device_parent(dev, parent);if (IS_ERR(kobj)) {error = PTR_ERR(kobj);goto parent_error;}if (kobj)dev->kobj.parent = kobj;/* use parent numa_node */if (parent && (dev_to_node(dev) == NUMA_NO_NODE))set_dev_node(dev, dev_to_node(parent));/* first, register with generic layer. *//* we require the name to be set before, and pass NULL */error = kobject_add(&dev->kobj, dev->kobj.parent, NULL);if (error) {glue_dir = get_glue_dir(dev);goto Error;}/* notify platform of device entry */error = device_platform_notify(dev, KOBJ_ADD);if (error)goto platform_error;error = device_create_file(dev, &dev_attr_uevent);if (error)goto attrError;error = device_add_class_symlinks(dev);if (error)goto SymlinkError;error = device_add_attrs(dev);if (error)goto AttrsError;error = bus_add_device(dev);if (error)goto BusError;error = dpm_sysfs_add(dev);if (error)goto DPMError;device_pm_add(dev);if (MAJOR(dev->devt)) {error = device_create_file(dev, &dev_attr_dev);if (error)goto DevAttrError;error = device_create_sys_dev_entry(dev);if (error)goto SysEntryError;devtmpfs_create_node(dev);}/* Notify clients of device addition. This call must come* after dpm_sysfs_add() and before kobject_uevent().*/if (dev->bus)blocking_notifier_call_chain(&dev->bus->p->bus_notifier,BUS_NOTIFY_ADD_DEVICE, dev);kobject_uevent(&dev->kobj, KOBJ_ADD);/** Check if any of the other devices (consumers) have been waiting for* this device (supplier) to be added so that they can create a device* link to it.** This needs to happen after device_pm_add() because device_link_add()* requires the supplier be registered before it's called.** But this also needs to happen before bus_probe_device() to make sure* waiting consumers can link to it before the driver is bound to the* device and the driver sync_state callback is called for this device.*/if (dev->fwnode && !dev->fwnode->dev) {dev->fwnode->dev = dev;fw_devlink_link_device(dev);}bus_probe_device(dev);if (parent)klist_add_tail(&dev->p->knode_parent,&parent->p->klist_children);if (dev->class) {mutex_lock(&dev->class->p->mutex);/* tie the class to the device */klist_add_tail(&dev->p->knode_class,&dev->class->p->klist_devices);/* notify any interfaces that the device is here */list_for_each_entry(class_intf,&dev->class->p->interfaces, node)if (class_intf->add_dev)class_intf->add_dev(dev, class_intf);mutex_unlock(&dev->class->p->mutex);}
done:put_device(dev);return error;SysEntryError:if (MAJOR(dev->devt))device_remove_file(dev, &dev_attr_dev);DevAttrError:device_pm_remove(dev);dpm_sysfs_remove(dev);DPMError:bus_remove_device(dev);BusError:device_remove_attrs(dev);AttrsError:device_remove_class_symlinks(dev);SymlinkError:device_remove_file(dev, &dev_attr_uevent);attrError:device_platform_notify(dev, KOBJ_REMOVE);
platform_error:kobject_uevent(&dev->kobj, KOBJ_REMOVE);glue_dir = get_glue_dir(dev);kobject_del(&dev->kobj);Error:cleanup_glue_dir(dev, glue_dir);
parent_error:put_device(parent);
name_error:kfree(dev->p);dev->p = NULL;goto done;
}二、device_create()、device_register()、device_add()使用中的区别
分别用device_create()、device_register()、device_add()来创建设备节点“/dev/led”，实现相同的功能能直观看到使用它们的区别。
1、device_create()
static class *led_class;static int __init led_init(void)
{int ret;dev_t devno;struct cdev *cdev;struct dev *dev;/* 注册设备号 */ret = alloc_chrdev_region(&devno, 0, 1, "led");if (ret < 0) return ret;/* 分配、初始化、注册cdev*/cdev = cdev_alloc();if (IS_ERR(cdev)) {ret = PTR_ERR(cdev);goto out_unregister_devno;}cdev_init(&cdev, &led_fops);cdev.owner = THIS_MODULE;ret = cdev_add(&cdev, devno, 1); if (ret) goto out_free_cdev;/* 创建设备类 */led_class = class_create(THIS_MODULE, "led_class");if (IS_ERR(led_class)) {ret = PTR_ERR(led_class);goto out_unregister_cdev;} /* 创建设备节点 */dev = device_create(led_class, NULL, devno, NULL, "led");if (IS_ERR(dev)) {ret = PTR_ERR(dev);goto out_del_class;}return 0;
}module_init(led_init);2、device_register()
static class *led_class;static int __init led_init(void)
{....../* 注册设备号 */....../* 分配、初始化、注册cdev*/....../* 创建设备类 */....../* 创建设备节点 */dev = kzalloc(sizeof(*dev), GFP_KERNEL);if (!dev) {ret = -ENOMEM;goto out_del_class;}dev->class = led_class; // 关联设备类dev->parent = NULL; dev->devt = devno; // 关联设备号dev_set_drvdata(dev, NULL); dev_set_name(dev, "led"); // 设置节点名字dev->release = device_create_release;ret = device_register(dev);if (ret) goto out_put_dev;return 0;
}module_init(led_init);3、device_add()
static class *led_class;static int __init led_init(void)
{....../* 注册设备号 */....../* 分配、初始化、注册cdev*/....../* 创建设备类 */....../* 创建设备节点 */dev = kzalloc(sizeof(*dev), GFP_KERNEL);if (!dev) {ret = -ENOMEM;goto out_del_class;}dev->class = led_class; // 关联设备类dev->parent = NULL; dev->devt = devno; // 关联设备号dev_set_drvdata(dev, NULL); dev_set_name(dev, "led"); // 设置节点名字dev->release = device_create_release;device_initialize(dev);ret = device_add(dev);if (ret) goto out_put_dev;return 0;
}module_init(led_init);
最后看下 dev_set_name 函数源码，它调用kobject_set_name_vargs去设置device节点名字
/* drivers/base/core.c */
int dev_set_name(struct device *dev, const char *fmt, ...)
{va_list vargs;int err;va_start(vargs, fmt);err = kobject_set_name_vargs(&dev->kobj, fmt, vargs);va_end(vargs);return err;
}
