The Basic Device Structure
~~~~~~~~~~~~~~~~~~~~~~~~~~
-struct device {
- struct list_head g_list;
- struct list_head node;
- struct list_head bus_list;
- struct list_head driver_list;
- struct list_head intf_list;
- struct list_head children;
- struct device * parent;
-
- char name[DEVICE_NAME_SIZE];
- char bus_id[BUS_ID_SIZE];
-
- spinlock_t lock;
- atomic_t refcount;
-
- struct bus_type * bus;
- struct driver_dir_entry dir;
-
- u32 class_num;
-
- struct device_driver *driver;
- void *driver_data;
- void *platform_data;
-
- u32 current_state;
- unsigned char *saved_state;
-
- void (*release)(struct device * dev);
-};
-
-Fields
-~~~~~~
-g_list: Node in the global device list.
-
-node: Node in device's parent's children list.
-
-bus_list: Node in device's bus's devices list.
-
-driver_list: Node in device's driver's devices list.
-
-intf_list: List of intf_data. There is one structure allocated for
- each interface that the device supports.
-
-children: List of child devices.
-
-parent: *** FIXME ***
-
-name: ASCII description of device.
- Example: " 3Com Corporation 3c905 100BaseTX [Boomerang]"
-
-bus_id: ASCII representation of device's bus position. This
- field should be a name unique across all devices on the
- bus type the device belongs to.
-
- Example: PCI bus_ids are in the form of
- <bus number>:<slot number>.<function number>
- This name is unique across all PCI devices in the system.
-
-lock: Spinlock for the device.
-
-refcount: Reference count on the device.
-
-bus: Pointer to struct bus_type that device belongs to.
-
-dir: Device's sysfs directory.
-
-class_num: Class-enumerated value of the device.
-
-driver: Pointer to struct device_driver that controls the device.
-
-driver_data: Driver-specific data.
-
-platform_data: Platform data specific to the device.
-
-current_state: Current power state of the device.
-
-saved_state: Pointer to saved state of the device. This is usable by
- the device driver controlling the device.
-
-release: Callback to free the device after all references have
- gone away. This should be set by the allocator of the
- device (i.e. the bus driver that discovered the device).
+See the kerneldoc for the struct device.
Programming Interface
Attributes
~~~~~~~~~~
struct device_attribute {
- struct attribute attr;
- ssize_t (*show)(struct device * dev, char * buf, size_t count, loff_t off);
- ssize_t (*store)(struct device * dev, const char * buf, size_t count, loff_t off);
+ struct attribute attr;
+ ssize_t (*show)(struct device *dev, struct device_attribute *attr,
+ char *buf);
+ ssize_t (*store)(struct device *dev, struct device_attribute *attr,
+ const char *buf, size_t count);
};
-Attributes of devices can be exported via drivers using a simple
-procfs-like interface.
+Attributes of devices can be exported by a device driver through sysfs.
Please see Documentation/filesystems/sysfs.txt for more information
on how sysfs works.
+As explained in Documentation/kobject.txt, device attributes must be be
+created before the KOBJ_ADD uevent is generated. The only way to realize
+that is by defining an attribute group.
+
Attributes are declared using a macro called DEVICE_ATTR:
#define DEVICE_ATTR(name,mode,show,store)
Example:
-DEVICE_ATTR(power,0644,show_power,store_power);
+static DEVICE_ATTR(type, 0444, show_type, NULL);
+static DEVICE_ATTR(power, 0644, show_power, store_power);
-This declares a structure of type struct device_attribute named
-'dev_attr_power'. This can then be added and removed to the device's
-directory using:
+This declares two structures of type struct device_attribute with respective
+names 'dev_attr_type' and 'dev_attr_power'. These two attributes can be
+organized as follows into a group:
-int device_create_file(struct device *device, struct device_attribute * entry);
-void device_remove_file(struct device * dev, struct device_attribute * attr);
+static struct attribute *dev_attrs[] = {
+ &dev_attr_type.attr,
+ &dev_attr_power.attr,
+ NULL,
+};
-Example:
+static struct attribute_group dev_attr_group = {
+ .attrs = dev_attrs,
+};
+
+static const struct attribute_group *dev_attr_groups[] = {
+ &dev_attr_group,
+ NULL,
+};
+
+This array of groups can then be associated with a device by setting the
+group pointer in struct device before device_register() is invoked:
+
+ dev->groups = dev_attr_groups;
+ device_register(dev);
-device_create_file(dev,&dev_attr_power);
-device_remove_file(dev,&dev_attr_power);
+The device_register() function will use the 'groups' pointer to create the
+device attributes and the device_unregister() function will use this pointer
+to remove the device attributes.
-The file name will be 'power' with a mode of 0644 (-rw-r--r--).
+Word of warning: While the kernel allows device_create_file() and
+device_remove_file() to be called on a device at any time, userspace has
+strict expectations on when attributes get created. When a new device is
+registered in the kernel, a uevent is generated to notify userspace (like
+udev) that a new device is available. If attributes are added after the
+device is registered, then userspace won't get notified and userspace will
+not know about the new attributes.
+This is important for device driver that need to publish additional
+attributes for a device at driver probe time. If the device driver simply
+calls device_create_file() on the device structure passed to it, then
+userspace will never be notified of the new attributes.
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