backlight: backlight: Add overview and update existing doc

Add overview chapter to backlight.c.

Update existing kernel-doc to follow a more consistent style and drop
kernel-doc for deprecated functions.

Signed-off-by: Sam Ravnborg <[e-mail]>
Reviewed-by: Daniel Thompson <[e-mail]>
Reviewed-by: Emil Velikov <[e-mail]>
Signed-off-by: Lee Jones <[e-mail]>

drivers/video/backlight/backlight.c

···
 #include <asm/backlight.h>
 #endif
 
+/**
+ * DOC: overview
+ *
+ * The backlight core supports implementing backlight drivers.
+ *
+ * A backlight driver registers a driver using
+ * devm_backlight_device_register(). The properties of the backlight
+ * driver such as type and max_brightness must be specified.
+ * When the core detect changes in for example brightness or power state
+ * the update_status() operation is called. The backlight driver shall
+ * implement this operation and use it to adjust backlight.
+ *
+ * Several sysfs attributes are provided by the backlight core::
+ *
+ * - brightness         R/W, set the requested brightness level
+ * - actual_brightness  RO, the brightness level used by the HW
+ * - max_brightness     RO, the maximum  brightness level supported
+ *
+ * See Documentation/ABI/stable/sysfs-class-backlight for the full list.
+ *
+ * The backlight can be adjusted using the sysfs interface, and
+ * the backlight driver may also support adjusting backlight using
+ * a hot-key or some other platform or firmware specific way.
+ *
+ * The driver must implement the get_brightness() operation if
+ * the HW do not support all the levels that can be specified in
+ * brightness, thus providing user-space access to the actual level
+ * via the actual_brightness attribute.
+ *
+ * When the backlight changes this is reported to user-space using
+ * an uevent connected to the actual_brightness attribute.
+ * When brightness is set by platform specific means, for example
+ * a hot-key to adjust backlight, the driver must notify the backlight
+ * core that brightness has changed using backlight_force_update().
+ *
+ * The backlight driver core receives notifications from fbdev and
+ * if the event is FB_EVENT_BLANK and if the value of blank, from the
+ * FBIOBLANK ioctrl, results in a change in the backlight state the
+ * update_status() operation is called.
+ */
+
 static struct list_head backlight_dev_list;
 static struct mutex backlight_dev_list_mutex;
 static struct blocking_notifier_head backlight_notifier;
···
 
 #if defined(CONFIG_FB) || (defined(CONFIG_FB_MODULE) && \
 			   defined(CONFIG_BACKLIGHT_CLASS_DEVICE_MODULE))
-/* This callback gets called when something important happens inside a
- * framebuffer driver. We're looking if that important event is blanking,
- * and if it is and necessary, we're switching backlight power as well ...
+/*
+ * fb_notifier_callback
+ *
+ * This callback gets called when something important happens inside a
+ * framebuffer driver. The backlight core only cares about FB_BLANK_UNBLANK
+ * which is reported to the driver using backlight_update_status()
+ * as a state change.
+ *
+ * There may be several fbdev's connected to the backlight device,
+ * in which case they are kept track of. A state change is only reported
+ * if there is a change in backlight for the specified fbdev.
  */
 static int fb_notifier_callback(struct notifier_block *self,
 				unsigned long event, void *data)
···
  * @reason: reason for update
  *
  * Updates the internal state of the backlight in response to a hardware event,
- * and generate a uevent to notify userspace
+ * and generates an uevent to notify userspace. A backlight driver shall call
+ * backlight_force_update() when the backlight is changed using, for example,
+ * a hot-key. The updated brightness is read using get_brightness() and the
+ * brightness value is reported using an uevent.
  */
 void backlight_force_update(struct backlight_device *bd,
 			    enum backlight_update_reason reason)
···
 }
 EXPORT_SYMBOL(backlight_force_update);
 
-/**
- * backlight_device_register - create and register a new object of
- *   backlight_device class.
- * @name: the name of the new object(must be the same as the name of the
- *   respective framebuffer device).
- * @parent: a pointer to the parent device
- * @devdata: an optional pointer to be stored for private driver use. The
- *   methods may retrieve it by using bl_get_data(bd).
- * @ops: the backlight operations structure.
- * @props: pointer to backlight's properties structure.
- *
- * Creates and registers new backlight device. Returns either an
- * ERR_PTR() or a pointer to the newly allocated device.
- */
+/* deprecated - use devm_backlight_device_register() */
 struct backlight_device *backlight_device_register(const char *name,
 	struct device *parent, void *devdata, const struct backlight_ops *ops,
 	const struct backlight_properties *props)
···
 }
 EXPORT_SYMBOL(backlight_device_register);
 
+/** backlight_device_get_by_type - find first backlight device of a type
+ * @type: the type of backlight device
+ *
+ * Look up the first backlight device of the specified type
+ *
+ * RETURNS:
+ *
+ * Pointer to backlight device if any was found. Otherwise NULL.
+ */
 struct backlight_device *backlight_device_get_by_type(enum backlight_type type)
 {
 	bool found = false;
···
 }
 EXPORT_SYMBOL(backlight_device_get_by_name);
 
-/**
- * backlight_device_unregister - unregisters a backlight device object.
- * @bd: the backlight device object to be unregistered and freed.
- *
- * Unregisters a previously registered via backlight_device_register object.
- */
+/* deprecated - use devm_backlight_device_unregister() */
 void backlight_device_unregister(struct backlight_device *bd)
 {
 	if (!bd)
···
  * backlight_register_notifier - get notified of backlight (un)registration
  * @nb: notifier block with the notifier to call on backlight (un)registration
  *
- * @return 0 on success, otherwise a negative error code
- *
  * Register a notifier to get notified when backlight devices get registered
  * or unregistered.
+ *
+ * RETURNS:
+ *
+ * 0 on success, otherwise a negative error code
  */
 int backlight_register_notifier(struct notifier_block *nb)
 {
···
  * backlight_unregister_notifier - unregister a backlight notifier
  * @nb: notifier block to unregister
  *
- * @return 0 on success, otherwise a negative error code
- *
  * Register a notifier to get notified when backlight devices get registered
  * or unregistered.
+ *
+ * RETURNS:
+ *
+ * 0 on success, otherwise a negative error code
  */
 int backlight_unregister_notifier(struct notifier_block *nb)
 {
···
 EXPORT_SYMBOL(backlight_unregister_notifier);
 
 /**
- * devm_backlight_device_register - resource managed backlight_device_register()
+ * devm_backlight_device_register - register a new backlight device
  * @dev: the device to register
  * @name: the name of the device
- * @parent: a pointer to the parent device
+ * @parent: a pointer to the parent device (often the same as @dev)
  * @devdata: an optional pointer to be stored for private driver use
  * @ops: the backlight operations structure
  * @props: the backlight properties
  *
- * @return a struct backlight on success, or an ERR_PTR on error
+ * Creates and registers new backlight device. When a backlight device
+ * is registered the configuration must be specified in the @props
+ * parameter. See description of &backlight_properties.
  *
- * Managed backlight_device_register(). The backlight_device returned
- * from this function are automatically freed on driver detach.
- * See backlight_device_register() for more information.
+ * RETURNS:
+ *
+ * struct backlight on success, or an ERR_PTR on error
  */
 struct backlight_device *devm_backlight_device_register(struct device *dev,
 	const char *name, struct device *parent, void *devdata,
···
 EXPORT_SYMBOL(devm_backlight_device_register);
 
 /**
- * devm_backlight_device_unregister - resource managed backlight_device_unregister()
+ * devm_backlight_device_unregister - unregister backlight device
  * @dev: the device to unregister
  * @bd: the backlight device to unregister
  *
- * Deallocated a backlight allocated with devm_backlight_device_register().
+ * Deallocates a backlight allocated with devm_backlight_device_register().
  * Normally this function will not need to be called and the resource management
- * code will ensure that the resource is freed.
+ * code will ensure that the resources are freed.
  */
 void devm_backlight_device_unregister(struct device *dev,
 				struct backlight_device *bd)
···
 }
 
 /**
- * devm_of_find_backlight - Resource-managed of_find_backlight()
- * @dev: Device
+ * devm_of_find_backlight - find backlight for a device
+ * @dev: the device
  *
- * Device managed version of of_find_backlight().
- * The reference on the backlight device is automatically
+ * This function looks for a property named 'backlight' on the DT node
+ * connected to @dev and looks up the backlight device. The lookup is
+ * device managed so the reference to the backlight device is automatically
  * dropped on driver detach.
+ *
+ * RETURNS:
+ *
+ * A pointer to the backlight device if found.
+ * Error pointer -EPROBE_DEFER if the DT property is set, but no backlight
+ * device is found. NULL if there's no backlight property.
  */
 struct backlight_device *devm_of_find_backlight(struct device *dev)
 {