[PATCH v5 10/19] backlight: add overview and update existing doc

Sam Ravnborg
2020-07-19T08:08:20Z

Add overview chapter to backlight.c.
Update existing kernel-doc to follow a more consistent
style and drop kernel-doc for deprecated functions.

v4:
- Include updated devm_of_find_backlight doc
(was accidently included in a later patch)

v3:
- Updated a few editorial details (Daniel)

v2:
- Sevaral editorial corrections that makes reading
much easier (Daniel)
- Spelling fixes (Daniel)
- updated intro chapter with a little more info

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Reviewed-by: Daniel Thompson <daniel.thompson@linaro.org>
Reviewed-by: Emil Velikov <emil.l.velikov@gmail.com>
Cc: Lee Jones <lee.jones@linaro.org>
Cc: Daniel Thompson <daniel.thompson@linaro.org>
Cc: Jingoo Han <jingoohan1@gmail.com>
---
drivers/video/backlight/backlight.c | 140 +++++++++++++++++++---------
1 file changed, 98 insertions(+), 42 deletions(-)

diff --git a/drivers/video/backlight/backlight.c b/drivers/video/backlight/backlight.c
index 18501956dd87..93ae8c63fe4c 100644
--- a/drivers/video/backlight/backlight.c
+++ b/drivers/video/backlight/backlight.c
@@ -22,6 +22,47 @@
#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_brighness 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 platfrom 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 ioclt, 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;
@@ -40,9 +81,17 @@ static const char *const backlight_scale_types[] = {

#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)
@@ -324,7 +373,10 @@ ATTRIBUTE_GROUPS(bl_device);
* @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)
@@ -337,20 +389,7 @@ void backlight_force_update(struct backlight_device *bd,
}
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)
@@ -417,6 +456,15 @@ struct backlight_device *backlight_device_register(const char *name,
}
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;
@@ -456,12 +504,7 @@ struct backlight_device *backlight_device_get_by_name(const char *name)
}
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)
@@ -509,10 +552,12 @@ static int devm_backlight_device_match(struct device *dev, void *res,
* 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)
{
@@ -524,10 +569,12 @@ EXPORT_SYMBOL(backlight_register_notifier);
* 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)
{
@@ -536,20 +583,22 @@ 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,
const struct backlight_ops *ops,
@@ -576,13 +625,13 @@ struct backlight_device *devm_backlight_device_register(struct device *dev,
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)
@@ -673,12 +722,19 @@ static void devm_backlight_release(void *data)
}

/**
- * 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)
{
--
2.25.1