[media] media-device.h: Improve documentation and update it

Now that we moved the content of the media-framework.txt into
the kerneldoc documentation, move the per-function specific
documentation to the corresponding functions and clean it up.

It would be good if we had already the markdown kernel-doc
patches merged upstream, but, while we doesn't have it,
let's make it less ugly at device-drivers.xml.

Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>

2 files changed, 339 insertions, 238 deletions

diff --git a/include/media/media-device.h b/include/media/media-device.h
index 6728528df9e2..65fdd44e05ef 100644
--- a/include/media/media-device.h
+++ b/include/media/media-device.h
@@ -33,23 +33,11 @@
 /**
  * DOC: Media Controller
  *
- * Linux kernel media framework
- * ============================
- *
- * This document describes the Linux kernel media framework, its data structures,
- * functions and their usage.
- *
- *
- * Introduction
- * ------------
- *
- * The media controller API is documented in DocBook format in
- * Documentation/DocBook/media/v4l/media-controller.xml. This document will focus
+ * The media controller userspace API is documented in DocBook format in
+ * Documentation/DocBook/media/v4l/media-controller.xml. This document focus
  * on the kernel-side implementation of the media framework.
  *
- *
- * Abstract media device model
- * ---------------------------
+ * * Abstract media device model:
  *
  * Discovering a device internal topology, and configuring it at runtime, is one
  * of the goals of the media framework. To achieve this, hardware devices are
@@ -72,195 +60,104 @@
  * pad to a sink pad.
  *
  *
- * Media device
- * ------------
+ * * Media device:
  *
- * A media device is represented by a struct media_device instance, defined in
+ * A media device is represented by a struct &media_device instance, defined in
  * include/media/media-device.h. Allocation of the structure is handled by the
- * media device driver, usually by embedding the media_device instance in a
+ * media device driver, usually by embedding the &media_device instance in a
  * larger driver-specific structure.
  *
  * Drivers register media device instances by calling
+ *	__media_device_register() via the macro media_device_register()
+ * and unregistered by calling
+ *	media_device_unregister().
  *
- * 	media_device_register(struct media_device *mdev);
- *
- * The caller is responsible for initializing the media_device structure before
- * registration. The following fields must be set:
- *
- *  - dev must point to the parent device (usually a pci_dev, usb_interface or
- *    platform_device instance).
- *
- *  - model must be filled with the device model name as a NUL-terminated UTF-8
- *    string. The device/model revision must not be stored in this field.
- *
- * The following fields are optional:
- *
- *  - serial is a unique serial number stored as a NUL-terminated ASCII string.
- *    The field is big enough to store a GUID in text form. If the hardware
- *    doesn't provide a unique serial number this field must be left empty.
- *
- *  - bus_info represents the location of the device in the system as a
- *    NUL-terminated ASCII string. For PCI/PCIe devices bus_info must be set to
- *    "PCI:" (or "PCIe:") followed by the value of pci_name(). For USB devices,
- *    the usb_make_path() function must be used. This field is used by
- *    applications to distinguish between otherwise identical devices that don't
- *    provide a serial number.
- *
- *  - hw_revision is the hardware device revision in a driver-specific format.
- *    When possible the revision should be formatted with the KERNEL_VERSION
- *    macro.
- *
- *  - driver_version is formatted with the KERNEL_VERSION macro. The version
- *    minor must be incremented when new features are added to the userspace API
- *    without breaking binary compatibility. The version major must be
- *    incremented when binary compatibility is broken.
- *
- * Upon successful registration a character device named media[0-9]+ is created.
- * The device major and minor numbers are dynamic. The model name is exported as
- * a sysfs attribute.
- *
- * Drivers unregister media device instances by calling
- *
- * 	media_device_unregister(struct media_device *mdev);
- *
- * Unregistering a media device that hasn't been registered is *NOT* safe.
- *
- *
- * Entities, pads and links
- * ------------------------
+ * * Entities, pads and links:
  *
  * - Entities
  *
- * Entities are represented by a struct media_entity instance, defined in
+ * Entities are represented by a struct &media_entity instance, defined in
  * include/media/media-entity.h. The structure is usually embedded into a
  * higher-level structure, such as a v4l2_subdev or video_device instance,
  * although drivers can allocate entities directly.
  *
  * Drivers initialize entity pads by calling
- *
- * 	media_entity_pads_init(struct media_entity *entity, u16 num_pads,
- * 			  struct media_pad *pads);
- *
- * If no pads are needed, drivers could directly fill entity->num_pads
- * with 0 and entity->pads with NULL or to call the above function that
- * will do the same.
- *
- * The media_entity name, type and flags fields should be initialized before
- * calling media_device_register_entity(). Entities embedded in higher-level
- * standard structures can have some of those fields set by the higher-level
- * framework.
- *
- * As the number of pads is known in advance, the pads array is not allocated
- * dynamically but is managed by the entity driver. Most drivers will embed the
- * pads array in a driver-specific structure, avoiding dynamic allocation.
- *
- * Drivers must set the direction of every pad in the pads array before calling
- * media_entity_pads_init. The function will initialize the other pads fields.
- *
- * Unlike the number of pads, the total number of links isn't always known in
- * advance by the entity driver. As an initial estimate, media_entity_pads_init
- * pre-allocates a number of links equal to the number of pads. The links array
- * will be reallocated if it grows beyond the initial estimate.
+ *	media_entity_pads_init().
  *
  * Drivers register entities with a media device by calling
+ *	media_device_register_entity()
+ * and unregistred by calling
+ *	media_device_unregister_entity().
  *
- * 	media_device_register_entity(struct media_device *mdev,
- * 				     struct media_entity *entity);
+ * - Interfaces
  *
- * Entities are identified by a unique positive integer ID. Drivers can provide an
- * ID by filling the media_entity id field prior to registration, or request the
- * media controller framework to assign an ID automatically. Drivers that provide
- * IDs manually must ensure that all IDs are unique. IDs are not guaranteed to be
- * contiguous even when they are all assigned automatically by the framework.
+ * Interfaces are represented by a struct &media_interface instance, defined in
+ * include/media/media-entity.h. Currently, only one type of interface is
+ * defined: a device node. Such interfaces are represented by a struct
+ * &media_intf_devnode.
  *
- * Drivers unregister entities by calling
- *
- * 	media_device_unregister_entity(struct media_entity *entity);
- *
- * Unregistering an entity will not change the IDs of the other entities, and the
- * ID will never be reused for a newly registered entity.
- *
- * When a media device is unregistered, all its entities are unregistered
- * automatically. No manual entities unregistration is then required.
- *
- * Drivers free resources associated with an entity by calling
- *
- * 	media_entity_cleanup(struct media_entity *entity);
- *
- * This function must be called during the cleanup phase after unregistering the
- * entity. Note that the media_entity instance itself must be freed explicitly by
- * the driver if required.
- *
- * Entities have flags that describe the entity capabilities and state.
- *
- * 	MEDIA_ENT_FL_DEFAULT indicates the default entity for a given type.
- * 	This can be used to report the default audio and video devices or the
- * 	default camera sensor.
- *
- * Logical entity groups can be defined by setting the group ID of all member
- * entities to the same non-zero value. An entity group serves no purpose in the
- * kernel, but is reported to userspace during entities enumeration.
- *
- * Media device drivers should define groups if several entities are logically
- * bound together. Example usages include reporting
- *
- * 	- ALSA, VBI and video nodes that carry the same media stream
- * 	- lens and flash controllers associated with a sensor
+ * Drivers initialize and create device node interfaces by calling
+ *	media_devnode_create()
+ * and remove them by calling:
+ *	media_devnode_remove().
  *
  * - Pads
  *
- * Pads are represented by a struct media_pad instance, defined in
+ * Pads are represented by a struct &media_pad instance, defined in
  * include/media/media-entity.h. Each entity stores its pads in a pads array
  * managed by the entity driver. Drivers usually embed the array in a
  * driver-specific structure.
  *
- * Pads are identified by their entity and their 0-based index in the pads array.
- * Both information are stored in the media_pad structure, making the media_pad
- * pointer the canonical way to store and pass link references.
+ * Pads are identified by their entity and their 0-based index in the pads
+ * array.
+ * Both information are stored in the &media_pad structure, making the
+ * &media_pad pointer the canonical way to store and pass link references.
  *
  * Pads have flags that describe the pad capabilities and state.
  *
- * 	MEDIA_PAD_FL_SINK indicates that the pad supports sinking data.
- * 	MEDIA_PAD_FL_SOURCE indicates that the pad supports sourcing data.
+ *	%MEDIA_PAD_FL_SINK indicates that the pad supports sinking data.
+ *	%MEDIA_PAD_FL_SOURCE indicates that the pad supports sourcing data.
  *
- * One and only one of MEDIA_PAD_FL_SINK and MEDIA_PAD_FL_SOURCE must be set for
- * each pad.
+ * NOTE: One and only one of %MEDIA_PAD_FL_SINK and %MEDIA_PAD_FL_SOURCE must
+ * be set for each pad.
  *
  * - Links
  *
- * Links are represented by a struct media_link instance, defined in
- * include/media/media-entity.h. Each entity stores all links originating at or
- * targeting any of its pads in a links array. A given link is thus stored
- * twice, once in the source entity and once in the target entity. The array is
- * pre-allocated and grows dynamically as needed.
+ * Links are represented by a struct &media_link instance, defined in
+ * include/media/media-entity.h. There are two types of links:
+ *
+ * 1. pad to pad links:
  *
- * Drivers create links by calling
+ * Associate two entities via their PADs. Each entity has a list that points
+ * to all links originating at or targeting any of its pads.
+ * A given link is thus stored twice, once in the source entity and once in
+ * the target entity.
  *
- * 	media_create_pad_link(struct media_entity *source, u16 source_pad,
- * 				 struct media_entity *sink,   u16 sink_pad,
- * 				 u32 flags);
+ * Drivers create pad to pad links by calling:
+ *	media_create_pad_link() and remove with media_entity_remove_links().
  *
- * An entry in the link array of each entity is allocated and stores pointers
- * to source and sink pads.
+ * 2. interface to entity links:
  *
- * Links have flags that describe the link capabilities and state.
+ * Associate one interface to a Link.
  *
- * 	MEDIA_LNK_FL_ENABLED indicates that the link is enabled and can be used
- * 	to transfer media data. When two or more links target a sink pad, only
- * 	one of them can be enabled at a time.
- * 	MEDIA_LNK_FL_IMMUTABLE indicates that the link enabled state can't be
- * 	modified at runtime. If MEDIA_LNK_FL_IMMUTABLE is set, then
- * 	MEDIA_LNK_FL_ENABLED must also be set since an immutable link is always
- * 	enabled.
+ * Drivers create interface to entity links by calling:
+ *	media_create_intf_link() and remove with media_remove_intf_links().
  *
+ * NOTE:
  *
- * Graph traversal
- * ---------------
+ * Links can only be created after having both ends already created.
+ *
+ * Links have flags that describe the link capabilities and state. The
+ * valid values are described at media_create_pad_link() and
+ * media_create_intf_link().
+ *
+ * Graph traversal:
  *
  * The media framework provides APIs to iterate over entities in a graph.
  *
- * To iterate over all entities belonging to a media device, drivers can use the
- * media_device_for_each_entity macro, defined in include/media/media-device.h.
+ * To iterate over all entities belonging to a media device, drivers can use
+ * the media_device_for_each_entity macro, defined in
+ * include/media/media-device.h.
  *
  * 	struct media_entity *entity;
  *
@@ -279,126 +176,82 @@
  * currently defined as 16.
  *
  * Drivers initiate a graph traversal by calling
- *
- * 	media_entity_graph_walk_start(struct media_entity_graph *graph,
- * 				      struct media_entity *entity);
+ *	media_entity_graph_walk_start()
  *
  * The graph structure, provided by the caller, is initialized to start graph
  * traversal at the given entity.
  *
  * Drivers can then retrieve the next entity by calling
- *
- * 	media_entity_graph_walk_next(struct media_entity_graph *graph);
+ *	media_entity_graph_walk_next()
  *
  * When the graph traversal is complete the function will return NULL.
  *
- * Graph traversal can be interrupted at any moment. No cleanup function call is
- * required and the graph structure can be freed normally.
+ * Graph traversal can be interrupted at any moment. No cleanup function call
+ * is required and the graph structure can be freed normally.
  *
  * Helper functions can be used to find a link between two given pads, or a pad
  * connected to another pad through an enabled link
+ *	media_entity_find_link() and media_entity_remote_pad()
  *
- * 	media_entity_find_link(struct media_pad *source,
- * 			       struct media_pad *sink);
- *
- * 	media_entity_remote_pad(struct media_pad *pad);
- *
- * Refer to the kerneldoc documentation for more information.
- *
- *
- * Use count and power handling
- * ----------------------------
- *
- * Due to the wide differences between drivers regarding power management needs,
- * the media controller does not implement power management. However, the
- * media_entity structure includes a use_count field that media drivers can use to
- * track the number of users of every entity for power management needs.
+ * Use count and power handling:
  *
- * The use_count field is owned by media drivers and must not be touched by entity
- * drivers. Access to the field must be protected by the media device graph_mutex
- * lock.
+ * Due to the wide differences between drivers regarding power management
+ * needs, the media controller does not implement power management. However,
+ * the &media_entity structure includes a use_count field that media drivers
+ * can use to track the number of users of every entity for power management
+ * needs.
  *
+ * The &media_entity.@use_count field is owned by media drivers and must not be
+ * touched by entity drivers. Access to the field must be protected by the
+ * &media_device.@graph_mutex lock.
  *
- * Links setup
- * -----------
+ * Links setup:
  *
  * Link properties can be modified at runtime by calling
+ *	media_entity_setup_link()
  *
- * 	media_entity_setup_link(struct media_link *link, u32 flags);
- *
- * The flags argument contains the requested new link flags.
- *
- * The only configurable property is the ENABLED link flag to enable/disable a
- * link. Links marked with the IMMUTABLE link flag can not be enabled or disabled.
- *
- * When a link is enabled or disabled, the media framework calls the
- * link_setup operation for the two entities at the source and sink of the link,
- * in that order. If the second link_setup call fails, another link_setup call is
- * made on the first entity to restore the original link flags.
- *
- * Media device drivers can be notified of link setup operations by setting the
- * media_device::link_notify pointer to a callback function. If provided, the
- * notification callback will be called before enabling and after disabling
- * links.
- *
- * Entity drivers must implement the link_setup operation if any of their links
- * is non-immutable. The operation must either configure the hardware or store
- * the configuration information to be applied later.
- *
- * Link configuration must not have any side effect on other links. If an enabled
- * link at a sink pad prevents another link at the same pad from being enabled,
- * the link_setup operation must return -EBUSY and can't implicitly disable the
- * first enabled link.
- *
- *
- * Pipelines and media streams
- * ---------------------------
+ * Pipelines and media streams:
  *
  * When starting streaming, drivers must notify all entities in the pipeline to
  * prevent link states from being modified during streaming by calling
- *
- * 	media_entity_pipeline_start(struct media_entity *entity,
- * 				    struct media_pipeline *pipe);
+ *	media_entity_pipeline_start().
  *
  * The function will mark all entities connected to the given entity through
  * enabled links, either directly or indirectly, as streaming.
  *
- * The media_pipeline instance pointed to by the pipe argument will be stored in
- * every entity in the pipeline. Drivers should embed the media_pipeline structure
- * in higher-level pipeline structures and can then access the pipeline through
- * the media_entity pipe field.
+ * The &media_pipeline instance pointed to by the pipe argument will be stored
+ * in every entity in the pipeline. Drivers should embed the &media_pipeline
+ * structure in higher-level pipeline structures and can then access the
+ * pipeline through the &media_entity pipe field.
  *
- * Calls to media_entity_pipeline_start() can be nested. The pipeline pointer must
- * be identical for all nested calls to the function.
+ * Calls to media_entity_pipeline_start() can be nested. The pipeline pointer
+ * must be identical for all nested calls to the function.
  *
  * media_entity_pipeline_start() may return an error. In that case, it will
  * clean up any of the changes it did by itself.
  *
  * When stopping the stream, drivers must notify the entities with
- *
- * 	media_entity_pipeline_stop(struct media_entity *entity);
+ *	media_entity_pipeline_stop().
  *
  * If multiple calls to media_entity_pipeline_start() have been made the same
- * number of media_entity_pipeline_stop() calls are required to stop streaming. The
- * media_entity pipe field is reset to NULL on the last nested stop call.
+ * number of media_entity_pipeline_stop() calls are required to stop streaming.
+ * The &media_entity pipe field is reset to NULL on the last nested stop call.
  *
- * Link configuration will fail with -EBUSY by default if either end of the link is
- * a streaming entity. Links that can be modified while streaming must be marked
- * with the MEDIA_LNK_FL_DYNAMIC flag.
+ * Link configuration will fail with -%EBUSY by default if either end of the
+ * link is a streaming entity. Links that can be modified while streaming must
+ * be marked with the %MEDIA_LNK_FL_DYNAMIC flag.
  *
  * If other operations need to be disallowed on streaming entities (such as
  * changing entities configuration parameters) drivers can explicitly check the
  * media_entity stream_count field to find out if an entity is streaming. This
  * operation must be done with the media_device graph_mutex held.
  *
- *
- * Link validation
- * ---------------
+ * Link validation:
  *
  * Link validation is performed by media_entity_pipeline_start() for any
  * entity which has sink pads in the pipeline. The
- * media_entity::link_validate() callback is used for that purpose. In
- * link_validate() callback, entity driver should check that the properties of
+ * &media_entity.@link_validate() callback is used for that purpose. In
+ * @link_validate() callback, entity driver should check that the properties of
  * the source pad of the connected entity and its own sink pad match. It is up
  * to the type of the entity (and in the end, the properties of the hardware)
  * what matching actually means.
@@ -484,13 +337,113 @@ struct media_device {
 /* media_devnode to media_device */
 #define to_media_device(node) container_of(node, struct media_device, devnode)
 
+/**
+ * __media_device_register() - Registers a media device element
+ *
+ * @mdev:	pointer to struct &media_device
+ * @owner:	should be filled with %THIS_MODULE
+ *
+ * Users, should, instead, call the media_device_register() macro.
+ *
+ * The caller is responsible for initializing the media_device structure before
+ * registration. The following fields must be set:
+ *
+ *  - dev must point to the parent device (usually a &pci_dev, &usb_interface or
+ *    &platform_device instance).
+ *
+ *  - model must be filled with the device model name as a NUL-terminated UTF-8
+ *    string. The device/model revision must not be stored in this field.
+ *
+ * The following fields are optional:
+ *
+ *  - serial is a unique serial number stored as a NUL-terminated ASCII string.
+ *    The field is big enough to store a GUID in text form. If the hardware
+ *    doesn't provide a unique serial number this field must be left empty.
+ *
+ *  - bus_info represents the location of the device in the system as a
+ *    NUL-terminated ASCII string. For PCI/PCIe devices bus_info must be set to
+ *    "PCI:" (or "PCIe:") followed by the value of pci_name(). For USB devices,
+ *    the usb_make_path() function must be used. This field is used by
+ *    applications to distinguish between otherwise identical devices that don't
+ *    provide a serial number.
+ *
+ *  - hw_revision is the hardware device revision in a driver-specific format.
+ *    When possible the revision should be formatted with the KERNEL_VERSION
+ *    macro.
+ *
+ *  - driver_version is formatted with the KERNEL_VERSION macro. The version
+ *    minor must be incremented when new features are added to the userspace API
+ *    without breaking binary compatibility. The version major must be
+ *    incremented when binary compatibility is broken.
+ *
+ * Notes:
+ *
+ * Upon successful registration a character device named media[0-9]+ is created.
+ * The device major and minor numbers are dynamic. The model name is exported as
+ * a sysfs attribute.
+ *
+ * Unregistering a media device that hasn't been registered is *NOT* safe.
+ */
 int __must_check __media_device_register(struct media_device *mdev,
 					 struct module *owner);
 #define media_device_register(mdev) __media_device_register(mdev, THIS_MODULE)
+
+/**
+ * __media_device_unregister() - Unegisters a media device element
+ *
+ * @mdev:	pointer to struct &media_device
+ */
 void media_device_unregister(struct media_device *mdev);
 
+/**
+ * media_device_register_entity() - registers a media entity inside a
+ *	previously registered media device.
+ *
+ * @mdev:	pointer to struct &media_device
+ * @entity:	pointer to struct &media_entity to be registered
+ *
+ * Entities are identified by a unique positive integer ID. The media
+ * controller framework will such ID automatically. IDs are not guaranteed
+ * to be contiguous, and the ID number can change on newer Kernel versions.
+ * So, neither the driver nor userspace should hardcode ID numbers to refer
+ * to the entities, but, instead, use the framework to find the ID, when
+ * needed.
+ *
+ * The media_entity name, type and flags fields should be initialized before
+ * calling media_device_register_entity(). Entities embedded in higher-level
+ * standard structures can have some of those fields set by the higher-level
+ * framework.
+ *
+ * If the device has pads, media_entity_pads_init() should be called before
+ * this function. Otherwise, the &media_entity.@pad and &media_entity.@num_pads
+ * should be zeroed before calling this function.
+ *
+ * Entities have flags that describe the entity capabilities and state:
+ *
+ * %MEDIA_ENT_FL_DEFAULT indicates the default entity for a given type.
+ *	This can be used to report the default audio and video devices or the
+ *	default camera sensor.
+ */
 int __must_check media_device_register_entity(struct media_device *mdev,
 					      struct media_entity *entity);
+
+/*
+ * media_device_unregister_entity() - unregisters a media entity.
+ *
+ * @entity:	pointer to struct &media_entity to be unregistered
+ *
+ * All links associated with the entity and all PADs are automatically
+ * unregistered from the media_device when this function is called.
+ *
+ * Unregistering an entity will not change the IDs of the other entities and
+ * the previoully used ID will never be reused for a newly registered entities.
+ *
+ * When a media device is unregistered, all its entities are unregistered
+ * automatically. No manual entities unregistration is then required.
+ *
+ * Note: the media_entity instance itself must be freed explicitly by
+ * the driver if required.
+ */
 void media_device_unregister_entity(struct media_entity *entity);
 struct media_device *media_device_get_devres(struct device *dev);
 struct media_device *media_device_find_devres(struct device *dev);
diff --git a/include/media/media-entity.h b/include/media/media-entity.h
index e9bc5857899c..51a7353effd0 100644
--- a/include/media/media-entity.h
+++ b/include/media/media-entity.h
@@ -343,18 +343,112 @@ void media_gobj_init(struct media_device *mdev,
 		    struct media_gobj *gobj);
 void media_gobj_remove(struct media_gobj *gobj);
 
+/**
+ * media_entity_pads_init() - Initialize the entity pads
+ *
+ * @entity:	entity where the pads belong
+ * @num_pads:	number of pads to be initialized
+ * @pads:	pads array
+ *
+ * If no pads are needed, drivers could either directly fill
+ * &media_entity->@num_pads with 0 and &media_entity->@pads with NULL or call
+ * this function that will do the same.
+ *
+ * As the number of pads is known in advance, the pads array is not allocated
+ * dynamically but is managed by the entity driver. Most drivers will embed the
+ * pads array in a driver-specific structure, avoiding dynamic allocation.
+ *
+ * Drivers must set the direction of every pad in the pads array before calling
+ * media_entity_pads_init(). The function will initialize the other pads fields.
+ */
 int media_entity_pads_init(struct media_entity *entity, u16 num_pads,
 		      struct media_pad *pads);
 
+/**
+ * media_entity_cleanup() - free resources associated with an entity
+ *
+ * @entity:	entity where the pads belong
+ *
+ * This function must be called during the cleanup phase after unregistering
+ * the entity (currently, it does nothing).
+ */
 static inline void media_entity_cleanup(struct media_entity *entity) {};
 
+/**
+ * media_create_pad_link() - creates a link between two entities.
+ *
+ * @source:	pointer to &media_entity of the source pad.
+ * @source_pad:	number of the source pad in the pads array
+ * @sink:	pointer to &media_entity of the sink pad.
+ * @sink_pad:	number of the sink pad in the pads array.
+ * @flags:	Link flags, as defined in include/uapi/linux/media.h.
+ *
+ * Valid values for flags:
+ * A %MEDIA_LNK_FL_ENABLED flag indicates that the link is enabled and can be
+ *	used to transfer media data. When two or more links target a sink pad,
+ *	only one of them can be enabled at a time.
+ *
+ * A %MEDIA_LNK_FL_IMMUTABLE flag indicates that the link enabled state can't
+ *	be modified at runtime. If %MEDIA_LNK_FL_IMMUTABLE is set, then
+ *	%MEDIA_LNK_FL_ENABLED must also be set since an immutable link is
+ *	always enabled.
+ *
+ * NOTE:
+ *
+ * Before calling this function, media_entity_pads_init() and
+ * media_device_register_entity() should be called previously for both ends.
+ */
 __must_check int media_create_pad_link(struct media_entity *source,
 			u16 source_pad, struct media_entity *sink,
 			u16 sink_pad, u32 flags);
 void __media_entity_remove_links(struct media_entity *entity);
+
+/**
+ * media_entity_remove_links() - remove all links associated with an entity
+ *
+ * @entity:	pointer to &media_entity
+ *
+ * Note: this is called automatically when an entity is unregistered via
+ * media_device_register_entity().
+ */
 void media_entity_remove_links(struct media_entity *entity);
 
 int __media_entity_setup_link(struct media_link *link, u32 flags);
+
+/**
+ * media_entity_setup_link() - changes the link flags properties in runtime
+ *
+ * @link:	pointer to &media_link
+ * @flags:	the requested new link flags
+ *
+ * The only configurable property is the %MEDIA_LNK_FL_ENABLED link flag
+ * flag to enable/disable a link. Links marked with the
+ * %MEDIA_LNK_FL_IMMUTABLE link flag can not be enabled or disabled.
+ *
+ * When a link is enabled or disabled, the media framework calls the
+ * link_setup operation for the two entities at the source and sink of the
+ * link, in that order. If the second link_setup call fails, another
+ * link_setup call is made on the first entity to restore the original link
+ * flags.
+ *
+ * Media device drivers can be notified of link setup operations by setting the
+ * media_device::link_notify pointer to a callback function. If provided, the
+ * notification callback will be called before enabling and after disabling
+ * links.
+ *
+ * Entity drivers must implement the link_setup operation if any of their links
+ * is non-immutable. The operation must either configure the hardware or store
+ * the configuration information to be applied later.
+ *
+ * Link configuration must not have any side effect on other links. If an
+ * enabled link at a sink pad prevents another link at the same pad from
+ * being enabled, the link_setup operation must return -EBUSY and can't
+ * implicitly disable the first enabled link.
+ *
+ * NOTE: the valid values of the flags for the link is the same as described
+ * on media_create_pad_link(), for pad to pad links or the same as described
+ * on media_create_intf_link(), for interface to entity links.
+ */
 int media_entity_setup_link(struct media_link *link, u32 flags);
 struct media_link *media_entity_find_link(struct media_pad *source,
 		struct media_pad *sink);
@@ -371,18 +465,72 @@ __must_check int media_entity_pipeline_start(struct media_entity *entity,
 					     struct media_pipeline *pipe);
 void media_entity_pipeline_stop(struct media_entity *entity);
 
+/**
+ * media_devnode_create() - creates and initializes a device node interface
+ *
+ * @mdev:	pointer to struct &media_device
+ * @type:	type of the interface, as given by MEDIA_INTF_T_* macros
+ *		as defined in the uapi/media/media.h header.
+ * @flags:	Interface flags as defined in uapi/media/media.h.
+ * @major:	Device node major number.
+ * @minor:	Device node minor number.
+ *
+ * Return: if succeeded, returns a pointer to the newly allocated
+ *	&media_intf_devnode pointer.
+ */
 struct media_intf_devnode *
 __must_check media_devnode_create(struct media_device *mdev,
 				  u32 type, u32 flags,
 				  u32 major, u32 minor);
+/**
+ * media_devnode_remove() - removes a device node interface
+ *
+ * @devnode:	pointer to &media_intf_devnode to be freed.
+ *
+ * When a device node interface is removed, all links to it are automatically
+ * removed.
+ */
 void media_devnode_remove(struct media_intf_devnode *devnode);
 struct media_link *
+
+/**
+ * media_create_intf_link() - creates a link between an entity and an interface
+ *
+ * @entity:	pointer to %media_entity
+ * @intf:	pointer to %media_interface
+ * @flags:	Link flags, as defined in include/uapi/linux/media.h.
+ *
+ *
+ * Valid values for flags:
+ * The %MEDIA_LNK_FL_ENABLED flag indicates that the interface is connected to
+ *	the entity hardware. That's the default value for interfaces. An
+ *	interface may be disabled if the hardware is busy due to the usage
+ *	of some other interface that it is currently controlling the hardware.
+ *	A typical example is an hybrid TV device that handle only one type of
+ *	stream on a given time. So, when the digital TV is streaming,
+ *	the V4L2 interfaces won't be enabled, as such device is not able to
+ *	also stream analog TV or radio.
+ *
+ * Note:
+ *
+ * Before calling this function, media_devnode_create() should be called for
+ * the interface and media_device_register_entity() should be called for the
+ * interface that will be part of the link.
+ */
 __must_check media_create_intf_link(struct media_entity *entity,
 				    struct media_interface *intf,
 				    u32 flags);
 void __media_remove_intf_link(struct media_link *link);
 void media_remove_intf_link(struct media_link *link);
 void __media_remove_intf_links(struct media_interface *intf);
+/**
+ * media_remove_intf_links() - remove all links associated with an interface
+ *
+ * @intf:	pointer to &media_interface
+ *
+ * Note: this is called automatically when an entity is unregistered via
+ * media_device_register_entity() and by media_devnode_remove().
+ */
 void media_remove_intf_links(struct media_interface *intf);