[media] v4l2-dev: add cross-references and improve markup

Add cross-references for the functions/structs and add
the markup tags to improve its display.
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>

Documentation/media/kapi/v4l2-dev.rst

 Video device creation
 =====================
 
-The actual device nodes in the /dev directory are created using the
-video_device struct (v4l2-dev.h). This struct can either be allocated
-dynamically or embedded in a larger struct.
+The actual device nodes in the ``/dev`` directory are created using the
+:c:type:`video_device` struct (``v4l2-dev.h``). This struct can either be
+allocated dynamically or embedded in a larger struct.
 
-To allocate it dynamically use:
+To allocate it dynamically use :cpp:func:`video_device_alloc`:
 
-.. code-block:: none
+.. code-block:: c
 
 	struct video_device *vdev = video_device_alloc();
 
@@ -16,100 +16,110 @@ To allocate it dynamically use:
 
 	vdev->release = video_device_release;
 
-If you embed it in a larger struct, then you must set the release()
+If you embed it in a larger struct, then you must set the ``release()``
 callback to your own function:
 
-.. code-block:: none
+.. code-block:: c
 
 	struct video_device *vdev = &my_vdev->vdev;
 
 	vdev->release = my_vdev_release;
 
-The release callback must be set and it is called when the last user
+The ``release()`` callback must be set and it is called when the last user
 of the video device exits.
 
-The default video_device_release() callback just calls kfree to free the
-allocated memory.
-
-There is also a video_device_release_empty() function that does nothing
-(is empty) and can be used if the struct is embedded and there is nothing
-to do when it is released.
-
-You should also set these fields:
-
-- v4l2_dev: must be set to the v4l2_device parent device.
-
-- name: set to something descriptive and unique.
-
-- vfl_dir: set this to VFL_DIR_RX for capture devices (VFL_DIR_RX has value 0,
-  so this is normally already the default), set to VFL_DIR_TX for output
-  devices and VFL_DIR_M2M for mem2mem (codec) devices.
-
-- fops: set to the v4l2_file_operations struct.
-
-- ioctl_ops: if you use the v4l2_ioctl_ops to simplify ioctl maintenance
-  (highly recommended to use this and it might become compulsory in the
-  future!), then set this to your v4l2_ioctl_ops struct. The vfl_type and
-  vfl_dir fields are used to disable ops that do not match the type/dir
-  combination. E.g. VBI ops are disabled for non-VBI nodes, and output ops
-  are disabled for a capture device. This makes it possible to provide
-  just one v4l2_ioctl_ops struct for both vbi and video nodes.
-
-- lock: leave to NULL if you want to do all the locking in the driver.
-  Otherwise you give it a pointer to a struct mutex_lock and before the
-  unlocked_ioctl file operation is called this lock will be taken by the
-  core and released afterwards. See the next section for more details.
-
-- queue: a pointer to the struct vb2_queue associated with this device node.
-  If queue is non-NULL, and queue->lock is non-NULL, then queue->lock is
-  used for the queuing ioctls (VIDIOC_REQBUFS, CREATE_BUFS, QBUF, DQBUF,
-  QUERYBUF, PREPARE_BUF, STREAMON and STREAMOFF) instead of the lock above.
-  That way the vb2 queuing framework does not have to wait for other ioctls.
-  This queue pointer is also used by the vb2 helper functions to check for
+The default :cpp:func:`video_device_release` callback currently
+just calls ``kfree`` to free the allocated memory.
+
+There is also a ::cpp:func:`video_device_release_empty` function that does
+nothing (is empty) and should be used if the struct is embedded and there
+is nothing to do when it is released.
+
+You should also set these fields of :c:type:`video_device`:
+
+- :c:type:`video_device`->v4l2_dev: must be set to the :c:type:`v4l2_device`
+  parent device.
+
+- :c:type:`video_device`->name: set to something descriptive and unique.
+
+- :c:type:`video_device`->vfl_dir: set this to ``VFL_DIR_RX`` for capture
+  devices (``VFL_DIR_RX`` has value 0, so this is normally already the
+  default), set to ``VFL_DIR_TX`` for output devices and ``VFL_DIR_M2M`` for mem2mem (codec) devices.
+
+- :c:type:`video_device`->fops: set to the :c:type:`v4l2_file_operations`
+  struct.
+
+- :c:type:`video_device`->ioctl_ops: if you use the :c:type:`v4l2_ioctl_ops`
+  to simplify ioctl maintenance (highly recommended to use this and it might
+  become compulsory in the future!), then set this to your
+  :c:type:`v4l2_ioctl_ops` struct. The :c:type:`video_device`->vfl_type and
+  :c:type:`video_device`->vfl_dir fields are used to disable ops that do not
+  match the type/dir combination. E.g. VBI ops are disabled for non-VBI nodes,
+  and output ops  are disabled for a capture device. This makes it possible to
+  provide just one :c:type:`v4l2_ioctl_ops struct` for both vbi and
+  video nodes.
+
+- :c:type:`video_device`->lock: leave to ``NULL`` if you want to do all the
+  locking  in the driver. Otherwise you give it a pointer to a struct
+  ``mutex_lock`` and before the :c:type:`video_device`->unlocked_ioctl
+  file operation is called this lock will be taken by the core and released
+  afterwards. See the next section for more details.
+
+- :c:type:`video_device`->queue: a pointer to the struct :c:type:`vb2_queue`
+  associated with this device node.
+  If queue is not ``NULL``, and queue->lock is not ``NULL``, then queue->lock
+  is used for the queuing ioctls (``VIDIOC_REQBUFS``, ``CREATE_BUFS``,
+  ``QBUF``, ``DQBUF``,  ``QUERYBUF``, ``PREPARE_BUF``, ``STREAMON`` and
+  ``STREAMOFF``) instead of the lock above.
+  That way the :ref:`vb2 <vb2_framework>` queuing framework does not have
+  to wait for other ioctls.   This queue pointer is also used by the
+  :ref:`vb2 <vb2_framework>` helper functions to check for
   queuing ownership (i.e. is the filehandle calling it allowed to do the
   operation).
 
-- prio: keeps track of the priorities. Used to implement VIDIOC_G/S_PRIORITY.
-  If left to NULL, then it will use the struct v4l2_prio_state in v4l2_device.
-  If you want to have a separate priority state per (group of) device node(s),
-  then you can point it to your own struct v4l2_prio_state.
-
-- dev_parent: you only set this if v4l2_device was registered with NULL as
-  the parent device struct. This only happens in cases where one hardware
-  device has multiple PCI devices that all share the same v4l2_device core.
-
-  The cx88 driver is an example of this: one core v4l2_device struct, but
-  it is used by both a raw video PCI device (cx8800) and a MPEG PCI device
-  (cx8802). Since the v4l2_device cannot be associated with two PCI devices
-  at the same time it is setup without a parent device. But when the struct
-  video_device is initialized you *do* know which parent PCI device to use and
-  so you set dev_device to the correct PCI device.
-
-If you use v4l2_ioctl_ops, then you should set .unlocked_ioctl to video_ioctl2
-in your v4l2_file_operations struct.
-
-Do not use .ioctl! This is deprecated and will go away in the future.
+- :c:type:`video_device`->prio: keeps track of the priorities. Used to
+  implement ``VIDIOC_G_PRIORITY`` and ``VIDIOC_S_PRIORITY``.
+  If left to ``NULL``, then it will use the struct :c:type:`v4l2_prio_state`
+  in :c:type:`v4l2_device`. If you want to have a separate priority state per
+  (group of) device node(s),   then you can point it to your own struct
+  :c:type:`v4l2_prio_state`.
+
+- :c:type:`video_device`->dev_parent: you only set this if v4l2_device was
+  registered with ``NULL`` as the parent ``device`` struct. This only happens
+  in cases where one hardware device has multiple PCI devices that all share
+  the same :c:type:`v4l2_device` core.
+
+  The cx88 driver is an example of this: one core :c:type:`v4l2_device` struct,
+  but   it is used by both a raw video PCI device (cx8800) and a MPEG PCI device
+  (cx8802). Since the :c:type:`v4l2_device` cannot be associated with two PCI
+  devices at the same time it is setup without a parent device. But when the
+  struct :c:type:`video_device` is initialized you **do** know which parent
+  PCI device to use and so you set ``dev_device`` to the correct PCI device.
+
+If you use :c:type:`v4l2_ioctl_ops`, then you should set
+:c:type:`video_device`->unlocked_ioctl to :cpp:func:`video_ioctl2` in your
+:c:type:`v4l2_file_operations` struct.
 
 In some cases you want to tell the core that a function you had specified in
-your v4l2_ioctl_ops should be ignored. You can mark such ioctls by calling this
-function before video_device_register is called:
-
-.. code-block:: none
+your :c:type:`v4l2_ioctl_ops` should be ignored. You can mark such ioctls by
+calling this function before :cpp:func:`video_register_device` is called:
 
-	void v4l2_disable_ioctl(struct video_device *vdev, unsigned int cmd);
+	:cpp:func:`v4l2_disable_ioctl <v4l2_disable_ioctl>`
+	(:c:type:`vdev <video_device>`, cmd).
 
 This tends to be needed if based on external factors (e.g. which card is
-being used) you want to turns off certain features in v4l2_ioctl_ops without
-having to make a new struct.
+being used) you want to turns off certain features in :c:type:`v4l2_ioctl_ops`
+without having to make a new struct.
 
-The v4l2_file_operations struct is a subset of file_operations. The main
-difference is that the inode argument is omitted since it is never used.
+The :c:type:`v4l2_file_operations` struct is a subset of file_operations.
+The main difference is that the inode argument is omitted since it is never
+used.
 
 If integration with the media framework is needed, you must initialize the
-media_entity struct embedded in the video_device struct (entity field) by
-calling media_entity_pads_init():
+:c:type:`media_entity` struct embedded in the :c:type:`video_device` struct
+(entity field) by calling :cpp:func:`media_entity_pads_init`:
 
-.. code-block:: none
+.. code-block:: c
 
 	struct media_pad *pad = &my_vdev->pad;
 	int err;
@@ -126,47 +136,52 @@ ioctls and locking
 ------------------
 
 The V4L core provides optional locking services. The main service is the
-lock field in struct video_device, which is a pointer to a mutex. If you set
-this pointer, then that will be used by unlocked_ioctl to serialize all ioctls.
-
-If you are using the videobuf2 framework, then there is a second lock that you
-can set: video_device->queue->lock. If set, then this lock will be used instead
-of video_device->lock to serialize all queuing ioctls (see the previous section
+lock field in struct :c:type:`video_device`, which is a pointer to a mutex.
+If you set this pointer, then that will be used by unlocked_ioctl to
+serialize all ioctls.
+
+If you are using the :ref:`videobuf2 framework <vb2_framework>`, then there
+is a second lock that you can set: :c:type:`video_device`->queue->lock. If
+set, then this lock will be used instead of :c:type:`video_device`->lock
+to serialize all queuing ioctls (see the previous section
 for the full list of those ioctls).
 
 The advantage of using a different lock for the queuing ioctls is that for some
 drivers (particularly USB drivers) certain commands such as setting controls
 can take a long time, so you want to use a separate lock for the buffer queuing
-ioctls. That way your VIDIOC_DQBUF doesn't stall because the driver is busy
+ioctls. That way your ``VIDIOC_DQBUF`` doesn't stall because the driver is busy
 changing the e.g. exposure of the webcam.
 
 Of course, you can always do all the locking yourself by leaving both lock
-pointers at NULL.
+pointers at ``NULL``.
 
-If you use the old videobuf then you must pass the video_device lock to the
-videobuf queue initialize function: if videobuf has to wait for a frame to
-arrive, then it will temporarily unlock the lock and relock it afterwards. If
-your driver also waits in the code, then you should do the same to allow other
+If you use the old :ref:`videobuf framework <vb_framework>` then you must
+pass the :c:type:`video_device`->lock to the videobuf queue initialize
+function: if videobuf has to wait for a frame to arrive, then it will
+temporarily unlock the lock and relock it afterwards. If your driver also
+waits in the code, then you should do the same to allow other
 processes to access the device node while the first process is waiting for
 something.
 
-In the case of videobuf2 you will need to implement the wait_prepare and
-wait_finish callbacks to unlock/lock if applicable. If you use the queue->lock
-pointer, then you can use the helper functions vb2_ops_wait_prepare/finish.
+In the case of :ref:`videobuf2 <vb2_framework>` you will need to implement the
+``wait_prepare()`` and ``wait_finish()`` callbacks to unlock/lock if applicable.
+If you use the ``queue->lock`` pointer, then you can use the helper functions
+:cpp:func:`vb2_ops_wait_prepare` and :cpp:func:`vb2_ops_wait_finish`.
 
 The implementation of a hotplug disconnect should also take the lock from
-video_device before calling v4l2_device_disconnect. If you are also using
-video_device->queue->lock, then you have to first lock video_device->queue->lock
-followed by video_device->lock. That way you can be sure no ioctl is running
-when you call v4l2_device_disconnect.
+:c:type:`video_device` before calling v4l2_device_disconnect. If you are also
+using :c:type:`video_device`->queue->lock, then you have to first lock
+:c:type:`video_device`->queue->lock followed by :c:type:`video_device`->lock.
+That way you can be sure no ioctl is running when you call
+:c:type:`v4l2_device_disconnect`.
 
-video_device registration
+Video device registration
 -------------------------
 
-Next you register the video device: this will create the character device
-for you.
+Next you register the video device with :cpp:func:`video_register_device`.
+This will create the character device for you.
 
-.. code-block:: none
+.. code-block:: c
 
 	err = video_register_device(vdev, VFL_TYPE_GRABBER, -1);
 	if (err) {
@@ -174,19 +189,20 @@ for you.
 		return err;
 	}
 
-If the v4l2_device parent device has a non-NULL mdev field, the video device
-entity will be automatically registered with the media device.
+If the :c:type:`v4l2_device` parent device has a not ``NULL`` mdev field,
+the video device entity will be automatically registered with the media
+device.
 
 Which device is registered depends on the type argument. The following
 types exist:
 
-VFL_TYPE_GRABBER: videoX for video input/output devices
-VFL_TYPE_VBI: vbiX for vertical blank data (i.e. closed captions, teletext)
-VFL_TYPE_RADIO: radioX for radio tuners
-VFL_TYPE_SDR: swradioX for Software Defined Radio tuners
+- ``VFL_TYPE_GRABBER``: ``/dev/videoX`` for video input/output devices
+- ``VFL_TYPE_VBI``: ``/dev/vbiX`` for vertical blank data (i.e. closed captions, teletext)
+- ``VFL_TYPE_RADIO``: ``/dev/radioX`` for radio tuners
+- ``VFL_TYPE_SDR``: ``/dev/swradioX`` for Software Defined Radio tuners
 
 The last argument gives you a certain amount of control over the device
-device node number used (i.e. the X in videoX). Normally you will pass -1
+device node number used (i.e. the X in ``videoX``). Normally you will pass -1
 to let the v4l2 framework pick the first free number. But sometimes users
 want to select a specific node number. It is common that drivers allow
 the user to select a specific device node number through a driver module
@@ -205,85 +221,90 @@ first free number.
 
 Since in this case you do not care about a warning about not being able
 to select the specified device node number, you can call the function
-video_register_device_no_warn() instead.
+:cpp:func:`video_register_device_no_warn` instead.
 
 Whenever a device node is created some attributes are also created for you.
-If you look in /sys/class/video4linux you see the devices. Go into e.g.
-video0 and you will see 'name', 'dev_debug' and 'index' attributes. The 'name'
-attribute is the 'name' field of the video_device struct. The 'dev_debug' attribute
-can be used to enable core debugging. See the next section for more detailed
-information on this.
+If you look in ``/sys/class/video4linux`` you see the devices. Go into e.g.
+``video0`` and you will see 'name', 'dev_debug' and 'index' attributes. The
+'name' attribute is the 'name' field of the video_device struct. The
+'dev_debug' attribute can be used to enable core debugging. See the next
+section for more detailed information on this.
 
 The 'index' attribute is the index of the device node: for each call to
-video_register_device() the index is just increased by 1. The first video
-device node you register always starts with index 0.
+:cpp:func:`video_register_device()` the index is just increased by 1. The
+first video device node you register always starts with index 0.
 
 Users can setup udev rules that utilize the index attribute to make fancy
-device names (e.g. 'mpegX' for MPEG video capture device nodes).
+device names (e.g. '``mpegX``' for MPEG video capture device nodes).
 
 After the device was successfully registered, then you can use these fields:
 
-- vfl_type: the device type passed to video_register_device.
-- minor: the assigned device minor number.
-- num: the device node number (i.e. the X in videoX).
-- index: the device index number.
+- :c:type:`video_device`->vfl_type: the device type passed to
+  :cpp:func:`video_register_device`.
+- :c:type:`video_device`->minor: the assigned device minor number.
+- :c:type:`video_device`->num: the device node number (i.e. the X in
+  ``videoX``).
+- :c:type:`video_device`->index: the device index number.
 
-If the registration failed, then you need to call video_device_release()
-to free the allocated video_device struct, or free your own struct if the
-video_device was embedded in it. The vdev->release() callback will never
-be called if the registration failed, nor should you ever attempt to
-unregister the device if the registration failed.
+If the registration failed, then you need to call
+:cpp:func:`video_device_release` to free the allocated :c:type:`video_device`
+struct, or free your own struct if the :c:type:`video_device` was embedded in
+it. The ``vdev->release()`` callback will never be called if the registration
+failed, nor should you ever attempt to unregister the device if the
+registration failed.
 
 video device debugging
 ----------------------
 
 The 'dev_debug' attribute that is created for each video, vbi, radio or swradio
-device in /sys/class/video4linux/<devX>/ allows you to enable logging of
+device in ``/sys/class/video4linux/<devX>/`` allows you to enable logging of
 file operations.
 
 It is a bitmask and the following bits can be set:
 
-.. code-block:: none
-
-	0x01: Log the ioctl name and error code. VIDIOC_(D)QBUF ioctls are only logged
-	      if bit 0x08 is also set.
-	0x02: Log the ioctl name arguments and error code. VIDIOC_(D)QBUF ioctls are
-	      only logged if bit 0x08 is also set.
-	0x04: Log the file operations open, release, read, write, mmap and
-	      get_unmapped_area. The read and write operations are only logged if
-	      bit 0x08 is also set.
-	0x08: Log the read and write file operations and the VIDIOC_QBUF and
-	      VIDIOC_DQBUF ioctls.
-	0x10: Log the poll file operation.
 
-video_device cleanup
+===== ================================================================
+Mask  Description
+===== ================================================================
+0x01  Log the ioctl name and error code. VIDIOC_(D)QBUF ioctls are
+      only logged if bit 0x08 is also set.
+0x02  Log the ioctl name arguments and error code. VIDIOC_(D)QBUF
+      ioctls are
+      only logged if bit 0x08 is also set.
+0x04  Log the file operations open, release, read, write, mmap and
+      get_unmapped_area. The read and write operations are only
+      logged if bit 0x08 is also set.
+0x08  Log the read and write file operations and the VIDIOC_QBUF and
+      VIDIOC_DQBUF ioctls.
+0x10  Log the poll file operation.
+===== ================================================================
+
+Video device cleanup
 --------------------
 
 When the video device nodes have to be removed, either during the unload
 of the driver or because the USB device was disconnected, then you should
-unregister them:
+unregister them with:
 
-.. code-block:: none
-
-	video_unregister_device(vdev);
+	:cpp:func:`video_unregister_device`
+	(:c:type:`vdev <video_device>`);
 
 This will remove the device nodes from sysfs (causing udev to remove them
-from /dev).
+from ``/dev``).
 
-After video_unregister_device() returns no new opens can be done. However,
-in the case of USB devices some application might still have one of these
-device nodes open. So after the unregister all file operations (except
+After :cpp:func:`video_unregister_device` returns no new opens can be done.
+However, in the case of USB devices some application might still have one of
+these device nodes open. So after the unregister all file operations (except
 release, of course) will return an error as well.
 
-When the last user of the video device node exits, then the vdev->release()
+When the last user of the video device node exits, then the ``vdev->release()``
 callback is called and you can do the final cleanup there.
 
 Don't forget to cleanup the media entity associated with the video device if
 it has been initialized:
 
-.. code-block:: none
-
-	media_entity_cleanup(&vdev->entity);
+	:cpp:func:`media_entity_cleanup <media_entity_cleanup>`
+	(&vdev->entity);
 
 This can be done from the release callback.
 
@@ -293,45 +314,44 @@ video_device helper functions
 
 There are a few useful helper functions:
 
-- file/video_device private data
+- file and :c:type:`video_device` private data
 
 You can set/get driver private data in the video_device struct using:
 
-.. code-block:: none
+	:cpp:func:`video_get_drvdata <video_get_drvdata>`
+	(:c:type:`vdev <video_device>`);
 
-	void *video_get_drvdata(struct video_device *vdev);
-	void video_set_drvdata(struct video_device *vdev, void *data);
+	:cpp:func:`video_set_drvdata <video_set_drvdata>`
+	(:c:type:`vdev <video_device>`);
 
-Note that you can safely call video_set_drvdata() before calling
-video_register_device().
+Note that you can safely call :cpp:func:`video_set_drvdata` before calling
+:cpp:func:`video_register_device`.
 
 And this function:
 
-.. code-block:: none
-
-	struct video_device *video_devdata(struct file *file);
+	:cpp:func:`video_devdata <video_devdata>`
+	(struct file \*file);
 
 returns the video_device belonging to the file struct.
 
-The video_drvdata function combines video_get_drvdata with video_devdata:
+The :cpp:func:`video_devdata` function combines :cpp:func:`video_get_drvdata`
+with :cpp:func:`video_devdata`:
 
-.. code-block:: none
+	:cpp:func:`video_drvdata <video_drvdata>`
+	(struct file \*file);
 
-	void *video_drvdata(struct file *file);
+You can go from a :c:type:`video_device` struct to the v4l2_device struct using:
 
-You can go from a video_device struct to the v4l2_device struct using:
-
-.. code-block:: none
+.. code-block:: c
 
 	struct v4l2_device *v4l2_dev = vdev->v4l2_dev;
 
 - Device node name
 
-The video_device node kernel name can be retrieved using
-
-.. code-block:: none
+The :c:type:`video_device` node kernel name can be retrieved using:
 
-	const char *video_device_node_name(struct video_device *vdev);
+	:cpp:func:`video_device_node_name <video_device_node_name>`
+	(:c:type:`vdev <video_device>`);
 
 The name is used as a hint by userspace tools such as udev. The function
 should be used where possible instead of accessing the video_device::num and

Documentation/media/kapi/v4l2-videobuf.rst

+.. _vb_framework:
+
 Videobuf Framework
 ==================
 

Documentation/media/kapi/v4l2-videobuf2.rst

+.. _vb2_framework:
+
 V4L2 videobuf2 kAPI
 ^^^^^^^^^^^^^^^^^^^