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 .. SPDX-License-Identifier: GPL-2.0
 
 Video device' s internal representation
 =======================================
 
 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 :c:func:`video_device_alloc`:
 
 .. code-block:: c
 
         struct video_device *vdev = video_device_alloc();
 
         if (vdev == NULL)
                 return -ENOMEM;
 
         vdev->release = video_device_release;
 
 If you embed it in a larger struct, then you must set the ``release()``
 callback to your own function:
 
 .. 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
 of the video device exits.
 
 The default :c:func:`video_device_release` callback currently
 just calls ``kfree`` to free the allocated memory.
 
 There is also a :c: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 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).
 
 - :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 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 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 :c: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 :c:type:`v4l2_ioctl_ops` should be ignored. You can mark such ioctls by
 calling this function before :c:func:`video_register_device` is called:
 
         :c: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 :c:type:`v4l2_ioctl_ops`
 without having to make a new struct.
 
 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
 :c:type:`media_entity` struct embedded in the :c:type:`video_device` struct
 (entity field) by calling :c:func:`media_entity_pads_init`:
 
 .. code-block:: c
 
         struct media_pad *pad = &my_vdev->pad;
         int err;
 
         err = media_entity_pads_init(&vdev->entity, 1, pad);
 
 The pads array must have been previously initialized. There is no need to
 manually set the struct media_entity type and name fields.
 
 A reference to the entity will be automatically acquired/released when the
 video device is opened/closed.
 
 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 :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
 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``.
 
 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 :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
 :c:func:`vb2_ops_wait_prepare` and :c:func:`vb2_ops_wait_finish`.
 
 The implementation of a hotplug disconnect should also take the lock from
 :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:func:`v4l2_device_disconnect`.
 
 Video device registration
 -------------------------
 
 Next you register the video device with :c:func:`video_register_device`.
 This will create the character device for you.
 
 .. code-block:: c
 
         err = video_register_device(vdev, VFL_TYPE_VIDEO, -1);
         if (err) {
                 video_device_release(vdev); /* or kfree(my_vdev); */
                 return err;
         }
 
 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:
 
 ========================== ====================  ==============================
 :c:type:`vfl_devnode_type` Device name           Usage
 ========================== ====================  ==============================
 ``VFL_TYPE_VIDEO``         ``/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_SUBDEV``        ``/dev/v4l-subdevX``  for V4L2 subdevices
 ``VFL_TYPE_SDR``           ``/dev/swradioX``     for Software Defined Radio
                                                  (SDR) tuners
 ``VFL_TYPE_TOUCH``         ``/dev/v4l-touchX``   for touch sensors
 ========================== ====================  ==============================
 
 The last argument gives you a certain amount of control over the 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
 option. That number is then passed to this function and video_register_device
 will attempt to select that device node number. If that number was already
 in use, then the next free device node number will be selected and it
 will send a warning to the kernel log.
 
 Another use-case is if a driver creates many devices. In that case it can
 be useful to place different video devices in separate ranges. For example,
 video capture devices start at 0, video output devices start at 16.
 So you can use the last argument to specify a minimum device node number
 and the v4l2 framework will try to pick the first free number that is equal
 or higher to what you passed. If that fails, then it will just pick the
 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
 :c: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.
 
 The 'index' attribute is the index of the device node: for each call to
 :c: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).
 
 After the device was successfully registered, then you can use these fields:
 
 - :c:type:`video_device`->vfl_type: the device type passed to
   :c: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
 :c: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
 file operations.
 
 It is a bitmask and the following bits can be set:
 
 .. tabularcolumns:: |p{5ex}|L|
 
 ===== ================================================================
 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.
 0x20  Log error and messages in the control operations.
 ===== ================================================================
 
 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 with:
 
         :c:func:`video_unregister_device`
         (:c:type:`vdev <video_device>`);
 
 This will remove the device nodes from sysfs (causing udev to remove them
 from ``/dev``).
 
 After :c: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()``
 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:
 
         :c:func:`media_entity_cleanup <media_entity_cleanup>`
         (&vdev->entity);
 
 This can be done from the release callback.
 
 
 helper functions
 ----------------
 
 There are a few useful helper functions:
 
 - file and :c:type:`video_device` private data
 
 You can set/get driver private data in the video_device struct using:
 
         :c:func:`video_get_drvdata <video_get_drvdata>`
         (:c:type:`vdev <video_device>`);
 
         :c:func:`video_set_drvdata <video_set_drvdata>`
         (:c:type:`vdev <video_device>`);
 
 Note that you can safely call :c:func:`video_set_drvdata` before calling
 :c:func:`video_register_device`.
 
 And this function:
 
         :c:func:`video_devdata <video_devdata>`
         (struct file \*file);
 
 returns the video_device belonging to the file struct.
 
 The :c:func:`video_devdata` function combines :c:func:`video_get_drvdata`
 with :c:func:`video_devdata`:
 
         :c:func:`video_drvdata <video_drvdata>`
         (struct file \*file);
 
 You can go from a :c:type:`video_device` struct to the v4l2_device struct using:
 
 .. code-block:: c
 
         struct v4l2_device *v4l2_dev = vdev->v4l2_dev;
 
 - Device node name
 
 The :c:type:`video_device` node kernel name can be retrieved using:
 
         :c: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
 video_device::minor fields.
 
 video_device functions and data structures
 ------------------------------------------
 
 .. kernel-doc:: include/media/v4l2-dev.h

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