xref: /kernel/linux/linux-6.6/Documentation/driver-api/media/v4l2-dev.rst

.. 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