376 lines
15 KiB
ReStructuredText
376 lines
15 KiB
ReStructuredText
.. SPDX-License-Identifier: GPL-2.0
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Video device' s internal representation
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=======================================
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The actual device nodes in the ``/dev`` directory are created using the
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:c:type:`video_device` struct (``v4l2-dev.h``). This struct can either be
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allocated dynamically or embedded in a larger struct.
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To allocate it dynamically use :c:func:`video_device_alloc`:
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.. code-block:: c
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struct video_device *vdev = video_device_alloc();
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if (vdev == NULL)
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return -ENOMEM;
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vdev->release = video_device_release;
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If you embed it in a larger struct, then you must set the ``release()``
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callback to your own function:
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.. code-block:: c
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struct video_device *vdev = &my_vdev->vdev;
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vdev->release = my_vdev_release;
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The ``release()`` callback must be set and it is called when the last user
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of the video device exits.
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The default :c:func:`video_device_release` callback currently
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just calls ``kfree`` to free the allocated memory.
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There is also a :c:func:`video_device_release_empty` function that does
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nothing (is empty) and should be used if the struct is embedded and there
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is nothing to do when it is released.
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You should also set these fields of :c:type:`video_device`:
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- :c:type:`video_device`->v4l2_dev: must be set to the :c:type:`v4l2_device`
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parent device.
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- :c:type:`video_device`->name: set to something descriptive and unique.
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- :c:type:`video_device`->vfl_dir: set this to ``VFL_DIR_RX`` for capture
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devices (``VFL_DIR_RX`` has value 0, so this is normally already the
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default), set to ``VFL_DIR_TX`` for output devices and ``VFL_DIR_M2M`` for mem2mem (codec) devices.
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- :c:type:`video_device`->fops: set to the :c:type:`v4l2_file_operations`
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struct.
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- :c:type:`video_device`->ioctl_ops: if you use the :c:type:`v4l2_ioctl_ops`
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to simplify ioctl maintenance (highly recommended to use this and it might
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become compulsory in the future!), then set this to your
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:c:type:`v4l2_ioctl_ops` struct. The :c:type:`video_device`->vfl_type and
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:c:type:`video_device`->vfl_dir fields are used to disable ops that do not
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match the type/dir combination. E.g. VBI ops are disabled for non-VBI nodes,
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and output ops are disabled for a capture device. This makes it possible to
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provide just one :c:type:`v4l2_ioctl_ops` struct for both vbi and
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video nodes.
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- :c:type:`video_device`->lock: leave to ``NULL`` if you want to do all the
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locking in the driver. Otherwise you give it a pointer to a struct
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``mutex_lock`` and before the :c:type:`video_device`->unlocked_ioctl
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file operation is called this lock will be taken by the core and released
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afterwards. See the next section for more details.
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- :c:type:`video_device`->queue: a pointer to the struct vb2_queue
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associated with this device node.
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If queue is not ``NULL``, and queue->lock is not ``NULL``, then queue->lock
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is used for the queuing ioctls (``VIDIOC_REQBUFS``, ``CREATE_BUFS``,
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``QBUF``, ``DQBUF``, ``QUERYBUF``, ``PREPARE_BUF``, ``STREAMON`` and
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``STREAMOFF``) instead of the lock above.
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That way the :ref:`vb2 <vb2_framework>` queuing framework does not have
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to wait for other ioctls. This queue pointer is also used by the
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:ref:`vb2 <vb2_framework>` helper functions to check for
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queuing ownership (i.e. is the filehandle calling it allowed to do the
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operation).
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- :c:type:`video_device`->prio: keeps track of the priorities. Used to
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implement ``VIDIOC_G_PRIORITY`` and ``VIDIOC_S_PRIORITY``.
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If left to ``NULL``, then it will use the struct v4l2_prio_state
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in :c:type:`v4l2_device`. If you want to have a separate priority state per
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(group of) device node(s), then you can point it to your own struct
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:c:type:`v4l2_prio_state`.
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- :c:type:`video_device`->dev_parent: you only set this if v4l2_device was
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registered with ``NULL`` as the parent ``device`` struct. This only happens
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in cases where one hardware device has multiple PCI devices that all share
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the same :c:type:`v4l2_device` core.
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The cx88 driver is an example of this: one core :c:type:`v4l2_device` struct,
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but it is used by both a raw video PCI device (cx8800) and a MPEG PCI device
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(cx8802). Since the :c:type:`v4l2_device` cannot be associated with two PCI
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devices at the same time it is setup without a parent device. But when the
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struct video_device is initialized you **do** know which parent
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PCI device to use and so you set ``dev_device`` to the correct PCI device.
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If you use :c:type:`v4l2_ioctl_ops`, then you should set
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:c:type:`video_device`->unlocked_ioctl to :c:func:`video_ioctl2` in your
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:c:type:`v4l2_file_operations` struct.
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In some cases you want to tell the core that a function you had specified in
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your :c:type:`v4l2_ioctl_ops` should be ignored. You can mark such ioctls by
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calling this function before :c:func:`video_register_device` is called:
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:c:func:`v4l2_disable_ioctl <v4l2_disable_ioctl>`
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(:c:type:`vdev <video_device>`, cmd).
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This tends to be needed if based on external factors (e.g. which card is
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being used) you want to turns off certain features in :c:type:`v4l2_ioctl_ops`
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without having to make a new struct.
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The :c:type:`v4l2_file_operations` struct is a subset of file_operations.
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The main difference is that the inode argument is omitted since it is never
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used.
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If integration with the media framework is needed, you must initialize the
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:c:type:`media_entity` struct embedded in the :c:type:`video_device` struct
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(entity field) by calling :c:func:`media_entity_pads_init`:
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.. code-block:: c
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struct media_pad *pad = &my_vdev->pad;
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int err;
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err = media_entity_pads_init(&vdev->entity, 1, pad);
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The pads array must have been previously initialized. There is no need to
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manually set the struct media_entity type and name fields.
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A reference to the entity will be automatically acquired/released when the
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video device is opened/closed.
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ioctls and locking
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------------------
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The V4L core provides optional locking services. The main service is the
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lock field in struct video_device, which is a pointer to a mutex.
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If you set this pointer, then that will be used by unlocked_ioctl to
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serialize all ioctls.
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If you are using the :ref:`videobuf2 framework <vb2_framework>`, then there
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is a second lock that you can set: :c:type:`video_device`->queue->lock. If
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set, then this lock will be used instead of :c:type:`video_device`->lock
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to serialize all queuing ioctls (see the previous section
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for the full list of those ioctls).
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The advantage of using a different lock for the queuing ioctls is that for some
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drivers (particularly USB drivers) certain commands such as setting controls
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can take a long time, so you want to use a separate lock for the buffer queuing
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ioctls. That way your ``VIDIOC_DQBUF`` doesn't stall because the driver is busy
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changing the e.g. exposure of the webcam.
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Of course, you can always do all the locking yourself by leaving both lock
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pointers at ``NULL``.
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If you use the old :ref:`videobuf framework <vb_framework>` then you must
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pass the :c:type:`video_device`->lock to the videobuf queue initialize
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function: if videobuf has to wait for a frame to arrive, then it will
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temporarily unlock the lock and relock it afterwards. If your driver also
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waits in the code, then you should do the same to allow other
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processes to access the device node while the first process is waiting for
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something.
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In the case of :ref:`videobuf2 <vb2_framework>` you will need to implement the
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``wait_prepare()`` and ``wait_finish()`` callbacks to unlock/lock if applicable.
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If you use the ``queue->lock`` pointer, then you can use the helper functions
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:c:func:`vb2_ops_wait_prepare` and :c:func:`vb2_ops_wait_finish`.
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The implementation of a hotplug disconnect should also take the lock from
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:c:type:`video_device` before calling v4l2_device_disconnect. If you are also
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using :c:type:`video_device`->queue->lock, then you have to first lock
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:c:type:`video_device`->queue->lock followed by :c:type:`video_device`->lock.
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That way you can be sure no ioctl is running when you call
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:c:func:`v4l2_device_disconnect`.
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Video device registration
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-------------------------
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Next you register the video device with :c:func:`video_register_device`.
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This will create the character device for you.
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.. code-block:: c
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err = video_register_device(vdev, VFL_TYPE_VIDEO, -1);
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if (err) {
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video_device_release(vdev); /* or kfree(my_vdev); */
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return err;
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}
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If the :c:type:`v4l2_device` parent device has a not ``NULL`` mdev field,
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the video device entity will be automatically registered with the media
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device.
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Which device is registered depends on the type argument. The following
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types exist:
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========================== ==================== ==============================
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:c:type:`vfl_devnode_type` Device name Usage
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========================== ==================== ==============================
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``VFL_TYPE_VIDEO`` ``/dev/videoX`` for video input/output devices
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``VFL_TYPE_VBI`` ``/dev/vbiX`` for vertical blank data (i.e.
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closed captions, teletext)
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``VFL_TYPE_RADIO`` ``/dev/radioX`` for radio tuners
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``VFL_TYPE_SUBDEV`` ``/dev/v4l-subdevX`` for V4L2 subdevices
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``VFL_TYPE_SDR`` ``/dev/swradioX`` for Software Defined Radio
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(SDR) tuners
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``VFL_TYPE_TOUCH`` ``/dev/v4l-touchX`` for touch sensors
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========================== ==================== ==============================
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The last argument gives you a certain amount of control over the device
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node number used (i.e. the X in ``videoX``). Normally you will pass -1
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to let the v4l2 framework pick the first free number. But sometimes users
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want to select a specific node number. It is common that drivers allow
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the user to select a specific device node number through a driver module
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option. That number is then passed to this function and video_register_device
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will attempt to select that device node number. If that number was already
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in use, then the next free device node number will be selected and it
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will send a warning to the kernel log.
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Another use-case is if a driver creates many devices. In that case it can
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be useful to place different video devices in separate ranges. For example,
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video capture devices start at 0, video output devices start at 16.
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So you can use the last argument to specify a minimum device node number
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and the v4l2 framework will try to pick the first free number that is equal
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or higher to what you passed. If that fails, then it will just pick the
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first free number.
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Since in this case you do not care about a warning about not being able
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to select the specified device node number, you can call the function
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:c:func:`video_register_device_no_warn` instead.
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Whenever a device node is created some attributes are also created for you.
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If you look in ``/sys/class/video4linux`` you see the devices. Go into e.g.
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``video0`` and you will see 'name', 'dev_debug' and 'index' attributes. The
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'name' attribute is the 'name' field of the video_device struct. The
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'dev_debug' attribute can be used to enable core debugging. See the next
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section for more detailed information on this.
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The 'index' attribute is the index of the device node: for each call to
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:c:func:`video_register_device()` the index is just increased by 1. The
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first video device node you register always starts with index 0.
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Users can setup udev rules that utilize the index attribute to make fancy
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device names (e.g. '``mpegX``' for MPEG video capture device nodes).
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After the device was successfully registered, then you can use these fields:
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- :c:type:`video_device`->vfl_type: the device type passed to
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:c:func:`video_register_device`.
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- :c:type:`video_device`->minor: the assigned device minor number.
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- :c:type:`video_device`->num: the device node number (i.e. the X in
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``videoX``).
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- :c:type:`video_device`->index: the device index number.
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If the registration failed, then you need to call
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:c:func:`video_device_release` to free the allocated :c:type:`video_device`
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struct, or free your own struct if the :c:type:`video_device` was embedded in
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it. The ``vdev->release()`` callback will never be called if the registration
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failed, nor should you ever attempt to unregister the device if the
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registration failed.
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video device debugging
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----------------------
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The 'dev_debug' attribute that is created for each video, vbi, radio or swradio
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device in ``/sys/class/video4linux/<devX>/`` allows you to enable logging of
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file operations.
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It is a bitmask and the following bits can be set:
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.. tabularcolumns:: |p{5ex}|L|
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===== ================================================================
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Mask Description
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===== ================================================================
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0x01 Log the ioctl name and error code. VIDIOC_(D)QBUF ioctls are
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only logged if bit 0x08 is also set.
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0x02 Log the ioctl name arguments and error code. VIDIOC_(D)QBUF
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ioctls are
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only logged if bit 0x08 is also set.
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0x04 Log the file operations open, release, read, write, mmap and
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get_unmapped_area. The read and write operations are only
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logged if bit 0x08 is also set.
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0x08 Log the read and write file operations and the VIDIOC_QBUF and
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VIDIOC_DQBUF ioctls.
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0x10 Log the poll file operation.
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0x20 Log error and messages in the control operations.
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===== ================================================================
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Video device cleanup
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--------------------
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When the video device nodes have to be removed, either during the unload
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of the driver or because the USB device was disconnected, then you should
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unregister them with:
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:c:func:`video_unregister_device`
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(:c:type:`vdev <video_device>`);
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This will remove the device nodes from sysfs (causing udev to remove them
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from ``/dev``).
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After :c:func:`video_unregister_device` returns no new opens can be done.
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However, in the case of USB devices some application might still have one of
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these device nodes open. So after the unregister all file operations (except
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release, of course) will return an error as well.
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When the last user of the video device node exits, then the ``vdev->release()``
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callback is called and you can do the final cleanup there.
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Don't forget to cleanup the media entity associated with the video device if
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it has been initialized:
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:c:func:`media_entity_cleanup <media_entity_cleanup>`
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(&vdev->entity);
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This can be done from the release callback.
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helper functions
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----------------
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There are a few useful helper functions:
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- file and :c:type:`video_device` private data
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You can set/get driver private data in the video_device struct using:
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:c:func:`video_get_drvdata <video_get_drvdata>`
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(:c:type:`vdev <video_device>`);
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:c:func:`video_set_drvdata <video_set_drvdata>`
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(:c:type:`vdev <video_device>`);
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Note that you can safely call :c:func:`video_set_drvdata` before calling
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:c:func:`video_register_device`.
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And this function:
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:c:func:`video_devdata <video_devdata>`
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(struct file \*file);
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returns the video_device belonging to the file struct.
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The :c:func:`video_devdata` function combines :c:func:`video_get_drvdata`
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with :c:func:`video_devdata`:
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:c:func:`video_drvdata <video_drvdata>`
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(struct file \*file);
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You can go from a :c:type:`video_device` struct to the v4l2_device struct using:
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.. code-block:: c
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struct v4l2_device *v4l2_dev = vdev->v4l2_dev;
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- Device node name
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The :c:type:`video_device` node kernel name can be retrieved using:
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:c:func:`video_device_node_name <video_device_node_name>`
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(:c:type:`vdev <video_device>`);
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The name is used as a hint by userspace tools such as udev. The function
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should be used where possible instead of accessing the video_device::num and
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video_device::minor fields.
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video_device functions and data structures
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------------------------------------------
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.. kernel-doc:: include/media/v4l2-dev.h
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