forked from Minki/linux
[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>
This commit is contained in:
Mauro Carvalho Chehab
parent
81d866fdcd
commit
243b6935ae
@ -1,13 +1,13 @@
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Video device creation
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=====================
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The actual device nodes in the /dev directory are created using the
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video_device struct (v4l2-dev.h). This struct can either be allocated
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dynamically or embedded in a larger struct.
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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:
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To allocate it dynamically use :cpp:func:`video_device_alloc`:
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.. code-block:: none
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.. code-block:: c
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struct video_device *vdev = video_device_alloc();
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@ -16,100 +16,110 @@ To allocate it dynamically use:
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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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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:: none
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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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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 video_device_release() callback just calls kfree to free the
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allocated memory.
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The default :cpp: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 video_device_release_empty() function that does nothing
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(is empty) and can be used if the struct is embedded and there is nothing
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to do when it is released.
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There is also a ::cpp: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:
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You should also set these fields of :c:type:`video_device`:
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- v4l2_dev: must be set to the v4l2_device parent 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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- name: set to something descriptive and unique.
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- :c:type:`video_device`->name: set to something descriptive and unique.
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- vfl_dir: set this to VFL_DIR_RX for capture devices (VFL_DIR_RX has value 0,
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so this is normally already the default), set to VFL_DIR_TX for output
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devices and VFL_DIR_M2M for mem2mem (codec) devices.
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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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- fops: set to the v4l2_file_operations struct.
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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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- ioctl_ops: if you use the v4l2_ioctl_ops to simplify ioctl maintenance
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(highly recommended to use this and it might become compulsory in the
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future!), then set this to your v4l2_ioctl_ops struct. The vfl_type and
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vfl_dir fields are used to disable ops that do not match the type/dir
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combination. E.g. VBI ops are disabled for non-VBI nodes, and output ops
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are disabled for a capture device. This makes it possible to provide
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just one v4l2_ioctl_ops struct for both vbi and video nodes.
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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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- lock: leave to NULL if you want to do all the locking in the driver.
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Otherwise you give it a pointer to a struct mutex_lock and before the
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unlocked_ioctl file operation is called this lock will be taken by the
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core and released afterwards. See the next section for more details.
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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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- queue: a pointer to the struct vb2_queue associated with this device node.
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If queue is non-NULL, and queue->lock is non-NULL, then queue->lock is
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used for the queuing ioctls (VIDIOC_REQBUFS, CREATE_BUFS, QBUF, DQBUF,
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QUERYBUF, PREPARE_BUF, STREAMON and STREAMOFF) instead of the lock above.
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That way the vb2 queuing framework does not have to wait for other ioctls.
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This queue pointer is also used by the vb2 helper functions to check for
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- :c:type:`video_device`->queue: a pointer to the struct :c:type:`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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- prio: keeps track of the priorities. Used to implement VIDIOC_G/S_PRIORITY.
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If left to NULL, then it will use the struct v4l2_prio_state in v4l2_device.
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If you want to have a separate priority state per (group of) device node(s),
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then you can point it to your own struct v4l2_prio_state.
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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 :c:type:`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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- dev_parent: you only set this if v4l2_device was registered with NULL as
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the parent device struct. This only happens in cases where one hardware
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device has multiple PCI devices that all share the same v4l2_device core.
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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 v4l2_device struct, but
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it is used by both a raw video PCI device (cx8800) and a MPEG PCI device
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(cx8802). Since the v4l2_device cannot be associated with two PCI devices
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at the same time it is setup without a parent device. But when the struct
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video_device is initialized you *do* know which parent PCI device to use and
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so you set dev_device to the correct PCI device.
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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 :c:type:`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 v4l2_ioctl_ops, then you should set .unlocked_ioctl to video_ioctl2
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in your v4l2_file_operations struct.
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Do not use .ioctl! This is deprecated and will go away in the future.
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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 :cpp: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 v4l2_ioctl_ops should be ignored. You can mark such ioctls by calling this
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function before video_device_register is called:
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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 :cpp:func:`video_register_device` is called:
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.. code-block:: none
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void v4l2_disable_ioctl(struct video_device *vdev, unsigned int cmd);
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:cpp: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 v4l2_ioctl_ops without
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having to make a new struct.
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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 v4l2_file_operations struct is a subset of file_operations. The main
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difference is that the inode argument is omitted since it is never used.
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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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media_entity struct embedded in the video_device struct (entity field) by
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calling media_entity_pads_init():
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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 :cpp:func:`media_entity_pads_init`:
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.. code-block:: none
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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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@ -126,47 +136,52 @@ 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. If you set
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this pointer, then that will be used by unlocked_ioctl to serialize all ioctls.
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lock field in struct :c:type:`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 videobuf2 framework, then there is a second lock that you
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can set: video_device->queue->lock. If set, then this lock will be used instead
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of video_device->lock to serialize all queuing ioctls (see the previous section
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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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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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pointers at ``NULL``.
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If you use the old videobuf then you must pass the video_device lock to the
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videobuf queue initialize function: if videobuf has to wait for a frame to
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arrive, then it will temporarily unlock the lock and relock it afterwards. If
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your driver also waits in the code, then you should do the same to allow other
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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 videobuf2 you will need to implement the wait_prepare and
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wait_finish callbacks to unlock/lock if applicable. If you use the queue->lock
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pointer, then you can use the helper functions vb2_ops_wait_prepare/finish.
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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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:cpp:func:`vb2_ops_wait_prepare` and :cpp: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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video_device before calling v4l2_device_disconnect. If you are also using
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video_device->queue->lock, then you have to first lock video_device->queue->lock
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followed by video_device->lock. That way you can be sure no ioctl is running
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when you call v4l2_device_disconnect.
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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:type:`v4l2_device_disconnect`.
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video_device registration
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Video device registration
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-------------------------
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Next you register the video device: this will create the character device
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for you.
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Next you register the video device with :cpp:func:`video_register_device`.
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This will create the character device for you.
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.. code-block:: none
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.. code-block:: c
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err = video_register_device(vdev, VFL_TYPE_GRABBER, -1);
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if (err) {
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@ -174,19 +189,20 @@ for you.
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return err;
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}
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If the v4l2_device parent device has a non-NULL mdev field, the video device
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entity will be automatically registered with the media device.
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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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VFL_TYPE_GRABBER: videoX for video input/output devices
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VFL_TYPE_VBI: vbiX for vertical blank data (i.e. closed captions, teletext)
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VFL_TYPE_RADIO: radioX for radio tuners
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VFL_TYPE_SDR: swradioX for Software Defined Radio tuners
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- ``VFL_TYPE_GRABBER``: ``/dev/videoX`` for video input/output devices
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- ``VFL_TYPE_VBI``: ``/dev/vbiX`` for vertical blank data (i.e. closed captions, teletext)
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- ``VFL_TYPE_RADIO``: ``/dev/radioX`` for radio tuners
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- ``VFL_TYPE_SDR``: ``/dev/swradioX`` for Software Defined Radio tuners
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The last argument gives you a certain amount of control over the device
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device node number used (i.e. the X in videoX). Normally you will pass -1
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device 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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@ -205,85 +221,90 @@ 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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video_register_device_no_warn() instead.
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:cpp: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 'name'
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attribute is the 'name' field of the video_device struct. The 'dev_debug' attribute
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can be used to enable core debugging. See the next section for more detailed
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information on this.
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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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video_register_device() the index is just increased by 1. The first video
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device node you register always starts with index 0.
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:cpp: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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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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- vfl_type: the device type passed to video_register_device.
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- minor: the assigned device minor number.
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- num: the device node number (i.e. the X in videoX).
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- index: the device index number.
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- :c:type:`video_device`->vfl_type: the device type passed to
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:cpp: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 video_device_release()
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to free the allocated video_device struct, or free your own struct if the
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video_device was embedded in it. The vdev->release() callback will never
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be called if the registration failed, nor should you ever attempt to
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unregister the device if the registration failed.
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If the registration failed, then you need to call
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:cpp: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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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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.. code-block:: none
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0x01: Log the ioctl name and error code. VIDIOC_(D)QBUF ioctls are only logged
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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.
|
||||
===== ================================================================
|
||||
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
|
||||
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
|
||||
The :c:type:`video_device` node kernel name can be retrieved using:
|
||||
|
||||
.. code-block:: none
|
||||
|
||||
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
|
||||
|
||||
@ -1,3 +1,5 @@
|
||||
.. _vb_framework:
|
||||
|
||||
Videobuf Framework
|
||||
==================
|
||||
|
||||
|
||||
@ -1,3 +1,5 @@
|
||||
.. _vb2_framework:
|
||||
|
||||
V4L2 videobuf2 kAPI
|
||||
^^^^^^^^^^^^^^^^^^^
|
||||
|
||||
|
||||
Loading…
Reference in New Issue
Block a user