diff options
Diffstat (limited to '')
| -rw-r--r-- | Documentation/driver-api/media/v4l2-dev.rst | 375 |
1 files changed, 375 insertions, 0 deletions
diff --git a/Documentation/driver-api/media/v4l2-dev.rst b/Documentation/driver-api/media/v4l2-dev.rst new file mode 100644 index 000000000..99e3b5fa7 --- /dev/null +++ b/Documentation/driver-api/media/v4l2-dev.rst @@ -0,0 +1,375 @@ +.. 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 |