| .. 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 |
| 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 |