1.. SPDX-License-Identifier: GPL-2.0 2 3Video device' s internal representation 4======================================= 5 6The actual device nodes in the ``/dev`` directory are created using the 7:c:type:`video_device` struct (``v4l2-dev.h``). This struct can either be 8allocated dynamically or embedded in a larger struct. 9 10To allocate it dynamically use :c:func:`video_device_alloc`: 11 12.. code-block:: c 13 14 struct video_device *vdev = video_device_alloc(); 15 16 if (vdev == NULL) 17 return -ENOMEM; 18 19 vdev->release = video_device_release; 20 21If you embed it in a larger struct, then you must set the ``release()`` 22callback to your own function: 23 24.. code-block:: c 25 26 struct video_device *vdev = &my_vdev->vdev; 27 28 vdev->release = my_vdev_release; 29 30The ``release()`` callback must be set and it is called when the last user 31of the video device exits. 32 33The default :c:func:`video_device_release` callback currently 34just calls ``kfree`` to free the allocated memory. 35 36There is also a :c:func:`video_device_release_empty` function that does 37nothing (is empty) and should be used if the struct is embedded and there 38is nothing to do when it is released. 39 40You should also set these fields of :c:type:`video_device`: 41 42- :c:type:`video_device`->v4l2_dev: must be set to the :c:type:`v4l2_device` 43 parent device. 44 45- :c:type:`video_device`->name: set to something descriptive and unique. 46 47- :c:type:`video_device`->vfl_dir: set this to ``VFL_DIR_RX`` for capture 48 devices (``VFL_DIR_RX`` has value 0, so this is normally already the 49 default), set to ``VFL_DIR_TX`` for output devices and ``VFL_DIR_M2M`` for mem2mem (codec) devices. 50 51- :c:type:`video_device`->fops: set to the :c:type:`v4l2_file_operations` 52 struct. 53 54- :c:type:`video_device`->ioctl_ops: if you use the :c:type:`v4l2_ioctl_ops` 55 to simplify ioctl maintenance (highly recommended to use this and it might 56 become compulsory in the future!), then set this to your 57 :c:type:`v4l2_ioctl_ops` struct. The :c:type:`video_device`->vfl_type and 58 :c:type:`video_device`->vfl_dir fields are used to disable ops that do not 59 match the type/dir combination. E.g. VBI ops are disabled for non-VBI nodes, 60 and output ops are disabled for a capture device. This makes it possible to 61 provide just one :c:type:`v4l2_ioctl_ops` struct for both vbi and 62 video nodes. 63 64- :c:type:`video_device`->lock: leave to ``NULL`` if you want to do all the 65 locking in the driver. Otherwise you give it a pointer to a struct 66 ``mutex_lock`` and before the :c:type:`video_device`->unlocked_ioctl 67 file operation is called this lock will be taken by the core and released 68 afterwards. See the next section for more details. 69 70- :c:type:`video_device`->queue: a pointer to the struct vb2_queue 71 associated with this device node. 72 If queue is not ``NULL``, and queue->lock is not ``NULL``, then queue->lock 73 is used for the queuing ioctls (``VIDIOC_REQBUFS``, ``CREATE_BUFS``, 74 ``QBUF``, ``DQBUF``, ``QUERYBUF``, ``PREPARE_BUF``, ``STREAMON`` and 75 ``STREAMOFF``) instead of the lock above. 76 That way the :ref:`vb2 <vb2_framework>` queuing framework does not have 77 to wait for other ioctls. This queue pointer is also used by the 78 :ref:`vb2 <vb2_framework>` helper functions to check for 79 queuing ownership (i.e. is the filehandle calling it allowed to do the 80 operation). 81 82- :c:type:`video_device`->prio: keeps track of the priorities. Used to 83 implement ``VIDIOC_G_PRIORITY`` and ``VIDIOC_S_PRIORITY``. 84 If left to ``NULL``, then it will use the struct v4l2_prio_state 85 in :c:type:`v4l2_device`. If you want to have a separate priority state per 86 (group of) device node(s), then you can point it to your own struct 87 :c:type:`v4l2_prio_state`. 88 89- :c:type:`video_device`->dev_parent: you only set this if v4l2_device was 90 registered with ``NULL`` as the parent ``device`` struct. This only happens 91 in cases where one hardware device has multiple PCI devices that all share 92 the same :c:type:`v4l2_device` core. 93 94 The cx88 driver is an example of this: one core :c:type:`v4l2_device` struct, 95 but it is used by both a raw video PCI device (cx8800) and a MPEG PCI device 96 (cx8802). Since the :c:type:`v4l2_device` cannot be associated with two PCI 97 devices at the same time it is setup without a parent device. But when the 98 struct video_device is initialized you **do** know which parent 99 PCI device to use and so you set ``dev_device`` to the correct PCI device. 100 101If you use :c:type:`v4l2_ioctl_ops`, then you should set 102:c:type:`video_device`->unlocked_ioctl to :c:func:`video_ioctl2` in your 103:c:type:`v4l2_file_operations` struct. 104 105In some cases you want to tell the core that a function you had specified in 106your :c:type:`v4l2_ioctl_ops` should be ignored. You can mark such ioctls by 107calling this function before :c:func:`video_register_device` is called: 108 109 :c:func:`v4l2_disable_ioctl <v4l2_disable_ioctl>` 110 (:c:type:`vdev <video_device>`, cmd). 111 112This tends to be needed if based on external factors (e.g. which card is 113being used) you want to turns off certain features in :c:type:`v4l2_ioctl_ops` 114without having to make a new struct. 115 116The :c:type:`v4l2_file_operations` struct is a subset of file_operations. 117The main difference is that the inode argument is omitted since it is never 118used. 119 120If integration with the media framework is needed, you must initialize the 121:c:type:`media_entity` struct embedded in the :c:type:`video_device` struct 122(entity field) by calling :c:func:`media_entity_pads_init`: 123 124.. code-block:: c 125 126 struct media_pad *pad = &my_vdev->pad; 127 int err; 128 129 err = media_entity_pads_init(&vdev->entity, 1, pad); 130 131The pads array must have been previously initialized. There is no need to 132manually set the struct media_entity type and name fields. 133 134A reference to the entity will be automatically acquired/released when the 135video device is opened/closed. 136 137ioctls and locking 138------------------ 139 140The V4L core provides optional locking services. The main service is the 141lock field in struct video_device, which is a pointer to a mutex. 142If you set this pointer, then that will be used by unlocked_ioctl to 143serialize all ioctls. 144 145If you are using the :ref:`videobuf2 framework <vb2_framework>`, then there 146is a second lock that you can set: :c:type:`video_device`->queue->lock. If 147set, then this lock will be used instead of :c:type:`video_device`->lock 148to serialize all queuing ioctls (see the previous section 149for the full list of those ioctls). 150 151The advantage of using a different lock for the queuing ioctls is that for some 152drivers (particularly USB drivers) certain commands such as setting controls 153can take a long time, so you want to use a separate lock for the buffer queuing 154ioctls. That way your ``VIDIOC_DQBUF`` doesn't stall because the driver is busy 155changing the e.g. exposure of the webcam. 156 157Of course, you can always do all the locking yourself by leaving both lock 158pointers at ``NULL``. 159 160In the case of :ref:`videobuf2 <vb2_framework>` you will need to implement the 161``wait_prepare()`` and ``wait_finish()`` callbacks to unlock/lock if applicable. 162If you use the ``queue->lock`` pointer, then you can use the helper functions 163:c:func:`vb2_ops_wait_prepare` and :c:func:`vb2_ops_wait_finish`. 164 165The implementation of a hotplug disconnect should also take the lock from 166:c:type:`video_device` before calling v4l2_device_disconnect. If you are also 167using :c:type:`video_device`->queue->lock, then you have to first lock 168:c:type:`video_device`->queue->lock followed by :c:type:`video_device`->lock. 169That way you can be sure no ioctl is running when you call 170:c:func:`v4l2_device_disconnect`. 171 172Video device registration 173------------------------- 174 175Next you register the video device with :c:func:`video_register_device`. 176This will create the character device for you. 177 178.. code-block:: c 179 180 err = video_register_device(vdev, VFL_TYPE_VIDEO, -1); 181 if (err) { 182 video_device_release(vdev); /* or kfree(my_vdev); */ 183 return err; 184 } 185 186If the :c:type:`v4l2_device` parent device has a not ``NULL`` mdev field, 187the video device entity will be automatically registered with the media 188device. 189 190Which device is registered depends on the type argument. The following 191types exist: 192 193========================== ==================== ============================== 194:c:type:`vfl_devnode_type` Device name Usage 195========================== ==================== ============================== 196``VFL_TYPE_VIDEO`` ``/dev/videoX`` for video input/output devices 197``VFL_TYPE_VBI`` ``/dev/vbiX`` for vertical blank data (i.e. 198 closed captions, teletext) 199``VFL_TYPE_RADIO`` ``/dev/radioX`` for radio tuners 200``VFL_TYPE_SUBDEV`` ``/dev/v4l-subdevX`` for V4L2 subdevices 201``VFL_TYPE_SDR`` ``/dev/swradioX`` for Software Defined Radio 202 (SDR) tuners 203``VFL_TYPE_TOUCH`` ``/dev/v4l-touchX`` for touch sensors 204========================== ==================== ============================== 205 206The last argument gives you a certain amount of control over the device 207node number used (i.e. the X in ``videoX``). Normally you will pass -1 208to let the v4l2 framework pick the first free number. But sometimes users 209want to select a specific node number. It is common that drivers allow 210the user to select a specific device node number through a driver module 211option. That number is then passed to this function and video_register_device 212will attempt to select that device node number. If that number was already 213in use, then the next free device node number will be selected and it 214will send a warning to the kernel log. 215 216Another use-case is if a driver creates many devices. In that case it can 217be useful to place different video devices in separate ranges. For example, 218video capture devices start at 0, video output devices start at 16. 219So you can use the last argument to specify a minimum device node number 220and the v4l2 framework will try to pick the first free number that is equal 221or higher to what you passed. If that fails, then it will just pick the 222first free number. 223 224Since in this case you do not care about a warning about not being able 225to select the specified device node number, you can call the function 226:c:func:`video_register_device_no_warn` instead. 227 228Whenever a device node is created some attributes are also created for you. 229If you look in ``/sys/class/video4linux`` you see the devices. Go into e.g. 230``video0`` and you will see 'name', 'dev_debug' and 'index' attributes. The 231'name' attribute is the 'name' field of the video_device struct. The 232'dev_debug' attribute can be used to enable core debugging. See the next 233section for more detailed information on this. 234 235The 'index' attribute is the index of the device node: for each call to 236:c:func:`video_register_device()` the index is just increased by 1. The 237first video device node you register always starts with index 0. 238 239Users can setup udev rules that utilize the index attribute to make fancy 240device names (e.g. '``mpegX``' for MPEG video capture device nodes). 241 242After the device was successfully registered, then you can use these fields: 243 244- :c:type:`video_device`->vfl_type: the device type passed to 245 :c:func:`video_register_device`. 246- :c:type:`video_device`->minor: the assigned device minor number. 247- :c:type:`video_device`->num: the device node number (i.e. the X in 248 ``videoX``). 249- :c:type:`video_device`->index: the device index number. 250 251If the registration failed, then you need to call 252:c:func:`video_device_release` to free the allocated :c:type:`video_device` 253struct, or free your own struct if the :c:type:`video_device` was embedded in 254it. The ``vdev->release()`` callback will never be called if the registration 255failed, nor should you ever attempt to unregister the device if the 256registration failed. 257 258video device debugging 259---------------------- 260 261The 'dev_debug' attribute that is created for each video, vbi, radio or swradio 262device in ``/sys/class/video4linux/<devX>/`` allows you to enable logging of 263file operations. 264 265It is a bitmask and the following bits can be set: 266 267.. tabularcolumns:: |p{5ex}|L| 268 269===== ================================================================ 270Mask Description 271===== ================================================================ 2720x01 Log the ioctl name and error code. VIDIOC_(D)QBUF ioctls are 273 only logged if bit 0x08 is also set. 2740x02 Log the ioctl name arguments and error code. VIDIOC_(D)QBUF 275 ioctls are 276 only logged if bit 0x08 is also set. 2770x04 Log the file operations open, release, read, write, mmap and 278 get_unmapped_area. The read and write operations are only 279 logged if bit 0x08 is also set. 2800x08 Log the read and write file operations and the VIDIOC_QBUF and 281 VIDIOC_DQBUF ioctls. 2820x10 Log the poll file operation. 2830x20 Log error and messages in the control operations. 284===== ================================================================ 285 286Video device cleanup 287-------------------- 288 289When the video device nodes have to be removed, either during the unload 290of the driver or because the USB device was disconnected, then you should 291unregister them with: 292 293 :c:func:`video_unregister_device` 294 (:c:type:`vdev <video_device>`); 295 296This will remove the device nodes from sysfs (causing udev to remove them 297from ``/dev``). 298 299After :c:func:`video_unregister_device` returns no new opens can be done. 300However, in the case of USB devices some application might still have one of 301these device nodes open. So after the unregister all file operations (except 302release, of course) will return an error as well. 303 304When the last user of the video device node exits, then the ``vdev->release()`` 305callback is called and you can do the final cleanup there. 306 307Don't forget to cleanup the media entity associated with the video device if 308it has been initialized: 309 310 :c:func:`media_entity_cleanup <media_entity_cleanup>` 311 (&vdev->entity); 312 313This can be done from the release callback. 314 315 316helper functions 317---------------- 318 319There are a few useful helper functions: 320 321- file and :c:type:`video_device` private data 322 323You can set/get driver private data in the video_device struct using: 324 325 :c:func:`video_get_drvdata <video_get_drvdata>` 326 (:c:type:`vdev <video_device>`); 327 328 :c:func:`video_set_drvdata <video_set_drvdata>` 329 (:c:type:`vdev <video_device>`); 330 331Note that you can safely call :c:func:`video_set_drvdata` before calling 332:c:func:`video_register_device`. 333 334And this function: 335 336 :c:func:`video_devdata <video_devdata>` 337 (struct file \*file); 338 339returns the video_device belonging to the file struct. 340 341The :c:func:`video_devdata` function combines :c:func:`video_get_drvdata` 342with :c:func:`video_devdata`: 343 344 :c:func:`video_drvdata <video_drvdata>` 345 (struct file \*file); 346 347You can go from a :c:type:`video_device` struct to the v4l2_device struct using: 348 349.. code-block:: c 350 351 struct v4l2_device *v4l2_dev = vdev->v4l2_dev; 352 353- Device node name 354 355The :c:type:`video_device` node kernel name can be retrieved using: 356 357 :c:func:`video_device_node_name <video_device_node_name>` 358 (:c:type:`vdev <video_device>`); 359 360The name is used as a hint by userspace tools such as udev. The function 361should be used where possible instead of accessing the video_device::num and 362video_device::minor fields. 363 364video_device functions and data structures 365------------------------------------------ 366 367.. kernel-doc:: include/media/v4l2-dev.h 368