1.. SPDX-License-Identifier: GFDL-1.1-no-invariants-or-later 2.. c:namespace:: V4L 3 4.. _open: 5 6*************************** 7Opening and Closing Devices 8*************************** 9 10.. _v4l2_hardware_control: 11 12Controlling a hardware peripheral via V4L2 13========================================== 14 15Hardware that is supported using the V4L2 uAPI often consists of multiple 16devices or peripherals, each of which have their own driver. 17 18The bridge driver exposes one or more V4L2 device nodes 19(see :ref:`v4l2_device_naming`). 20 21There are other drivers providing support for other components of 22the hardware, which may also expose device nodes, called V4L2 sub-devices. 23 24When such V4L2 sub-devices are exposed, they allow controlling those 25other hardware components - usually connected via a serial bus (like 26I²C, SMBus or SPI). Depending on the bridge driver, those sub-devices 27can be controlled indirectly via the bridge driver or explicitly via 28the :ref:`Media Controller <media_controller>` and via the 29:ref:`V4L2 sub-devices <subdev>`. 30 31The devices that require the use of the 32:ref:`Media Controller <media_controller>` are called **MC-centric** 33devices. The devices that are fully controlled via V4L2 device nodes 34are called **video-node-centric**. 35 36Userspace can check if a V4L2 hardware peripheral is MC-centric by 37calling :ref:`VIDIOC_QUERYCAP` and checking the 38:ref:`device_caps field <device-capabilities>`. 39 40If the device returns ``V4L2_CAP_IO_MC`` flag at ``device_caps``, 41then it is MC-centric, otherwise, it is video-node-centric. 42 43It is required for MC-centric drivers to identify the V4L2 44sub-devices and to configure the pipelines via the 45:ref:`media controller API <media_controller>` before using the peripheral. 46Also, the sub-devices' configuration shall be controlled via the 47:ref:`sub-device API <subdev>`. 48 49.. note:: 50 51 A video-node-centric may still provide media-controller and 52 sub-device interfaces as well. 53 54 However, in that case the media-controller and the sub-device 55 interfaces are read-only and just provide information about the 56 device. The actual configuration is done via the video nodes. 57 58.. _v4l2_device_naming: 59 60V4L2 Device Node Naming 61======================= 62 63V4L2 drivers are implemented as kernel modules, loaded manually by the 64system administrator or automatically when a device is first discovered. 65The driver modules plug into the ``videodev`` kernel module. It provides 66helper functions and a common application interface specified in this 67document. 68 69Each driver thus loaded registers one or more device nodes with major 70number 81. Minor numbers are allocated dynamically unless the kernel 71is compiled with the kernel option CONFIG_VIDEO_FIXED_MINOR_RANGES. 72In that case minor numbers are allocated in ranges depending on the 73device node type. 74 75The device nodes supported by the Video4Linux subsystem are: 76 77======================== ==================================================== 78Default device node name Usage 79======================== ==================================================== 80``/dev/videoX`` Video and metadata for capture/output devices 81``/dev/vbiX`` Vertical blank data (i.e. closed captions, teletext) 82``/dev/radioX`` Radio tuners and modulators 83``/dev/swradioX`` Software Defined Radio tuners and modulators 84``/dev/v4l-touchX`` Touch sensors 85``/dev/v4l-subdevX`` Video sub-devices (used by sensors and other 86 components of the hardware peripheral)\ [#]_ 87======================== ==================================================== 88 89Where ``X`` is a non-negative integer. 90 91.. note:: 92 93 1. The actual device node name is system-dependent, as udev rules may apply. 94 2. There is no guarantee that ``X`` will remain the same for the same 95 device, as the number depends on the device driver's probe order. 96 If you need an unique name, udev default rules produce 97 ``/dev/v4l/by-id/`` and ``/dev/v4l/by-path/`` directories containing 98 links that can be used uniquely to identify a V4L2 device node:: 99 100 $ tree /dev/v4l 101 /dev/v4l 102 ├── by-id 103 │ └── usb-OmniVision._USB_Camera-B4.04.27.1-video-index0 -> ../../video0 104 └── by-path 105 └── pci-0000:00:14.0-usb-0:2:1.0-video-index0 -> ../../video0 106 107.. [#] **V4L2 sub-device nodes** (e. g. ``/dev/v4l-subdevX``) use a different 108 set of system calls, as covered at :ref:`subdev`. 109 110Many drivers support "video_nr", "radio_nr" or "vbi_nr" module 111options to select specific video/radio/vbi node numbers. This allows the 112user to request that the device node is named e.g. /dev/video5 instead 113of leaving it to chance. When the driver supports multiple devices of 114the same type more than one device node number can be assigned, 115separated by commas: 116 117.. code-block:: none 118 119 # modprobe mydriver video_nr=0,1 radio_nr=0,1 120 121In ``/etc/modules.conf`` this may be written as: 122 123:: 124 125 options mydriver video_nr=0,1 radio_nr=0,1 126 127When no device node number is given as module option the driver supplies 128a default. 129 130Normally udev will create the device nodes in /dev automatically for 131you. If udev is not installed, then you need to enable the 132CONFIG_VIDEO_FIXED_MINOR_RANGES kernel option in order to be able to 133correctly relate a minor number to a device node number. I.e., you need 134to be certain that minor number 5 maps to device node name video5. With 135this kernel option different device types have different minor number 136ranges. These ranges are listed in :ref:`devices`. 137 138The creation of character special files (with mknod) is a privileged 139operation and devices cannot be opened by major and minor number. That 140means applications cannot *reliably* scan for loaded or installed 141drivers. The user must enter a device name, or the application can try 142the conventional device names. 143 144.. _related: 145 146Related Devices 147=============== 148 149Devices can support several functions. For example video capturing, VBI 150capturing and radio support. 151 152The V4L2 API creates different V4L2 device nodes for each of these functions. 153 154The V4L2 API was designed with the idea that one device node could 155support all functions. However, in practice this never worked: this 156'feature' was never used by applications and many drivers did not 157support it and if they did it was certainly never tested. In addition, 158switching a device node between different functions only works when 159using the streaming I/O API, not with the 160:c:func:`read()`/\ :c:func:`write()` API. 161 162Today each V4L2 device node supports just one function. 163 164Besides video input or output the hardware may also support audio 165sampling or playback. If so, these functions are implemented as ALSA PCM 166devices with optional ALSA audio mixer devices. 167 168One problem with all these devices is that the V4L2 API makes no 169provisions to find these related V4L2 device nodes. Some really complex 170hardware use the Media Controller (see :ref:`media_controller`) which can 171be used for this purpose. But several drivers do not use it, and while some 172code exists that uses sysfs to discover related V4L2 device nodes (see 173libmedia_dev in the 174`v4l-utils <http://git.linuxtv.org/cgit.cgi/v4l-utils.git/>`__ git 175repository), there is no library yet that can provide a single API 176towards both Media Controller-based devices and devices that do not use 177the Media Controller. If you want to work on this please write to the 178linux-media mailing list: 179`https://linuxtv.org/lists.php <https://linuxtv.org/lists.php>`__. 180 181Multiple Opens 182============== 183 184V4L2 devices can be opened more than once. [#f1]_ When this is supported 185by the driver, users can for example start a "panel" application to 186change controls like brightness or audio volume, while another 187application captures video and audio. In other words, panel applications 188are comparable to an ALSA audio mixer application. Just opening a V4L2 189device should not change the state of the device. [#f2]_ 190 191Once an application has allocated the memory buffers needed for 192streaming data (by calling the :ref:`VIDIOC_REQBUFS` 193or :ref:`VIDIOC_CREATE_BUFS` ioctls, or 194implicitly by calling the :c:func:`read()` or 195:c:func:`write()` functions) that application (filehandle) 196becomes the owner of the device. It is no longer allowed to make changes 197that would affect the buffer sizes (e.g. by calling the 198:ref:`VIDIOC_S_FMT <VIDIOC_G_FMT>` ioctl) and other applications are 199no longer allowed to allocate buffers or start or stop streaming. The 200EBUSY error code will be returned instead. 201 202Merely opening a V4L2 device does not grant exclusive access. [#f3]_ 203Initiating data exchange however assigns the right to read or write the 204requested type of data, and to change related properties, to this file 205descriptor. Applications can request additional access privileges using 206the priority mechanism described in :ref:`app-pri`. 207 208Shared Data Streams 209=================== 210 211V4L2 drivers should not support multiple applications reading or writing 212the same data stream on a device by copying buffers, time multiplexing 213or similar means. This is better handled by a proxy application in user 214space. 215 216Functions 217========= 218 219To open and close V4L2 devices applications use the 220:c:func:`open()` and :c:func:`close()` function, 221respectively. Devices are programmed using the 222:ref:`ioctl() <func-ioctl>` function as explained in the following 223sections. 224 225.. [#f1] 226 There are still some old and obscure drivers that have not been 227 updated to allow for multiple opens. This implies that for such 228 drivers :c:func:`open()` can return an ``EBUSY`` error code 229 when the device is already in use. 230 231.. [#f2] 232 Unfortunately, opening a radio device often switches the state of the 233 device to radio mode in many drivers. This behavior should be fixed 234 eventually as it violates the V4L2 specification. 235 236.. [#f3] 237 Drivers could recognize the ``O_EXCL`` open flag. Presently this is 238 not required, so applications cannot know if it really works. 239