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diff --git a/Documentation/userspace-api/media/v4l/open.rst b/Documentation/userspace-api/media/v4l/open.rst new file mode 100644 index 000000000..18bfb9b81 --- /dev/null +++ b/Documentation/userspace-api/media/v4l/open.rst @@ -0,0 +1,238 @@ +.. SPDX-License-Identifier: GFDL-1.1-no-invariants-or-later +.. c:namespace:: V4L + +.. _open: + +*************************** +Opening and Closing Devices +*************************** + +.. _v4l2_hardware_control: + +Controlling a hardware peripheral via V4L2 +========================================== + +Hardware that is supported using the V4L2 uAPI often consists of multiple +devices or peripherals, each of which have their own driver. + +The bridge driver exposes one or more V4L2 device nodes +(see :ref:`v4l2_device_naming`). + +There are other drivers providing support for other components of +the hardware, which may also expose device nodes, called V4L2 sub-devices. + +When such V4L2 sub-devices are exposed, they allow controlling those +other hardware components - usually connected via a serial bus (like +I²C, SMBus or SPI). Depending on the bridge driver, those sub-devices +can be controlled indirectly via the bridge driver or explicitly via +the :ref:`Media Controller <media_controller>` and via the +:ref:`V4L2 sub-devices <subdev>`. + +The devices that require the use of the +:ref:`Media Controller <media_controller>` are called **MC-centric** +devices. The devices that are fully controlled via V4L2 device nodes +are called **video-node-centric**. + +Userspace can check if a V4L2 hardware peripheral is MC-centric by +calling :ref:`VIDIOC_QUERYCAP` and checking the +:ref:`device_caps field <device-capabilities>`. + +If the device returns ``V4L2_CAP_IO_MC`` flag at ``device_caps``, +then it is MC-centric, otherwise, it is video-node-centric. + +It is required for MC-centric drivers to identify the V4L2 +sub-devices and to configure the pipelines via the +:ref:`media controller API <media_controller>` before using the peripheral. +Also, the sub-devices' configuration shall be controlled via the +:ref:`sub-device API <subdev>`. + +.. note:: + + A video-node-centric may still provide media-controller and + sub-device interfaces as well. + + However, in that case the media-controller and the sub-device + interfaces are read-only and just provide information about the + device. The actual configuration is done via the video nodes. + +.. _v4l2_device_naming: + +V4L2 Device Node Naming +======================= + +V4L2 drivers are implemented as kernel modules, loaded manually by the +system administrator or automatically when a device is first discovered. +The driver modules plug into the ``videodev`` kernel module. It provides +helper functions and a common application interface specified in this +document. + +Each driver thus loaded registers one or more device nodes with major +number 81. Minor numbers are allocated dynamically unless the kernel +is compiled with the kernel option CONFIG_VIDEO_FIXED_MINOR_RANGES. +In that case minor numbers are allocated in ranges depending on the +device node type. + +The device nodes supported by the Video4Linux subsystem are: + +======================== ==================================================== +Default device node name Usage +======================== ==================================================== +``/dev/videoX`` Video and metadata for capture/output devices +``/dev/vbiX`` Vertical blank data (i.e. closed captions, teletext) +``/dev/radioX`` Radio tuners and modulators +``/dev/swradioX`` Software Defined Radio tuners and modulators +``/dev/v4l-touchX`` Touch sensors +``/dev/v4l-subdevX`` Video sub-devices (used by sensors and other + components of the hardware peripheral)\ [#]_ +======================== ==================================================== + +Where ``X`` is a non-negative integer. + +.. note:: + + 1. The actual device node name is system-dependent, as udev rules may apply. + 2. There is no guarantee that ``X`` will remain the same for the same + device, as the number depends on the device driver's probe order. + If you need an unique name, udev default rules produce + ``/dev/v4l/by-id/`` and ``/dev/v4l/by-path/`` directories containing + links that can be used uniquely to identify a V4L2 device node:: + + $ tree /dev/v4l + /dev/v4l + ├── by-id + │ └── usb-OmniVision._USB_Camera-B4.04.27.1-video-index0 -> ../../video0 + └── by-path + └── pci-0000:00:14.0-usb-0:2:1.0-video-index0 -> ../../video0 + +.. [#] **V4L2 sub-device nodes** (e. g. ``/dev/v4l-subdevX``) use a different + set of system calls, as covered at :ref:`subdev`. + +Many drivers support "video_nr", "radio_nr" or "vbi_nr" module +options to select specific video/radio/vbi node numbers. This allows the +user to request that the device node is named e.g. /dev/video5 instead +of leaving it to chance. When the driver supports multiple devices of +the same type more than one device node number can be assigned, +separated by commas: + +.. code-block:: none + + # modprobe mydriver video_nr=0,1 radio_nr=0,1 + +In ``/etc/modules.conf`` this may be written as: + +:: + + options mydriver video_nr=0,1 radio_nr=0,1 + +When no device node number is given as module option the driver supplies +a default. + +Normally udev will create the device nodes in /dev automatically for +you. If udev is not installed, then you need to enable the +CONFIG_VIDEO_FIXED_MINOR_RANGES kernel option in order to be able to +correctly relate a minor number to a device node number. I.e., you need +to be certain that minor number 5 maps to device node name video5. With +this kernel option different device types have different minor number +ranges. These ranges are listed in :ref:`devices`. + +The creation of character special files (with mknod) is a privileged +operation and devices cannot be opened by major and minor number. That +means applications cannot *reliably* scan for loaded or installed +drivers. The user must enter a device name, or the application can try +the conventional device names. + +.. _related: + +Related Devices +=============== + +Devices can support several functions. For example video capturing, VBI +capturing and radio support. + +The V4L2 API creates different V4L2 device nodes for each of these functions. + +The V4L2 API was designed with the idea that one device node could +support all functions. However, in practice this never worked: this +'feature' was never used by applications and many drivers did not +support it and if they did it was certainly never tested. In addition, +switching a device node between different functions only works when +using the streaming I/O API, not with the +:c:func:`read()`/\ :c:func:`write()` API. + +Today each V4L2 device node supports just one function. + +Besides video input or output the hardware may also support audio +sampling or playback. If so, these functions are implemented as ALSA PCM +devices with optional ALSA audio mixer devices. + +One problem with all these devices is that the V4L2 API makes no +provisions to find these related V4L2 device nodes. Some really complex +hardware use the Media Controller (see :ref:`media_controller`) which can +be used for this purpose. But several drivers do not use it, and while some +code exists that uses sysfs to discover related V4L2 device nodes (see +libmedia_dev in the +`v4l-utils <http://git.linuxtv.org/cgit.cgi/v4l-utils.git/>`__ git +repository), there is no library yet that can provide a single API +towards both Media Controller-based devices and devices that do not use +the Media Controller. If you want to work on this please write to the +linux-media mailing list: +`https://linuxtv.org/lists.php <https://linuxtv.org/lists.php>`__. + +Multiple Opens +============== + +V4L2 devices can be opened more than once. [#f1]_ When this is supported +by the driver, users can for example start a "panel" application to +change controls like brightness or audio volume, while another +application captures video and audio. In other words, panel applications +are comparable to an ALSA audio mixer application. Just opening a V4L2 +device should not change the state of the device. [#f2]_ + +Once an application has allocated the memory buffers needed for +streaming data (by calling the :ref:`VIDIOC_REQBUFS` +or :ref:`VIDIOC_CREATE_BUFS` ioctls, or +implicitly by calling the :c:func:`read()` or +:c:func:`write()` functions) that application (filehandle) +becomes the owner of the device. It is no longer allowed to make changes +that would affect the buffer sizes (e.g. by calling the +:ref:`VIDIOC_S_FMT <VIDIOC_G_FMT>` ioctl) and other applications are +no longer allowed to allocate buffers or start or stop streaming. The +EBUSY error code will be returned instead. + +Merely opening a V4L2 device does not grant exclusive access. [#f3]_ +Initiating data exchange however assigns the right to read or write the +requested type of data, and to change related properties, to this file +descriptor. Applications can request additional access privileges using +the priority mechanism described in :ref:`app-pri`. + +Shared Data Streams +=================== + +V4L2 drivers should not support multiple applications reading or writing +the same data stream on a device by copying buffers, time multiplexing +or similar means. This is better handled by a proxy application in user +space. + +Functions +========= + +To open and close V4L2 devices applications use the +:c:func:`open()` and :c:func:`close()` function, +respectively. Devices are programmed using the +:ref:`ioctl() <func-ioctl>` function as explained in the following +sections. + +.. [#f1] + There are still some old and obscure drivers that have not been + updated to allow for multiple opens. This implies that for such + drivers :c:func:`open()` can return an ``EBUSY`` error code + when the device is already in use. + +.. [#f2] + Unfortunately, opening a radio device often switches the state of the + device to radio mode in many drivers. This behavior should be fixed + eventually as it violates the V4L2 specification. + +.. [#f3] + Drivers could recognize the ``O_EXCL`` open flag. Presently this is + not required, so applications cannot know if it really works. |