Right now, only kAPI documentation describes the device naming. However, such description is needed at the uAPI too. Add it, and describe how to get an unique identify for a given device. Signed-off-by: Mauro Carvalho Chehab <mchehab@xxxxxxxxxxxxxxxx> Acked-by: Hans Verkuil <hans.verkuil@xxxxxxxxx> Acked-by: Sakari Ailus <sakari.ailus@xxxxxxxxxxxxxxx> --- Documentation/media/uapi/v4l/open.rst | 53 ++++++++++++++++++++++++++++++----- 1 file changed, 46 insertions(+), 7 deletions(-) diff --git a/Documentation/media/uapi/v4l/open.rst b/Documentation/media/uapi/v4l/open.rst index afd116edb40d..de2144277f55 100644 --- a/Documentation/media/uapi/v4l/open.rst +++ b/Documentation/media/uapi/v4l/open.rst @@ -7,14 +7,16 @@ Opening and Closing Devices *************************** -Device Naming -============= +.. _v4l2_device_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. +V4L2 Device Node Naming +======================= + +V4L2 :term:`drivers <driver>` 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 and a minor number between 0 and 255. Minor numbers are @@ -23,6 +25,43 @@ option CONFIG_VIDEO_FIXED_MINOR_RANGES. In that case minor numbers are allocated in ranges depending on the device node type (video, radio, etc.). +The existing :term:`device node` types defined at V4L2 spec are: + +======================== ====================================================== +Default device node name Usage +======================== ====================================================== +``/dev/videoX`` Video input/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 +======================== ====================================================== + +Where ``X`` is a non-negative number. + +.. 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 + + 3. :term:`V4L2 sub-device` nodes (e. g. ``/dev/v4l-sudevX``) provide a + different API and aren't considered as + :term:`V4L2 device nodes <v4l2 device node>`. + + They are 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 -- 2.13.6
