media: open.rst: better document device node naming

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 identifier for a given device.
Acked-by: Hans Verkuil <hans.verkuil@cisco.com>
Reviewed-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>

Documentation/userspace-api/media/v4l/open.rst

@@ -7,12 +7,14 @@ Opening and Closing Devices
 ***************************
 
 
-Device Naming
-=============
+.. _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
+The driver modules plug into the ``videodev`` kernel module. It provides
 helper functions and a common application interface specified in this
 document.
 
@@ -23,6 +25,41 @@ 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 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