linux-kernel - [PATCH v3 2/7] media: open.rst: better document device node naming

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Message-Id: <154729518e406bac4fdd53ce22e027936ece7b35.1503673702.git.mchehab@s-opensource.com>
Date:   Fri, 25 Aug 2017 12:11:52 -0300
From:   Mauro Carvalho Chehab <mchehab@...pensource.com>
To:     Linux Doc Mailing List <linux-doc@...r.kernel.org>,
        Linux Media Mailing List <linux-media@...r.kernel.org>
Cc:     Mauro Carvalho Chehab <mchehab@...pensource.com>,
        Mauro Carvalho Chehab <mchehab@...radead.org>,
        linux-kernel@...r.kernel.org, Jonathan Corbet <corbet@....net>
Subject: [PATCH v3 2/7] 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 identify for a given device.

Signed-off-by: Mauro Carvalho Chehab <mchehab@...pensource.com>
---
 Documentation/media/uapi/v4l/open.rst | 39 ++++++++++++++++++++++++++++++++---
 1 file changed, 36 insertions(+), 3 deletions(-)

diff --git a/Documentation/media/uapi/v4l/open.rst b/Documentation/media/uapi/v4l/open.rst
index afd116edb40d..e7160f667499 100644
--- a/Documentation/media/uapi/v4l/open.rst
+++ b/Documentation/media/uapi/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 +25,37 @@ 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 V4L2 device node types 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
+``/dev/swradioX``	 Software Defined Radio tuners
+``/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's not warranty 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/`` that can be used to
+      uniquely 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
+
+
 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
-- 
2.13.3