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Subject[PATCH v6 2/7] media: open.rst: better document device node naming
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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.

Acked-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>
---
Documentation/media/uapi/v4l/open.rst | 44 ++++++++++++++++++++++++++++++++---
1 file changed, 41 insertions(+), 3 deletions(-)

diff --git a/Documentation/media/uapi/v4l/open.rst b/Documentation/media/uapi/v4l/open.rst
index afd116edb40d..7e7aad784388 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 +25,42 @@ 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 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. **V4L2 sub-device nodes** (e. g. ``/dev/v4l-sudevX``) provide a
+ different API and aren't considered as V4L2 device nodes.
+
+ 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
--
2.13.5
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