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SubjectRe: [PATCH v5 2/7] media: open.rst: better document device node naming
Hi Mauro,

On Mon, Aug 28, 2017 at 09:53:56AM -0300, Mauro Carvalho Chehab wrote:
> 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 | 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..fc0037091814 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 and modulators
> +``/dev/swradioX`` Software Defined Radio tuners and modulators
> +``/dev/v4l-touchX`` Touch sensors

Should we document V4L2 sub-device nodes here as well? They are implemented
by the V4L2 core as well as the other device node types.

Their purpose is somewhat different, though, and I think we'll need to make
that explicit somehow.

> +======================== ======================================================
> +
> +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 warranty that ``X`` will remain the same for the same

s/warranty/guarantee/

> + 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/`` directoiries containing

"directories"

> + 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
> +
> +
> 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

--
Kind regards,

Sakari Ailus
e-mail: sakari.ailus@iki.fi

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