linux-kernel - Re: [PATCH v5 2/7] media: open.rst: better document device node naming

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Date:   Tue, 29 Aug 2017 11:34:06 +0300
From:   Sakari Ailus <sakari.ailus@....fi>
To:     Mauro Carvalho Chehab <mchehab@...pensource.com>
Cc:     Linux Doc Mailing List <linux-doc@...r.kernel.org>,
        Linux Media Mailing List <linux-media@...r.kernel.org>,
        Mauro Carvalho Chehab <mchehab@...radead.org>,
        linux-kernel@...r.kernel.org, Jonathan Corbet <corbet@....net>,
        Hans Verkuil <hans.verkuil@...co.com>
Subject: Re: [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@...co.com>
> 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..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@....fi