Date: Fri, 17 Mar 2017 14:49:27 -0300
From: Mauro Carvalho Chehab <mchehab@s-opensource.com>
To: Russell King - ARM Linux <linux@armlinux.org.uk>
Cc: Philipp Zabel <p.zabel@pengutronix.de>,
        Hans Verkuil <hverkuil@xs4all.nl>, Sakari Ailus <sakari.ailus@iki.fi>,
        Steve Longerbeam <slongerbeam@gmail.com>, robh+dt@kernel.org,
        mark.rutland@arm.com, shawnguo@kernel.org, kernel@pengutronix.de,
        fabio.estevam@nxp.com, mchehab@kernel.org, nick@shmanahar.org,
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        shuah@kernel.org, sakari.ailus@linux.intel.com, pavel@ucw.cz,
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        linux-arm-kernel@lists.infradead.org, linux-media@vger.kernel.org,
        devel@driverdev.osuosl.org,
        Steve Longerbeam <steve_longerbeam@mentor.com>,
        Jacek Anaszewski <j.anaszewski@samsung.com>
Subject: Re: [PATCH v4 14/36] [media] v4l2-mc: add a function to inherit
 controls from a pipeline
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Em Fri, 17 Mar 2017 12:16:08 +0000
Russell King - ARM Linux <linux@armlinux.org.uk> escreveu:

> On Fri, Mar 17, 2017 at 01:02:07PM +0100, Philipp Zabel wrote:
> > I think most of the simple, fixed pipeline use cases could be handled by
> > libv4l2, by allowing to pass a v4l2 subdevice path to v4l2_open. If that
> > function internally would set up the media links to the
> > nearest /dev/video interface, propagate format, resolution and frame
> > intervals if necessary, and return an fd to the video device, there'd be
> > no additional complexity for the users beyond selecting the v4l2_subdev
> > instead of the video device.  
> 
> ... which would then require gstreamer to be modified too. The gstreamer
> v4l2 plugin looks for /dev/video* or /dev/v4l2/video* devices and monitors
> these for changes, so gstreamer applications know which capture devices
> are available:
> 
>   const gchar *paths[] = { "/dev", "/dev/v4l2", NULL };
>   const gchar *names[] = { "video", NULL };
> 
>   /* Add some depedency, so the dynamic features get updated upon changes in
>    * /dev/video* */
>   gst_plugin_add_dependency (plugin,
>       NULL, paths, names, GST_PLUGIN_DEPENDENCY_FLAG_FILE_NAME_IS_PREFIX);
> 
> I haven't checked yet whether sys/v4l2/gstv4l2deviceprovider.c knows
> anything about the v4l2 subdevs.

Not only gstreamer do that, but all simple V4L2 applications, although
on most of them, you can either pass a command line argument or setup
the patch via GUI.

Btw, I've no idea from where gstreamer took /dev/v4l2 :-)
I'm yet to find a distribution using it.

On the other hand, /dev/v4l/by-patch and /dev/v4l/by-id are usual directories
where V4L2 devices can be found, and should provide persistent names. So, IMHO,
gst should prefer those names, when they exist:

$ tree /dev/v4l
/dev/v4l
├── by-id
│   ├── usb-046d_HD_Pro_Webcam_C920_55DA1CCF-video-index0 -> ../../video1
│   └── usb-Sunplus_mMobile_Inc_USB_Web-CAM-video-index0 -> ../../video0
└── by-path
    ├── platform-3f980000.usb-usb-0:1.2:1.0-video-index0 -> ../../video1
    └── platform-3f980000.usb-usb-0:1.5:1.0-video-index0 -> ../../video0


Thanks,
Mauro