linux-kernel - Re: [PATCH v4 14/36] [media] v4l2-mc: add a function to inherit controls from a pipeline

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Date:   Fri, 17 Mar 2017 14:49:27 -0300
From:   Mauro Carvalho Chehab <mchehab@...pensource.com>
To:     Russell King - ARM Linux <linux@...linux.org.uk>
Cc:     Philipp Zabel <p.zabel@...gutronix.de>,
        Hans Verkuil <hverkuil@...all.nl>,
        Sakari Ailus <sakari.ailus@....fi>,
        Steve Longerbeam <slongerbeam@...il.com>, robh+dt@...nel.org,
        mark.rutland@....com, shawnguo@...nel.org, kernel@...gutronix.de,
        fabio.estevam@....com, mchehab@...nel.org, nick@...anahar.org,
        markus.heiser@...marIT.de,
        laurent.pinchart+renesas@...asonboard.com, bparrot@...com,
        geert@...ux-m68k.org, arnd@...db.de, sudipm.mukherjee@...il.com,
        minghsiu.tsai@...iatek.com, tiffany.lin@...iatek.com,
        jean-christophe.trotin@...com, horms+renesas@...ge.net.au,
        niklas.soderlund+renesas@...natech.se, robert.jarzmik@...e.fr,
        songjun.wu@...rochip.com, andrew-ct.chen@...iatek.com,
        gregkh@...uxfoundation.org, shuah@...nel.org,
        sakari.ailus@...ux.intel.com, pavel@....cz,
        devicetree@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, linux-media@...r.kernel.org,
        devel@...verdev.osuosl.org,
        Steve Longerbeam <steve_longerbeam@...tor.com>,
        Jacek Anaszewski <j.anaszewski@...sung.com>
Subject: Re: [PATCH v4 14/36] [media] v4l2-mc: add a function to inherit
 controls from a pipeline

Em Fri, 17 Mar 2017 12:16:08 +0000
Russell King - ARM Linux <linux@...linux.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