[Bug 790265] New: v4l2src: Give preference to contiguous memory format over non contiguous one while setting format to /dev/video

[GStreamer (GNOME Bugzilla)]

https://bugzilla.gnome.org/show_bug.cgi?id=790265

            Bug ID: 790265
           Summary: v4l2src: Give preference to contiguous memory format
                    over non contiguous one while setting format to
                    /dev/video
    Classification: Platform
           Product: GStreamer
           Version: 1.8.3
                OS: Linux
            Status: NEW
          Severity: enhancement
          Priority: Normal
         Component: gst-plugins-good
          Assignee: gstreamer-bugs at lists.freedesktop.org
          Reporter: devarsht at xilinx.com
        QA Contact: gstreamer-bugs at lists.freedesktop.org
     GNOME version: ---

Created attachment 363457
  --> https://bugzilla.gnome.org/attachment.cgi?id=363457&action=edit
Patch we made to chage preference order for choosing contiguous video format
over non-contiguous one

Hello,

According to my understanding gstreamer element v4l2src is programmed to prefer
non-contiguous video format over contiguous one, i.e if you specify
GST_VIDEO_FORMAT_NV12 in pipeline, then v4l2src will program the /dev/video
node with V4L2_PIX_FMT_NV12M and not V4L2_PIX_FMT_NV12 if both are available
from capture device point of view.

(I think this preference is set in
https://cgit.freedesktop.org/gstreamer/gst-plugins-
good/tree/sys/v4l2/gstv4l2object.c line 1822)

Now this was breaking the functionality of our use-case as one of the peer
element had some issues with V4L2_PIX_FMT_NV12M so we had to change the
preference order with attached patch and then we were able to use
V4L2_PIX_FMT_NV12.

In my opinion think the better solutino here would be to give flexibility to
user to choose whichever format he/she wants to use (like contiguous or
non-contiguous), so can you please guide me on what is the best way to do this
?

In my view I see below possible things:
1) Add a Boolean property prefer_contiguous_format in v4l2src plugin.
2) Add separate GST_VIDEO_FORMAT macro for multiplanar formats.
3) Add some property in caps through which user can select preference for
contiguous video format if available.

Based upon your suggestion I would try to provide an updated patch.

Thanks,
Devarsh

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