[Bug 796986] New: v4l2convert can't import padded DMA buffers

[GStreamer (GNOME Bugzilla)]

https://bugzilla.gnome.org/show_bug.cgi?id=796986

            Bug ID: 796986
           Summary: v4l2convert can't import padded DMA buffers
    Classification: Platform
           Product: GStreamer
           Version: git master
                OS: Linux
            Status: NEW
          Severity: normal
          Priority: Normal
         Component: gst-plugins-good
          Assignee: gstreamer-bugs at lists.freedesktop.org
          Reporter: p.zabel at pengutronix.de
        QA Contact: gstreamer-bugs at lists.freedesktop.org
     GNOME version: ---

gst_v4l2_object_set_format already handles VIDIOC_S_FMT returning larger than
requested buffers, and sets GstVideoMeta appropriately for the added padding.
This happens for example for v4l2videodec, which can produce buffers padded to
macroblock alignment.
Currently such padded DMA buffers can't be imported info v4l2convert, as the
OUTPUT queue format is set to the visible size, which causes VIDIOC_QBUF to
fail with the incompatible larger DMA buffers.

A gst_v4l2_object_set_format variant is needed that gets passed the padding
information obtained from GstVideoMeta, to actually request a compatible format
at the importing OUTPUT queue. I'm unsure whether the parameter added to this
_set_format variant should be GstVideoMeta directly, a derived GstVideoInfo, or
even GstVideoAlignment.
Setting a padded queue format, and then setting the processed rectangle via the
crop selection would allow v4l2convert to handle imported DMA buffers that are
larger in either width or height than the negotiated visible size.

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