[Bug 746834] New: v4l2sink: driver is not queried for minimum number of buffers when propose_allocation is not called

[GStreamer (GNOME Bugzilla)]

https://bugzilla.gnome.org/show_bug.cgi?id=746834

            Bug ID: 746834
           Summary: v4l2sink: driver is not queried for minimum number of
                    buffers when propose_allocation is not called
    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: tobias.modschiedler at cetitec.com
        QA Contact: gstreamer-bugs at lists.freedesktop.org
     GNOME version: ---

Created attachment 300372
  --> https://bugzilla.gnome.org/attachment.cgi?id=300372&action=edit
Proposed solution.

When propose_allocation() is not passed through to a v4l2sink, the driver is
never queried for its requirements regarding the minimum number of buffers to
use. Thus the buffer pool does not have this information.

I am pretty new to GStreamer, but my expectation would be that the driver is
always asked about this. The attached patch enables exactly this behavior: It
calls gst_v4l2_get_attribute() directly after the device is opened, to get
either V4L2_CID_MIN_BUFFERS_FOR_OUTPUT or V4L2_CID_MIN_BUFFERS_FOR_CAPTURE.
This number is then used as a default value for min_buffers when creating the
buffer pool.

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