[Bug 736072] New: v4l2: set min_latency for output device according to required minimum number of buffers

[GStreamer (bugzilla.gnome.org)]

https://bugzilla.gnome.org/show_bug.cgi?id=736072
  GStreamer | gst-plugins-good | git

           Summary: v4l2: set min_latency for output device according to
                    required minimum number of buffers
    Classification: Platform
           Product: GStreamer
           Version: git
        OS/Version: Linux
            Status: UNCONFIRMED
          Severity: normal
          Priority: Normal
         Component: gst-plugins-good
        AssignedTo: gstreamer-bugs at lists.freedesktop.org
        ReportedBy: aurelien.zanelli at parrot.com
         QAContact: gstreamer-bugs at lists.freedesktop.org
     GNOME version: ---


Since we can get the minimum number of buffers needed by an output device to
work, use it to set min_latency which will determine how many buffers are
queued.


Device I use need at least 3 buffers to work correctly. Without this patch
poo->min_latency is set to GST_V4L2_MIN_BUFFERS == 2.
At allocation time, V4L2 device doesn't complain because we request more than 3
buffers sinc v4l2 bufferpool min_buffers is set to 6.
But when streaming only 2 buffers will be queued at a time since we will
dequeue a buffer when pool->num_queued reachs pool->min_latency.

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