[Bug 764319] New: videorate : avoid useless buffer copy un drop-only mode

[GStreamer (GNOME Bugzilla)]

https://bugzilla.gnome.org/show_bug.cgi?id=764319

            Bug ID: 764319
           Summary: videorate : avoid useless buffer copy un drop-only
                    mode
    Classification: Platform
           Product: GStreamer
           Version: git master
                OS: All
            Status: NEW
          Severity: normal
          Priority: Normal
         Component: gst-plugins-base
          Assignee: gstreamer-bugs at lists.freedesktop.org
          Reporter: fred.bertolus at gmail.com
        QA Contact: gstreamer-bugs at lists.freedesktop.org
     GNOME version: ---

Created attachment 324956
  --> https://bugzilla.gnome.org/attachment.cgi?id=324956&action=edit
Proposed patch

The videorate element has a reference to the last buffer in videorate->prevbuf.
In gst_video_rate_flush_prev, before push the buffer, videorate has to change
the timestamp of the buffer so it make it writable with
gst_buffer_make_writable. Before that, videorate take another reference on the
buffer because one of these ref will be transfered at the push pad and the
other one is for the videorate->prevbuf.

So when gst_buffer_make_writable is call, the ref count of the buffer is at
least 2 and the buffer will be copied. If the memory is not sharable, there
will be a deep copy.

This is the expected behavior but it's not optimal in the drop-only mode.
In drop only mode, videorate will never duplicate a buffer so it don't have to
keep a reference on the previous buffer. In fact, video rate don't keep the
previous buffer : just after calling gst_video_rate_flush_prev, it release its
ref to the buffer :

/* No need to keep the buffer around for longer */
gst_buffer_replace (&videorate->prevbuf, NULL);

However this is done after taking the ref for the push pad and after the
gst_buffer_make_writable, so the copy is already done, and the videorate never
use this copy.

The proposed patch release the useless reference to prevbuf before make the
buffer writable, in the case we are in drop-only mode.

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