[Bug 79223] New: extra vsync when reading back pixels in xbmc

bugzilla-daemon at freedesktop.org bugzilla-daemon at freedesktop.org
Sun May 25 12:10:07 PDT 2014


          Priority: medium
            Bug ID: 79223
          Assignee: dri-devel at lists.freedesktop.org
           Summary: extra vsync when reading back pixels in xbmc
          Severity: normal
    Classification: Unclassified
                OS: All
          Reporter: pierre-bugzilla at ossman.eu
          Hardware: Other
            Status: NEW
           Version: unspecified
         Component: DRM/Radeon
           Product: DRI

Something is causing an extra wait for vsync in xbmc when it needs to read back
pixels. This seems to be specific to the radeon driver as I'm not seeing it
with nouveau.

Test scenario:

 - Configure xbmc to wait for vsync

 - Install something that activates xbmc's capture code (e.g. xbmc's boblight

 - Play a video whose framerate > screen refresh / 2

You'll see the frame rate reliably locked to screen refresh / 2 under these
circumstances. A major problem as you generally want to run with a 24 Hz
refresh rate for 24 Hz films.

I'm unable to pinpoint exactly the offending GL command as xbmc's rendering is
quite complex. I hope you're more experienced in finding these things. :)

The rough sequence of events is:

1. xbmc renders the scene
2. Wait for vsync
3. Map PBO and memcpy data. (from previous frame)
4. Render small version of scene
5. glReadPixels into PBO

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