[krillin] overlays are not displayed onscreen in some positions

This always happens when the driver rejects part of the list, and accepts the other part.

 # | Type |
 0 | OVERLAY |
 1 | GL_RENDER |
 2 | FB_TARGET |

The gl-rendered layer is displayed, but the overlay layer is not.

The decision to accept or reject a particular buffer seems based on stride and position. If all are accepted as overlays, or if all are rejected and drawn with GL, then we don't see a problem. If part of the list is accepted and part is rejected, then the driver does not display the buffers that it has accepted. Still digging down in the driver.

So after a fair amount of digging into the kernel, it seems that when the composition mode is mixed, the buffer that corresponds to the fb target ends up as the top layer in the display processor controller. If I dig down and force this layer to be 50% alpha by modifying the right register, I can see the other client rendering underneath.

So, on this device the 'overlays' are actually 'underlays' in that the fb target layer is always presented as the top-most layer, and the overlays make it to the screen by being alpha-blended with the topmost layer. Mir was clearing this using glClearColor(0.0, 0.0, 0.0, 1.0), and so the fb target layer was blocking the underlays from appearing beneath. We should clear with glClearColor(0.0, 0.0, 0.0, 0.0) in the fallback renderer instead.

Fix committed into lp:mir at revision None, scheduled for release in mir, milestone 0.9.0

This bug was fixed in the package mir - 0.9.0+15.04.20141125-0ubuntu1

---------------
mir (0.9.0+15.04.20141125-0ubuntu1) vivid; urgency=medium

  [ Alberto Aguirre ]
  * New upstream release 0.9.0 (https://launchpad.net/mir/+milestone/0.9.0)
    - Enhancements:
      . New simpler API to configure and run a mir server.
      . The event loop is now based on GLib's main loop library instead of
        Boost.Asio.
      . For Android platforms, the server now sends buffer fence fds to its
        clients instead of potentially stalling the compositor thread waiting
        for them to be signalled.
      . New client debug interface to translate from surface to screen
        coordinates.
    - ABI summary: Servers need rebuilding, but clients do not;
      . Mirclient ABI unchanged at 8
      . Mircommon ABI bumped to 3
      . Mirplatform ABI bumped to 4
      . Mirserver ABI bumped to 27
    - Bug fixes:
      . Add a debug interface to translate from surface to screen coordinates
        (LP: #1346633)
      . Ensure a buffer requested by a surface is not delivered
        after the surface is deleted (LP: #1376324)
      . Overlays are not displayed onscreen in some positions (LP: #1378326)
      . Server aborts when an exception is thrown from the main thread
        (LP: #1378740)
      . Fix race causing lost alarm notifications (LP: #1381925)
      . Avoid lifecycle notifications racing with connection release
        (LP: #1386646)
      . Improve error checking and reporting for the client library
       (LP: #1390388)
      . Mir demo-shell now detects power button using proper Linux scan codes
       (LP: #1303817)
      . A prompt session with an invalid application pid should be an error
        (LP: #1377968)
      . When XDG_RUNTIME_DIR is defined but pointing to a non-existing
        directory use "/tmp" (LP: #1304873)
      . [regression] demo-shell bypass is not used on fullscreen surfaces if
        there are windowed surfaces behind (LP: #1378706)
      . Mir upgrade through dist-upgrade installs incorrect platform
        (LP: #1378995)
      . Fix Mir progressbar example using internal glibc defines(LP: #239272)
      . Stop the default_lifecycle_event_handler raising SIGHUP while
        disconnecting (LP: #1386185)
      . [regression] Mir fails to build with MIR_ENABLE_TESTS=OFF (LP: #1388539)
      . [regression] mir_demo_server_basic does not start (LP: #1391923)

  [ Ubuntu daily release ]
  * New rebuild forced
 -- Ubuntu daily release <email address hidden> Tue, 25 Nov 2014 17:49:24 +0000