[Mesa-dev] [Bug 96872] glReadPixels returns black pixels on first display in software mode

[bugzilla-daemon at freedesktop.org]

https://bugs.freedesktop.org/show_bug.cgi?id=96872

            Bug ID: 96872
           Summary: glReadPixels returns black pixels on first display in
                    software mode
           Product: Mesa
           Version: 10.5
          Hardware: Other
                OS: All
            Status: NEW
          Severity: normal
          Priority: medium
         Component: Mesa core
          Assignee: mesa-dev at lists.freedesktop.org
          Reporter: b7.10110111 at gmail.com
        QA Contact: mesa-dev at lists.freedesktop.org

Created attachment 124967
  --> https://bugs.freedesktop.org/attachment.cgi?id=124967&action=edit
Test program

The attached program uses glReadPixels after glClear and glFinish. On normal
HW-accelerated Mesa, this gives the expected result: the data defined by
glClearColor are received (the program just prints "Test passed"). This also
works normally if e.g. NVIDIA proprietary driver is in use, but Mesa libGL is
used to draw things. Similarly for Xephyr, where software rendering is the only
possible option.

But if I use e.g. intel driver but set LIBGL_ALWAYS_SOFTWARE=1 environment
variable for the test program, I get the following output:

Wrong data received from glReadPixels(): expected ff ff ff ff, got 00 00 00 00
Test passed

I.e. on first call to runTest() (which is a GLUT display function)
glReadPixels() returns black transparent pixels, and only on the second it
works as expected.

Was testing this on Intel Atom N550. The same result is observable on i915G
graphics.
The OS I tested this on is Kubuntu 14.04 with LTS enablement stack (packages
with "-vivid" suffix in names).

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