Created attachment 124967 [details] 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).
I can also reproduce this with FBDEV driver being in use and default renderer.
test passes with Mesa 17.1.0-devel (git-2f7a7ae), resolving as fixed
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