Hi,
I'm currently trying to figure out why glean/texCube reproducably fails on my
R420. That test basically has a number of sections that look like this
loop(..) {
render();
loop(..) {
glReadPixels(..);
check_values();
}
}
The outer loop continuously overwrites the same region of the framebuffer over
and over again.
In the third iteration of the outer loop in the "TestReflectionMap" section,
this test reads a stale pixel, i.e. the value returned from glReadPixels is
the value from the second iteration.
Each of the following changes individually fixes the problem:
1) Do not overwrite the same region of the framebuffer in every iteration;
instead, use a different framebuffer region for every iteration.
2) Add a sleep(1) before glReadPixels.
3) Add a sleep(1) after glReadPixels.
4) Call wbinvd() in the DRM at the end of radeon_cp_idle() (after the call to
radeon_do_cp_idle), so that radeon_span.c ends up triggering a wbinvd before
the actual read takes place.
To me, this looks like glReadPixels reads a stale value from a cache
somewhere. But which cache, and why? To add to the mystery, Glean has a lot
of tests that look very similar, and they work just fine (though they might
have a sufficiently different cache access pattern that might hide the
symptom). Is there something I'm missing? And finally: What's the proper way
to fix a problem like this?
I'd appreciate all the help I can get in understanding this problem, since
I've never dealt with something like it before.
Thanks,
Nicolai
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