https://bugs.freedesktop.org/show_bug.cgi?id=38800
--- Comment #16 from Michel Dänzer michel@daenzer.net 2011-07-05 10:37:21 PDT --- (In reply to comment #15)
Whatever resolution I choose, perf top shows me that 50% of my used CPU time is spent in r600_fence_finish from the r600g DRI driver.
That makes some sense, as r600_fence_finish() is currently more or less a polling loop. This CPU usage is probably glFinish() waiting for the frame to finish rendering.
(BTW, IIRC the intel drivers used to have a broken glFinish() implementation which didn't always wait for all rendering to finish. I wonder if your tests might have been affected by that)
I also see about 5% of my used CPU time in radeon_fp_native_mode from the kernel driver; again, this is resolution independent.
Hmm, this might indicate it's constantly trying to get a native mode for the fake LVDS. Have you tried force-disabling it, to see if that makes any difference?
I'm forced to conclude that it's not CPU-side slowdown - is there any way (like Intel's intel_gpu_top and intel_gpu_dump) to see roughly what the GPU is up to?
Not yet, unfortunately.
However, I think we first need to narrow down further where exactly the CPU is blocking, and why.