I think I've got a concrete test here for something I've been trying to track down for a while now: it appears that when the GPU is too busy, batches get dropped and sometimes the GPU hangs. This really became clear when I was testing a patch for citybench to "improve" the swap throttling -- a broken version that I submitted caused no throttling to occur at all. The app, which renders a predefined set of frames as fast as possible, would now frequently stutter in the middle. The results were insanely fast (40-60% better!). But it didn't make sense for there to be a stutter if the set of frames rendered is predefined. So, I wrote this little patch series to try to test my guess that my batches were getting dropped. The plan is: Emit a dword write every (render) batch, and after a while go look and see if they all landed. Corresponding to the stutters is: Mesa: Initializing x86-64 optimizations batch 42 didn't report: 0xd0d0d0d0 instead of 0x0000002a batch 43 didn't report: 0xd0d0d0d0 instead of 0x0000002b batch 44 didn't report: 0xd0d0d0d0 instead of 0x0000002c batch 45 didn't report: 0xd0d0d0d0 instead of 0x0000002d batch 46 didn't report: 0xd0d0d0d0 instead of 0x0000002e batch 47 didn't report: 0xd0d0d0d0 instead of 0x0000002f batch 48 didn't report: 0xd0d0d0d0 instead of 0x00000030 batch 49 didn't report: 0xd0d0d0d0 instead of 0x00000031 batch 50 didn't report: 0xd0d0d0d0 instead of 0x00000032 batch 51 didn't report: 0xd0d0d0d0 instead of 0x00000033 batch 52 didn't report: 0xd0d0d0d0 instead of 0x00000034 batch 53 didn't report: 0xd0d0d0d0 instead of 0x00000035 shutting up The cool thing is apparently I don't need citybench, either. This paste came from vblank_mode=0 glxgears fullscreened on my 1600x900 panel (default size doesn't reproduce the failure). This is gen7, danvet's kernel de67cba65944f26c0f147035bd62e30c5f456b96 rebased on top of cherry-picks of the 4 ivb workaround patches. A revert of a71d8d94525e8fd855c0466fb586ae1cb008f3a2 doesn't help. I've also run this on a test libdrm that returns NULL pointers on map failure and asserts that exec and pwrite don't return errors.
