drm/i915: Fix mmio vs. CS flip race on ILK+
Starting from ILK, mmio flips also cause a flip done interrupt to be signalled. This means if we first do a set_base and follow it immediately with the CS flip, we might mistake the flip done interrupt caused by the set_base as the flip done interrupt caused by the CS flip. The hardware has a flip counter which increments every time a mmio or CS flip is issued. It basically counts the number of DSPSURF register writes. This means we can sample the counter before we put the CS flip into the ring, and then when we get a flip done interrupt we can check whether the CS flip has actually performed the surface address update, or if the interrupt was caused by a previous but yet unfinished mmio flip. Even with the flip counter we still have a race condition of the CS flip base address update happens after the mmio flip done interrupt was raised but not yet processed by the driver. When the interrupt is eventually processed, the flip counter will already indicate that the CS flip has been executed, but it would not actually complete until the next start of vblank. We can use the DSPSURFLIVE register to check whether the hardware is actually scanning out of the buffer we expect, or if we managed hit this race window. This covers all the cases where the CS flip actually changes the base address. If the base address remains unchanged, we might still complete the CS flip before it has actually completed. But since the address didn't change anyway, the premature flip completion can't result in userspace overwriting data that's still being scanned out. CTG already has the flip counter and DSPSURFLIVE registers, and although the flip done interrupt is still limited to CS flips alone, the code now also checks the flip counter on CTG as well. v2: s/dspsurf/gtt_offset/ (Chris) Testcase: igt/kms_mmio_vs_cs_flip/setcrtc_vs_cs_flip Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=73027Signed-off-by:Ville Syrjälä <ville.syrjala@linux.intel.com> Reviewed-by:
Rodrigo Vivi <rodrigo.vivi@gmail.com> [danvet: Add g4x_ prefix to flip_count_after_eq.] Signed-off-by:
Daniel Vetter <daniel.vetter@ffwll.ch>
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