drm/i915/ringbuffer: Flush writes before RING_TAIL update
Be paranoid and make sure we flush any and all writes out of the WCB before performing the UC mmio to update the RING_TAIL. (An UC write should itself be enough to do the flush, hence the paranoia here.) Quite infrequently, we see problems where the GPU seems to overshoot the RING_TAIL and so executes garbage, hence the speculation. References: https://bugs.freedesktop.org/show_bug.cgi?id=111598 References: https://bugs.freedesktop.org/show_bug.cgi?id=111417 References: https://bugs.freedesktop.org/show_bug.cgi?id=111034Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk> Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com> Link: https://patchwork.freedesktop.org/patch/msgid/20190909113018.13300-1-chris@chris-wilson.co.uk
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