Our I915g (early gen3, the oldest machine we have in the farm) is still
reporting occasional incoherency performing the following operations:

  1) write through GGTT (indirect write into memory)
  2) write through either CPU or WC (direct write into memory)
  3) read from GGTT (indirect read)

Instead of reporting the value from (2), the read from GGTT reports the
earlier value written via the GGTT. We have made sure that the writes are
flushed from the CPU (commit 3a32497f ("drm/i915/selftests: Provide
full mb() around clflush") and commit add00e6d ("drm/i915: Flush the
WCB following a WC write")), but still see the error, just less
frequently. The only remaining cache that might be affected here is a
chipset cache, so flush that as well.

Testcase: igt/drv_selftest/live_coherency #gdg
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180717092655.28417-1-chris@chris-wilson.co.uk