[PATCH 1/5] drm/i915: Be paranoid and post the writes to stop the rings

Chris Wilson chris at chris-wilson.co.uk
Sat Dec 16 21:33:08 UTC 2017

Although the mmio are uncached and so should be flushed on every write,
be paranoid and do a mmio read after setting the ring head/tail to be
sure they have taken effect before moving on.

Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
Cc: Mika Kuoppala <mika.kuoppala at linux.intel.com>
Cc: Joonas Lahtinen <joonas.lahtinen at linux.intel.com>
 drivers/gpu/drm/i915/intel_uncore.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/drivers/gpu/drm/i915/intel_uncore.c b/drivers/gpu/drm/i915/intel_uncore.c
index 89547b614aa6..7af78043105f 100644
--- a/drivers/gpu/drm/i915/intel_uncore.c
+++ b/drivers/gpu/drm/i915/intel_uncore.c
@@ -1522,9 +1522,11 @@ static void gen3_stop_engine(struct intel_engine_cs *engine)
 	I915_WRITE_FW(RING_HEAD(base), I915_READ_FW(RING_TAIL(base)));
+	POSTING_READ_FW(RING_HEAD(base)); /* paranoia */
 	I915_WRITE_FW(RING_HEAD(base), 0);
 	I915_WRITE_FW(RING_TAIL(base), 0);
 	/* The ring must be empty before it is disabled */
 	I915_WRITE_FW(RING_CTL(base), 0);

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