So CI is reporting a few flips on Skylake for intel_engines_are_idle(),
so apply a double check that the ring is valid before inspecting the
registers. However, we keep the forcewake omission in play to avoid the
Sandybridge failure.

Fixes: 0b702dca2658 ("drm/i915: Avoid waking the engines just to check if they 
are idle")
Signed-off-by: Chris Wilson <[email protected]>
Cc: Mika Kuoppala <[email protected]>
---
 drivers/gpu/drm/i915/intel_engine_cs.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/drivers/gpu/drm/i915/intel_engine_cs.c 
b/drivers/gpu/drm/i915/intel_engine_cs.c
index 4f244019560d..e3614e483f40 100644
--- a/drivers/gpu/drm/i915/intel_engine_cs.c
+++ b/drivers/gpu/drm/i915/intel_engine_cs.c
@@ -987,7 +987,8 @@ static bool ring_is_idle(struct intel_engine_cs *engine)
         * If the engine is not awake, both reads return 0 as we do so without
         * forcewake.
         */
-       if ((I915_READ_FW(RING_HEAD(engine->mmio_base)) & HEAD_ADDR) !=
+       if (I915_READ_FW(RING_CTL(engine->mmio_base)) & RING_VALID &&
+           (I915_READ_FW(RING_HEAD(engine->mmio_base)) & HEAD_ADDR) !=
            (I915_READ_FW(RING_TAIL(engine->mmio_base)) & TAIL_ADDR))
                idle = false;
 
-- 
2.20.1

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