An interesting snippet from Sandybridge's prm:
"Although a Ring Buffer can be enabled in the non-empty state, it must
not be disabled unless it is empty. Attempting to disable a Ring Buffer
in the non-empty state is UNDEFINED."
Let's avoid the undefined behaviour as we disable the rings prior to
reset and resume.
Signed-off-by: Chris Wilson <chris@xxxxxxxxxxxxxxxxxx>
Cc: Mika Kuoppala <mika.kuoppala@xxxxxxxxxxxxxxx>
---
drivers/gpu/drm/i915/intel_ringbuffer.c | 4 +++-
drivers/gpu/drm/i915/intel_uncore.c | 4 +++-
2 files changed, 6 insertions(+), 2 deletions(-)
diff --git a/drivers/gpu/drm/i915/intel_ringbuffer.c b/drivers/gpu/drm/i915/intel_ringbuffer.c
index 05e01446b00b..3f2073a9d37a 100644
--- a/drivers/gpu/drm/i915/intel_ringbuffer.c
+++ b/drivers/gpu/drm/i915/intel_ringbuffer.c
@@ -480,10 +480,12 @@ static bool stop_ring(struct intel_engine_cs *engine)
}
}
- I915_WRITE_CTL(engine, 0);
I915_WRITE_HEAD(engine, 0);
I915_WRITE_TAIL(engine, 0);
+ /* The ring must be empty before it is disabled */
+ I915_WRITE_CTL(engine, 0);
+
return (I915_READ_HEAD(engine) & HEAD_ADDR) == 0;
}
diff --git a/drivers/gpu/drm/i915/intel_uncore.c b/drivers/gpu/drm/i915/intel_uncore.c
index 20e3c65c0999..ac688ee6fe4e 100644
--- a/drivers/gpu/drm/i915/intel_uncore.c
+++ b/drivers/gpu/drm/i915/intel_uncore.c
@@ -1387,10 +1387,12 @@ static void gen3_stop_engine(struct intel_engine_cs *engine)
DRM_DEBUG_DRIVER("%s: timed out on STOP_RING\n",
engine->name);
- I915_WRITE_FW(RING_CTL(base), 0);
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);
+
/* Check acts as a post */
if (I915_READ_FW(RING_HEAD(base)) != 0)
DRM_DEBUG_DRIVER("%s: ring head not parked\n",
--
2.15.0.rc2
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