[Intel-gfx] [PATCH v3] drm/i915: Adjust size of PIPE_CONTROL used for gen8 render seqno write

[Michał Winiarski]

On gen8+ size of PIPE_CONTROL with Post Sync Operation should be
6 dwords. When we're using older 5-dword variant it's possible to
observe inconsistent values written by 6-dword PIPE_CONTROL with Post
Sync Operation from user batches.

v2: Fix BAT failures
v3: Comments on alignment and thrashing high dword of seqno (Chris)

Testcase: igt/gem_pipe_control_store_loop/*-qword-write
Cc: Chris Wilson <chris at chris-wilson.co.uk>
Signed-off-by: Michał Winiarski <michal.winiarski at intel.com>
---
 drivers/gpu/drm/i915/intel_lrc.c | 10 ++++++++--
 1 file changed, 8 insertions(+), 2 deletions(-)

diff --git a/drivers/gpu/drm/i915/intel_lrc.c b/drivers/gpu/drm/i915/intel_lrc.c
index 6fcbf6b..4c7ebc4 100644
--- a/drivers/gpu/drm/i915/intel_lrc.c
+++ b/drivers/gpu/drm/i915/intel_lrc.c
@@ -1925,15 +1925,18 @@ static int gen8_emit_request_render(struct drm_i915_gem_request *request)
 	struct intel_ringbuffer *ringbuf = request->ringbuf;
 	int ret;
 
-	ret = intel_logical_ring_begin(request, 6 + WA_TAIL_DWORDS);
+	ret = intel_logical_ring_begin(request, 8 + WA_TAIL_DWORDS);
 	if (ret)
 		return ret;
 
+	/* We're using qword write, seqno should be aligned to 8 bytes. */
+	BUILD_BUG_ON(I915_GEM_HWS_INDEX & 1);
+
 	/* w/a for post sync ops following a GPGPU operation we
 	 * need a prior CS_STALL, which is emitted by the flush
 	 * following the batch.
 	 */
-	intel_logical_ring_emit(ringbuf, GFX_OP_PIPE_CONTROL(5));
+	intel_logical_ring_emit(ringbuf, GFX_OP_PIPE_CONTROL(6));
 	intel_logical_ring_emit(ringbuf,
 				(PIPE_CONTROL_GLOBAL_GTT_IVB |
 				 PIPE_CONTROL_CS_STALL |
@@ -1941,7 +1944,10 @@ static int gen8_emit_request_render(struct drm_i915_gem_request *request)
 	intel_logical_ring_emit(ringbuf, hws_seqno_address(request->ring));
 	intel_logical_ring_emit(ringbuf, 0);
 	intel_logical_ring_emit(ringbuf, i915_gem_request_get_seqno(request));
+	/* We're thrashing one dword of HWS. */
+	intel_logical_ring_emit(ringbuf, 0);
 	intel_logical_ring_emit(ringbuf, MI_USER_INTERRUPT);
+	intel_logical_ring_emit(ringbuf, MI_NOOP);
 	return intel_logical_ring_advance_and_submit(request);
 }
 
-- 
2.7.1