[Intel-gfx] [PATCH] drm/i915/execlists: Unshadow MI_USER_INTERRUPT

[Chris Wilson]

Given an immediate preemption event on re-enabling arbitration
(MI_ARB_ON_OFF | MI_ARB_ENABLE) it appears that the HW may forget about
the ongoing MI_USER_INTERRUPT, losing the interrupt in the process. If
we happen to be waiting on that interrupt at the time, the system may
then grind to a halt.

My presumption is that there is an effective shadow inside the CS as it
parses and buffers the commands, and if we push the MI_USER_INTERRUPT
out of the immediate parse buffer it is not lost by the arbitration
check.

Testcase: igt/gem_concurrent_blit
Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
---
Plausible? I need a few hours to confirm my hunch.
-Chris
---
 drivers/gpu/drm/i915/gt/intel_lrc.c | 5 ++---
 1 file changed, 2 insertions(+), 3 deletions(-)

diff --git a/drivers/gpu/drm/i915/gt/intel_lrc.c b/drivers/gpu/drm/i915/gt/intel_lrc.c
index 851e62ddcb87..526cb9231d58 100644
--- a/drivers/gpu/drm/i915/gt/intel_lrc.c
+++ b/drivers/gpu/drm/i915/gt/intel_lrc.c
@@ -2271,14 +2271,13 @@ static u32 *gen8_emit_fini_breadcrumb(struct i915_request *request, u32 *cs)
 				  request->fence.seqno,
 				  request->timeline->hwsp_offset,
 				  0);
+	*cs++ = MI_USER_INTERRUPT;
 
 	cs = gen8_emit_ggtt_write(cs,
 				  intel_engine_next_hangcheck_seqno(request->engine),
 				  I915_GEM_HWS_HANGCHECK_ADDR,
 				  MI_FLUSH_DW_STORE_INDEX);
 
-
-	*cs++ = MI_USER_INTERRUPT;
 	*cs++ = MI_ARB_ON_OFF | MI_ARB_ENABLE;
 
 	request->tail = intel_ring_offset(request, cs);
@@ -2297,13 +2296,13 @@ static u32 *gen8_emit_fini_breadcrumb_rcs(struct i915_request *request, u32 *cs)
 				      PIPE_CONTROL_DC_FLUSH_ENABLE |
 				      PIPE_CONTROL_FLUSH_ENABLE |
 				      PIPE_CONTROL_CS_STALL);
+	*cs++ = MI_USER_INTERRUPT;
 
 	cs = gen8_emit_ggtt_write_rcs(cs,
 				      intel_engine_next_hangcheck_seqno(request->engine),
 				      I915_GEM_HWS_HANGCHECK_ADDR,
 				      PIPE_CONTROL_STORE_DATA_INDEX);
 
-	*cs++ = MI_USER_INTERRUPT;
 	*cs++ = MI_ARB_ON_OFF | MI_ARB_ENABLE;
 
 	request->tail = intel_ring_offset(request, cs);
-- 
2.20.1