drm/i915/execlists: Drop preemption arbitrations points along the ring

Limit the arbitration (where preemption may occur) to inside the batch,
and prevent it from happening on the pipecontrols/flushes we use to
write the breadcrumb seqno. Once the user batch is complete, we have
nothing left to do but serialise and emit the breadcrumb; switching
contexts at this point is futile so don't.
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Michał Winiarski <michal.winiarski@intel.com>
Cc: Michel Thierry <michel.thierry@intel.com>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Michel Thierry <michel.thierry@intel.com>
Reviewed-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180503195416.22498-1-chris@chris-wilson.co.uk

drivers/gpu/drm/i915/intel_lrc.c

@@ -1933,7 +1933,7 @@ static int gen8_emit_bb_start(struct i915_request *rq,
 		rq->ctx->ppgtt->pd_dirty_rings &= ~intel_engine_flag(rq->engine);
 	}
 
-	cs = intel_ring_begin(rq, 4);
+	cs = intel_ring_begin(rq, 6);
 	if (IS_ERR(cs))
 		return PTR_ERR(cs);
 
@@ -1962,6 +1962,9 @@ static int gen8_emit_bb_start(struct i915_request *rq,
 		(flags & I915_DISPATCH_RS ? MI_BATCH_RESOURCE_STREAMER : 0);
 	*cs++ = lower_32_bits(offset);
 	*cs++ = upper_32_bits(offset);
+
+	*cs++ = MI_ARB_ON_OFF | MI_ARB_DISABLE;
+	*cs++ = MI_NOOP;
 	intel_ring_advance(rq, cs);
 
 	return 0;
@@ -2104,7 +2107,7 @@ static void gen8_emit_breadcrumb(struct i915_request *request, u32 *cs)
 	cs = gen8_emit_ggtt_write(cs, request->global_seqno,
 				  intel_hws_seqno_address(request->engine));
 	*cs++ = MI_USER_INTERRUPT;
-	*cs++ = MI_NOOP;
+	*cs++ = MI_ARB_ON_OFF | MI_ARB_ENABLE;
 	request->tail = intel_ring_offset(request, cs);
 	assert_ring_tail_valid(request->ring, request->tail);
 
@@ -2120,7 +2123,7 @@ static void gen8_emit_breadcrumb_rcs(struct i915_request *request, u32 *cs)
 	cs = gen8_emit_ggtt_write_rcs(cs, request->global_seqno,
 				      intel_hws_seqno_address(request->engine));
 	*cs++ = MI_USER_INTERRUPT;
-	*cs++ = MI_NOOP;
+	*cs++ = MI_ARB_ON_OFF | MI_ARB_ENABLE;
 	request->tail = intel_ring_offset(request, cs);
 	assert_ring_tail_valid(request->ring, request->tail);