drm/i915: Use ordered seqno write interrupt generation on gen8+ execlists

Broadwell and later currently use the same unordered command sequence to
update the seqno in the HWS status page and then assert the user
interrupt. We should apply the w/a from legacy (where we do an mmio
read to delay the seqno read after the interrupt), but this is not
enough to enforce coherent seqno visibilty on Skylake. Rather than
search for the proper post-interrupt seqno barrier, use a strongly
ordered command sequence to write the seqno, then assert the user
interrupt from the ring.

v2: Move around the wa tail dwords to avoid adding duplicate code.

v3: Add references, comments on workarounds and bit5 check.

References: https://bugs.freedesktop.org/show_bug.cgi?id=93693
Testcase: igt/gem_ring_sync_loop #skl
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Mika Kuoppala <mika.kuoppala@intel.com>
Reviewed-by: Mika Kuoppala <mika.kuoppala@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1453297415-17793-1-git-send-email-mika.kuoppala@intel.com

drivers/gpu/drm/i915/intel_lrc.c

@@ -760,23 +760,34 @@ static int logical_ring_wait_for_space(struct drm_i915_gem_request *req,
  * on a queue waiting for the ELSP to be ready to accept a new context submission. At that
  * point, the tail *inside* the context is updated and the ELSP written to.
  */
-static void
+static int
 intel_logical_ring_advance_and_submit(struct drm_i915_gem_request *request)
 {
-	struct intel_engine_cs *ring = request->ring;
+	struct intel_ringbuffer *ringbuf = request->ringbuf;
 	struct drm_i915_private *dev_priv = request->i915;
 
-	intel_logical_ring_advance(request->ringbuf);
+	intel_logical_ring_advance(ringbuf);
+	request->tail = ringbuf->tail;
 
-	request->tail = request->ringbuf->tail;
+	/*
+	 * Here we add two extra NOOPs as padding to avoid
+	 * lite restore of a context with HEAD==TAIL.
+	 *
+	 * Caller must reserve WA_TAIL_DWORDS for us!
+	 */
+	intel_logical_ring_emit(ringbuf, MI_NOOP);
+	intel_logical_ring_emit(ringbuf, MI_NOOP);
+	intel_logical_ring_advance(ringbuf);
 
-	if (intel_ring_stopped(ring))
-		return;
+	if (intel_ring_stopped(request->ring))
+		return 0;
 
 	if (dev_priv->guc.execbuf_client)
 		i915_guc_submit(dev_priv->guc.execbuf_client, request);
 	else
 		execlists_context_queue(request);
+
+	return 0;
 }
 
 static void __wrap_ring_buffer(struct intel_ringbuffer *ringbuf)
@@ -1845,44 +1856,65 @@ static void bxt_a_set_seqno(struct intel_engine_cs *ring, u32 seqno)
 	intel_flush_status_page(ring, I915_GEM_HWS_INDEX);
 }
 
+/*
+ * Reserve space for 2 NOOPs at the end of each request to be
+ * used as a workaround for not being allowed to do lite
+ * restore with HEAD==TAIL (WaIdleLiteRestore).
+ */
+#define WA_TAIL_DWORDS 2
+
+static inline u32 hws_seqno_address(struct intel_engine_cs *engine)
+{
+	return engine->status_page.gfx_addr + I915_GEM_HWS_INDEX_ADDR;
+}
+
 static int gen8_emit_request(struct drm_i915_gem_request *request)
 {
 	struct intel_ringbuffer *ringbuf = request->ringbuf;
-	struct intel_engine_cs *ring = ringbuf->ring;
-	u32 cmd;
 	int ret;
 
-	/*
-	 * Reserve space for 2 NOOPs at the end of each request to be
-	 * used as a workaround for not being allowed to do lite
-	 * restore with HEAD==TAIL (WaIdleLiteRestore).
-	 */
-	ret = intel_logical_ring_begin(request, 8);
+	ret = intel_logical_ring_begin(request, 6 + WA_TAIL_DWORDS);
 	if (ret)
 		return ret;
 
-	cmd = MI_STORE_DWORD_IMM_GEN4;
-	cmd |= MI_GLOBAL_GTT;
+	/* w/a: bit 5 needs to be zero for MI_FLUSH_DW address. */
+	BUILD_BUG_ON(I915_GEM_HWS_INDEX_ADDR & (1 << 5));
 
-	intel_logical_ring_emit(ringbuf, cmd);
 	intel_logical_ring_emit(ringbuf,
-				(ring->status_page.gfx_addr +
-				(I915_GEM_HWS_INDEX << MI_STORE_DWORD_INDEX_SHIFT)));
+				(MI_FLUSH_DW + 1) | MI_FLUSH_DW_OP_STOREDW);
+	intel_logical_ring_emit(ringbuf,
+				hws_seqno_address(request->ring) |
+				MI_FLUSH_DW_USE_GTT);
 	intel_logical_ring_emit(ringbuf, 0);
 	intel_logical_ring_emit(ringbuf, i915_gem_request_get_seqno(request));
 	intel_logical_ring_emit(ringbuf, MI_USER_INTERRUPT);
 	intel_logical_ring_emit(ringbuf, MI_NOOP);
-	intel_logical_ring_advance_and_submit(request);
+	return intel_logical_ring_advance_and_submit(request);
+}
 
-	/*
-	 * Here we add two extra NOOPs as padding to avoid
-	 * lite restore of a context with HEAD==TAIL.
-	 */
-	intel_logical_ring_emit(ringbuf, MI_NOOP);
-	intel_logical_ring_emit(ringbuf, MI_NOOP);
-	intel_logical_ring_advance(ringbuf);
+static int gen8_emit_request_render(struct drm_i915_gem_request *request)
+{
+	struct intel_ringbuffer *ringbuf = request->ringbuf;
+	int ret;
 
-	return 0;
+	ret = intel_logical_ring_begin(request, 6 + WA_TAIL_DWORDS);
+	if (ret)
+		return ret;
+
+	/* 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,
+				(PIPE_CONTROL_GLOBAL_GTT_IVB |
+				 PIPE_CONTROL_CS_STALL |
+				 PIPE_CONTROL_QW_WRITE));
+	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));
+	intel_logical_ring_emit(ringbuf, MI_USER_INTERRUPT);
+	return intel_logical_ring_advance_and_submit(request);
 }
 
 static int intel_lr_context_render_state_init(struct drm_i915_gem_request *req)
@@ -2069,6 +2101,7 @@ static int logical_render_ring_init(struct drm_device *dev)
 	ring->init_context = gen8_init_rcs_context;
 	ring->cleanup = intel_fini_pipe_control;
 	ring->emit_flush = gen8_emit_flush_render;
+	ring->emit_request = gen8_emit_request_render;
 
 	ring->dev = dev;
 

drivers/gpu/drm/i915/intel_ringbuffer.h

@@ -426,6 +426,7 @@ intel_write_status_page(struct intel_engine_cs *ring,
  * The area from dword 0x30 to 0x3ff is available for driver usage.
  */
 #define I915_GEM_HWS_INDEX		0x30
+#define I915_GEM_HWS_INDEX_ADDR (I915_GEM_HWS_INDEX << MI_STORE_DWORD_INDEX_SHIFT)
 #define I915_GEM_HWS_SCRATCH_INDEX	0x40
 #define I915_GEM_HWS_SCRATCH_ADDR (I915_GEM_HWS_SCRATCH_INDEX << MI_STORE_DWORD_INDEX_SHIFT)