[PATCH v3] drm/i915/execlists: Move WA_TAIL_DWORDS to callee

* [PATCH v3] drm/i915/execlists: Move WA_TAIL_DWORDS to callee
@ 2016-02-05 19:31 Dave Gordon
  2016-02-05 22:56 ` Rodrigo Vivi
  2016-02-08 12:58 ` ✓ Fi.CI.BAT: success for " Patchwork
  0 siblings, 2 replies; 6+ messages in thread
From: Dave Gordon @ 2016-02-05 19:31 UTC (permalink / raw)
  To: intel-gfx; +Cc: Ben Widawsky

From: Chris Wilson <chris@chris-wilson.co.uk>

Currently emit-request starts writing to the ring and reserves space
for a workaround to be emitted later whilst submitting the request. It
is easier to read if the caller only allocates sufficient space for
its access (then the reader can quickly verify that the ring begin
allocates the exact space for the number of dwords emitted) and closes
the access to the ring. During submit, if we need to add the workaround,
we can reacquire ring access, in the assurance that we reserved space
for ourselves when beginning the request.

v3:
    Reinstated #define for WA_TAIL_DWORDS so we could accommodate GPUs
    that required different amounts of padding, generalised NOOP fill
    [Rodrigo Vivi], added W/A space to reserved amount and updated
    code comments [Dave Gordon],

Originally-by: Rodrigo Vivi <rodrigo.vivi@gmail.com>
Rewritten-by: Chris Wilson <chris@chris-wilson.co.uk>
Further-tweaked-by: Dave Gordon <david.s.gordon@intel.com>

Signed-off-by: Dave Gordon <david.s.gordon@intel.com>
Cc: Rodrigo Vivi <rodrigo.vivi@gmail.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Dave Gordon <david.s.gordon@intel.com>
Cc: Ben Widawsky <benjamin.widawsky@intel.com>
---
 drivers/gpu/drm/i915/intel_lrc.c | 80 ++++++++++++++++++++++++----------------
 1 file changed, 49 insertions(+), 31 deletions(-)

diff --git a/drivers/gpu/drm/i915/intel_lrc.c b/drivers/gpu/drm/i915/intel_lrc.c
index 3a03646..8b278f1 100644
--- a/drivers/gpu/drm/i915/intel_lrc.c
+++ b/drivers/gpu/drm/i915/intel_lrc.c
@@ -225,6 +225,13 @@ enum {
 #define GEN8_CTX_ID_SHIFT 32
 #define CTX_RCS_INDIRECT_CTX_OFFSET_DEFAULT  0x17
 
+/*
+ * 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 int intel_lr_context_pin(struct intel_context *ctx,
 				struct intel_engine_cs *engine);
 static void lrc_setup_hardware_status_page(struct intel_engine_cs *ring,
@@ -462,10 +469,9 @@ static void execlists_context_unqueue(struct intel_engine_cs *ring)
 		 */
 		if (req0->elsp_submitted) {
 			/*
-			 * Apply the wa NOOPS to prevent ring:HEAD == req:TAIL
-			 * as we resubmit the request. See gen8_emit_request()
-			 * for where we prepare the padding after the end of the
-			 * request.
+			 * Consume the W/A NOOPs to prevent ring:HEAD == req:TAIL as
+			 * we resubmit the request. See intel_logical_ring_submit()
+			 * where we prepare the padding after the end of the request.
 			 */
 			struct intel_ringbuffer *ringbuf;
 
@@ -752,33 +758,47 @@ static int logical_ring_wait_for_space(struct drm_i915_gem_request *req,
 }
 
 /*
- * intel_logical_ring_advance_and_submit() - advance the tail and submit the workload
- * @request: Request to advance the logical ringbuffer of.
+ * intel_logical_ring_submit() - submit the workload (to GuC or execlist queue)
+ * @request: Request to submit
  *
- * The tail is updated in our logical ringbuffer struct, not in the actual context. What
- * really happens during submission is that the context and current tail will be placed
- * 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.
+ * The tail is updated in our logical ringbuffer struct, not in the actual
+ * context. What really happens during submission is that the context and
+ * current tail will be placed 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 by the submitting agent i.e.
+ * either the driver (in execlist mode), or the GuC (in GuC-submission mode).
  */
 static int
-intel_logical_ring_advance_and_submit(struct drm_i915_gem_request *request)
+intel_logical_ring_submit(struct drm_i915_gem_request *request)
 {
 	struct intel_ringbuffer *ringbuf = request->ringbuf;
 	struct drm_i915_private *dev_priv = request->i915;
 	struct intel_engine_cs *engine = request->ring;
 
-	intel_logical_ring_advance(ringbuf);
 	request->tail = 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!
+	 * Fill in a few NOOPs after the end of the request proper,
+	 * as a buffer between requests to be used as a workaround
+	 * for not being allowed to do lite restore with HEAD==TAIL.
+	 * (WaIdleLiteRestore). These words may be consumed by the
+	 * submission mechanism if a context is *re*submitted while
+	 * (apparently) still active; otherwise, they will be left
+	 * as a harmless preamble to the next request.
 	 */
-	intel_logical_ring_emit(ringbuf, MI_NOOP);
-	intel_logical_ring_emit(ringbuf, MI_NOOP);
-	intel_logical_ring_advance(ringbuf);
+	if (WA_TAIL_DWORDS > 0) {
+		int ret, i;
+
+		/* Safe because we reserved the space earlier */
+		ret = intel_logical_ring_begin(request, WA_TAIL_DWORDS);
+		if (WARN_ON(ret != 0))
+			return ret;
+
+		for (i = 0; i < WA_TAIL_DWORDS; ++i)
+			intel_logical_ring_emit(ringbuf, MI_NOOP);
+
+		intel_logical_ring_advance(ringbuf);
+	}
 
 	if (intel_ring_stopped(engine))
 		return 0;
@@ -907,7 +927,8 @@ int intel_logical_ring_reserve_space(struct drm_i915_gem_request *request)
 	 * adding any commands to it then there might not actually be
 	 * sufficient room for the submission commands.
 	 */
-	intel_ring_reserved_space_reserve(request->ringbuf, MIN_SPACE_FOR_ADD_REQUEST);
+	intel_ring_reserved_space_reserve(request->ringbuf,
+			MIN_SPACE_FOR_ADD_REQUEST + WA_TAIL_DWORDS);
 
 	return intel_logical_ring_begin(request, 0);
 }
@@ -1875,13 +1896,6 @@ 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;
@@ -1892,7 +1906,7 @@ static int gen8_emit_request(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, 6);
 	if (ret)
 		return ret;
 
@@ -1908,7 +1922,9 @@ static int gen8_emit_request(struct drm_i915_gem_request *request)
 	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);
-	return intel_logical_ring_advance_and_submit(request);
+	intel_logical_ring_advance(ringbuf);
+
+	return intel_logical_ring_submit(request);
 }
 
 static int gen8_emit_request_render(struct drm_i915_gem_request *request)
@@ -1916,7 +1932,7 @@ 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, 6);
 	if (ret)
 		return ret;
 
@@ -1933,7 +1949,9 @@ static int gen8_emit_request_render(struct drm_i915_gem_request *request)
 	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);
+	intel_logical_ring_advance(ringbuf);
+
+	return intel_logical_ring_submit(request);
 }
 
 static int intel_lr_context_render_state_init(struct drm_i915_gem_request *req)
-- 
1.9.1

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