drm/i915: Make request's wait-for-space explicit

At the start of building a request, we would wait for roughly enough
space to fit the average request (to reduce the likelihood of having to
wait and abort partway through request construction). To achieve we
would try to begin a 0-length command packet, this just adds extra
confusion so make the wait-for-space explicit, as in the next patch we
want to move it from the backend to the i915_gem_request_alloc() so it
can ensure that the wait-for-space is the first operation in building a
new request.
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20171115151204.8105-2-chris@chris-wilson.co.uk

drivers/gpu/drm/i915/intel_lrc.c

@@ -1180,7 +1180,7 @@ static int execlists_request_alloc(struct drm_i915_gem_request *request)
 {
 	struct intel_engine_cs *engine = request->engine;
 	struct intel_context *ce = &request->ctx->engine[engine->id];
-	u32 *cs;
+	int ret;
 
 	GEM_BUG_ON(!ce->pin_count);
 
@@ -1190,9 +1190,9 @@ static int execlists_request_alloc(struct drm_i915_gem_request *request)
 	 */
 	request->reserved_space += EXECLISTS_REQUEST_SIZE;
 
-	cs = intel_ring_begin(request, 0);
-	if (IS_ERR(cs))
-		return PTR_ERR(cs);
+	ret = intel_ring_wait_for_space(request->ring, request->reserved_space);
+	if (ret)
+		return ret;
 
 	/* Note that after this point, we have committed to using
 	 * this request as it is being used to both track the

drivers/gpu/drm/i915/intel_ringbuffer.c

@@ -1578,7 +1578,7 @@ void intel_legacy_submission_resume(struct drm_i915_private *dev_priv)
 
 static int ring_request_alloc(struct drm_i915_gem_request *request)
 {
-	u32 *cs;
+	int ret;
 
 	GEM_BUG_ON(!request->ctx->engine[request->engine->id].pin_count);
 
@@ -1588,37 +1588,24 @@ static int ring_request_alloc(struct drm_i915_gem_request *request)
 	 */
 	request->reserved_space += LEGACY_REQUEST_SIZE;
 
-	cs = intel_ring_begin(request, 0);
-	if (IS_ERR(cs))
-		return PTR_ERR(cs);
+	ret = intel_ring_wait_for_space(request->ring, request->reserved_space);
+	if (ret)
+		return ret;
 
 	request->reserved_space -= LEGACY_REQUEST_SIZE;
 	return 0;
 }
 
-static noinline int wait_for_space(struct drm_i915_gem_request *req,
-				   unsigned int bytes)
+static noinline int wait_for_space(struct intel_ring *ring, unsigned int bytes)
 {
-	struct intel_ring *ring = req->ring;
 	struct drm_i915_gem_request *target;
 	long timeout;
 
-	lockdep_assert_held(&req->i915->drm.struct_mutex);
+	lockdep_assert_held(&ring->vma->vm->i915->drm.struct_mutex);
 
 	if (intel_ring_update_space(ring) >= bytes)
 		return 0;
 
-	/*
-	 * Space is reserved in the ringbuffer for finalising the request,
-	 * as that cannot be allowed to fail. During request finalisation,
-	 * reserved_space is set to 0 to stop the overallocation and the
-	 * assumption is that then we never need to wait (which has the
-	 * risk of failing with EINTR).
-	 *
-	 * See also i915_gem_request_alloc() and i915_add_request().
-	 */
-	GEM_BUG_ON(!req->reserved_space);
-
 	list_for_each_entry(target, &ring->request_list, ring_link) {
 		/* Would completion of this request free enough space? */
 		if (bytes <= __intel_ring_space(target->postfix,
@@ -1642,6 +1629,22 @@ static noinline int wait_for_space(struct drm_i915_gem_request *req,
 	return 0;
 }
 
+int intel_ring_wait_for_space(struct intel_ring *ring, unsigned int bytes)
+{
+	GEM_BUG_ON(bytes > ring->effective_size);
+	if (unlikely(bytes > ring->effective_size - ring->emit))
+		bytes += ring->size - ring->emit;
+
+	if (unlikely(bytes > ring->space)) {
+		int ret = wait_for_space(ring, bytes);
+		if (unlikely(ret))
+			return ret;
+	}
+
+	GEM_BUG_ON(ring->space < bytes);
+	return 0;
+}
+
 u32 *intel_ring_begin(struct drm_i915_gem_request *req,
 		      unsigned int num_dwords)
 {
@@ -1681,7 +1684,20 @@ u32 *intel_ring_begin(struct drm_i915_gem_request *req,
 	}
 
 	if (unlikely(total_bytes > ring->space)) {
-		int ret = wait_for_space(req, total_bytes);
+		int ret;
+
+		/*
+		 * Space is reserved in the ringbuffer for finalising the
+		 * request, as that cannot be allowed to fail. During request
+		 * finalisation, reserved_space is set to 0 to stop the
+		 * overallocation and the assumption is that then we never need
+		 * to wait (which has the risk of failing with EINTR).
+		 *
+		 * See also i915_gem_request_alloc() and i915_add_request().
+		 */
+		GEM_BUG_ON(!req->reserved_space);
+
+		ret = wait_for_space(ring, total_bytes);
 		if (unlikely(ret))
 			return ERR_PTR(ret);
 	}

drivers/gpu/drm/i915/intel_ringbuffer.h

@@ -660,6 +660,7 @@ void intel_legacy_submission_resume(struct drm_i915_private *dev_priv);
 
 int __must_check intel_ring_cacheline_align(struct drm_i915_gem_request *req);
 
+int intel_ring_wait_for_space(struct intel_ring *ring, unsigned int bytes);
 u32 __must_check *intel_ring_begin(struct drm_i915_gem_request *req,
 				   unsigned int n);