drm/i915: Add *_ring_begin() to request allocation

Now that the *_ring_begin() functions no longer call the request allocation
code, it is finally safe for the request allocation code to call *_ring_begin().
This is important to guarantee that the space reserved for the subsequent
i915_add_request() call does actually get reserved.

v2: Renamed functions according to review feedback (Tomas Elf).

For: VIZ-5115
Signed-off-by: John Harrison <John.C.Harrison@Intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/i915_gem.c

@@ -2679,19 +2679,20 @@ int i915_gem_request_alloc(struct intel_engine_cs *ring,
 	 * i915_add_request() call can't fail. Note that the reserve may need
 	 * to be redone if the request is not actually submitted straight
 	 * away, e.g. because a GPU scheduler has deferred it.
-	 *
-	 * Note further that this call merely notes the reserve request. A
-	 * subsequent call to *_ring_begin() is required to actually ensure
-	 * that the reservation is available. Without the begin, if the
-	 * request creator immediately submitted the request without adding
-	 * any commands to it then there might not actually be sufficient
-	 * room for the submission commands. Unfortunately, the current
-	 * *_ring_begin() implementations potentially call back here to
-	 * i915_gem_request_alloc(). Thus calling _begin() here would lead to
-	 * infinite recursion! Until that back call path is removed, it is
-	 * necessary to do a manual _begin() outside.
 	 */
-	intel_ring_reserved_space_reserve(req->ringbuf, MIN_SPACE_FOR_ADD_REQUEST);
+	if (i915.enable_execlists)
+		ret = intel_logical_ring_reserve_space(req);
+	else
+		ret = intel_ring_reserve_space(req);
+	if (ret) {
+		/*
+		 * At this point, the request is fully allocated even if not
+		 * fully prepared. Thus it can be cleaned up using the proper
+		 * free code.
+		 */
+		i915_gem_request_cancel(req);
+		return ret;
+	}
 
 	*req_out = ring->outstanding_lazy_request = req;
 	return 0;

drivers/gpu/drm/i915/intel_lrc.c

@@ -834,6 +834,21 @@ static int intel_logical_ring_begin(struct drm_i915_gem_request *req,
 	return 0;
 }
 
+int intel_logical_ring_reserve_space(struct drm_i915_gem_request *request)
+{
+	/*
+	 * The first call merely notes the reserve request and is common for
+	 * all back ends. The subsequent localised _begin() call actually
+	 * ensures that the reservation is available. Without the begin, if
+	 * the request creator immediately submitted the request without
+	 * 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);
+
+	return intel_logical_ring_begin(request, 0);
+}
+
 /**
  * execlists_submission() - submit a batchbuffer for execution, Execlists style
  * @dev: DRM device.

drivers/gpu/drm/i915/intel_lrc.h

@@ -37,6 +37,7 @@
 
 /* Logical Rings */
 int intel_logical_ring_alloc_request_extras(struct drm_i915_gem_request *request);
+int intel_logical_ring_reserve_space(struct drm_i915_gem_request *request);
 void intel_logical_ring_stop(struct intel_engine_cs *ring);
 void intel_logical_ring_cleanup(struct intel_engine_cs *ring);
 int intel_logical_rings_init(struct drm_device *dev);

drivers/gpu/drm/i915/intel_ringbuffer.c

@@ -2202,24 +2202,27 @@ int intel_ring_alloc_request_extras(struct drm_i915_gem_request *request)
 	return 0;
 }
 
+int intel_ring_reserve_space(struct drm_i915_gem_request *request)
+{
+	/*
+	 * The first call merely notes the reserve request and is common for
+	 * all back ends. The subsequent localised _begin() call actually
+	 * ensures that the reservation is available. Without the begin, if
+	 * the request creator immediately submitted the request without
+	 * 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);
+
+	return intel_ring_begin(request, 0);
+}
+
 void intel_ring_reserved_space_reserve(struct intel_ringbuffer *ringbuf, int size)
 {
-	/* NB: Until request management is fully tidied up and the OLR is
-	 * removed, there are too many ways for get false hits on this
-	 * anti-recursion check! */
-	/*WARN_ON(ringbuf->reserved_size);*/
+	WARN_ON(ringbuf->reserved_size);
 	WARN_ON(ringbuf->reserved_in_use);
 
 	ringbuf->reserved_size = size;
-
-	/*
-	 * Really need to call _begin() here but that currently leads to
-	 * recursion problems! This will be fixed later but for now just
-	 * return and hope for the best. Note that there is only a real
-	 * problem if the create of the request never actually calls _begin()
-	 * but if they are not submitting any work then why did they create
-	 * the request in the first place?
-	 */
 }
 
 void intel_ring_reserved_space_cancel(struct intel_ringbuffer *ringbuf)

drivers/gpu/drm/i915/intel_ringbuffer.h

@@ -484,6 +484,7 @@ intel_ring_get_request(struct intel_engine_cs *ring)
  * will always have sufficient room to do its stuff. The request creation
  * code calls this automatically.
  */
+int intel_ring_reserve_space(struct drm_i915_gem_request *request);
 void intel_ring_reserved_space_reserve(struct intel_ringbuffer *ringbuf, int size);
 /* Cancel the reservation, e.g. because the request is being discarded. */
 void intel_ring_reserved_space_cancel(struct intel_ringbuffer *ringbuf);