drm/i915: use semaphores for the display plane

In theory this will have performance and power improvements. Performance
because we don't need to stall when the scanout BO is busy, and power
because we don't have to stall when the BO is busy (and the ring can
even go to sleep if the HW supports it).

v2:
squash 2 patches into 1 (me)
un-inline the enable_semaphores function (Daniel)
remove comment about SNB hangs from i915_gem_object_sync (Chris)
rename intel_enable_semaphores to i915_semaphore_is_enabled (me)
removed page flip comment; "no why" (Chris)

To address other comments from Daniel (irc):
update the comment to say 'vt-d is crap, don't enable semaphores'
  - I think you misinterpreted Chris' comment, it already exists.
checking out whether we can pageflip on the render ring on ivb (didn't
work on early silicon)
  - We don't want to enable workarounds for early silicon unless we have
    to.
  - I can't find any references in the docs about this.
optionally use it if the fb is already busy on the render ring
  - This should be how the code already worked, unless I am
    misunderstanding your meaning.

Signed-off-by: Ben Widawsky <ben@bwidawsk.net>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

1 files changed, 44 insertions, 7 deletions

diff --git a/drivers/gpu/drm/i915/i915_gem.c b/drivers/gpu/drm/i915/i915_gem.c
index ad71751..d75a657 100644
--- a/drivers/gpu/drm/i915/i915_gem.c
+++ b/drivers/gpu/drm/i915/i915_gem.c
@@ -1953,6 +1953,48 @@ i915_gem_object_wait_rendering(struct drm_i915_gem_object *obj)
 	return 0;
 }
 
+int
+i915_gem_object_sync(struct drm_i915_gem_object *obj,
+		     struct intel_ring_buffer *to)
+{
+	struct intel_ring_buffer *from = obj->ring;
+	u32 seqno;
+	int ret, idx;
+
+	if (from == NULL || to == from)
+		return 0;
+
+	if (!i915_semaphore_is_enabled(obj->base.dev))
+		return i915_gem_object_wait_rendering(obj);
+
+	idx = intel_ring_sync_index(from, to);
+
+	seqno = obj->last_rendering_seqno;
+	if (seqno <= from->sync_seqno[idx])
+		return 0;
+
+	if (seqno == from->outstanding_lazy_request) {
+		struct drm_i915_gem_request *request;
+
+		request = kzalloc(sizeof(*request), GFP_KERNEL);
+		if (request == NULL)
+			return -ENOMEM;
+
+		ret = i915_add_request(from, NULL, request);
+		if (ret) {
+			kfree(request);
+			return ret;
+		}
+
+		seqno = request->seqno;
+	}
+
+	from->sync_seqno[idx] = seqno;
+
+	return to->sync_to(to, from, seqno - 1);
+
+}
+
 static void i915_gem_object_finish_gtt(struct drm_i915_gem_object *obj)
 {
 	u32 old_write_domain, old_read_domains;
@@ -2926,11 +2968,6 @@ int i915_gem_object_set_cache_level(struct drm_i915_gem_object *obj,
  * Prepare buffer for display plane (scanout, cursors, etc).
  * Can be called from an uninterruptible phase (modesetting) and allows
  * any flushes to be pipelined (for pageflips).
- *
- * For the display plane, we want to be in the GTT but out of any write
- * domains. So in many ways this looks like set_to_gtt_domain() apart from the
- * ability to pipeline the waits, pinning and any additional subtleties
- * that may differentiate the display plane from ordinary buffers.
  */
 int
 i915_gem_object_pin_to_display_plane(struct drm_i915_gem_object *obj,
@@ -2945,8 +2982,8 @@ i915_gem_object_pin_to_display_plane(struct drm_i915_gem_object *obj,
 		return ret;
 
 	if (pipelined != obj->ring) {
-		ret = i915_gem_object_wait_rendering(obj);
-		if (ret == -ERESTARTSYS)
+		ret = i915_gem_object_sync(obj, pipelined);
+		if (ret)
 			return ret;
 	}