drm/i915: Wait upon the last request seqno, rather than a future seqno

In commit 69c2fc89
Author: Chris Wilson <chris@chris-wilson.co.uk>
Date:   Fri Jul 20 12:41:03 2012 +0100

    drm/i915: Remove the per-ring write list

the explicit flush was removed from i915_ring_idle(). However, we
continued to wait upon the next seqno which now did not correspond to
any request (except for the unusual condition of a failure to queue a
request after execbuffer) and so would wait indefinitely.

This has an important side-effect that i915_gpu_idle() does not cause
the seqno to be incremented. This is vital if we are to be able to idle
the GPU to handle seqno wraparound, as in subsequent patches.
Signed-off-by: NChris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: NMika Kuoppala <mika.kuoppala@intel.com>
Signed-off-by: NDaniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/i915_gem.c

@@ -2462,10 +2462,29 @@ i915_gem_object_unbind(struct drm_i915_gem_object *obj)
 
 static int i915_ring_idle(struct intel_ring_buffer *ring)
 {
-	if (list_empty(&ring->active_list))
+	u32 seqno;
+	int ret;
+
+	/* We need to add any requests required to flush the objects */
+	if (!list_empty(&ring->active_list)) {
+		seqno = list_entry(ring->active_list.prev,
+				   struct drm_i915_gem_object,
+				   ring_list)->last_read_seqno;
+
+		ret = i915_gem_check_olr(ring, seqno);
+		if (ret)
+			return ret;
+	}
+
+	/* Wait upon the last request to be completed */
+	if (list_empty(&ring->request_list))
 		return 0;
 
-	return i915_wait_seqno(ring, i915_gem_next_request_seqno(ring));
+	seqno = list_entry(ring->request_list.prev,
+			   struct drm_i915_gem_request,
+			   list)->seqno;
+
+	return i915_wait_seqno(ring, seqno);
 }
 
 int i915_gpu_idle(struct drm_device *dev)