drm/1915: ringbuffer wait for idle function

Added a new function which waits for the ringbuffer space to be equal to
(total - 8). This is the empty condition of the ringbuffer, and
equivalent to head==tail.

Also modified two users of this functionality elsewhere in the code.
Signed-off-by: Ben Widawsky <ben@bwidawsk.net>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>

drivers/gpu/drm/i915/i915_dma.c

@@ -571,7 +571,7 @@ static int i915_quiescent(struct drm_device *dev)
 	struct intel_ring_buffer *ring = LP_RING(dev->dev_private);
 
 	i915_kernel_lost_context(dev);
-	return intel_wait_ring_buffer(ring, ring->size - 8);
+	return intel_wait_ring_idle(ring);
 }
 
 static int i915_flush_ioctl(struct drm_device *dev, void *data,

drivers/gpu/drm/i915/intel_ringbuffer.c

@@ -873,7 +873,7 @@ void intel_cleanup_ring_buffer(struct intel_ring_buffer *ring)
 
 	/* Disable the ring buffer. The ring must be idle at this point */
 	dev_priv = ring->dev->dev_private;
-	ret = intel_wait_ring_buffer(ring, ring->size - 8);
+	ret = intel_wait_ring_idle(ring);
 	if (ret)
 		DRM_ERROR("failed to quiesce %s whilst cleaning up: %d\n",
 			  ring->name, ret);

drivers/gpu/drm/i915/intel_ringbuffer.h

@@ -164,7 +164,13 @@ intel_read_status_page(struct intel_ring_buffer *ring,
 #define I915_BREADCRUMB_INDEX		0x21
 
 void intel_cleanup_ring_buffer(struct intel_ring_buffer *ring);
+
 int __must_check intel_wait_ring_buffer(struct intel_ring_buffer *ring, int n);
+static inline int intel_wait_ring_idle(struct intel_ring_buffer *ring)
+{
+	return intel_wait_ring_buffer(ring, ring->space - 8);
+}
+
 int __must_check intel_ring_begin(struct intel_ring_buffer *ring, int n);
 
 static inline void intel_ring_emit(struct intel_ring_buffer *ring,