drm/i915: Introduce DRM_I915_THROTTLE_JIFFIES

As Daniel commented on

commit b7ffe1362c5f468b853223acc9268804aa92afc8
Author: Chris Wilson <chris@chris-wilson.co.uk>
Date:   Mon Apr 27 13:41:24 2015 +0100

    drm/i915: Free RPS boosts for all laggards

it is better to be explicit when sharing hardcoded values such as
throttle/boost timeouts. Make it so!

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/i915_drv.h

@@ -279,6 +279,12 @@ struct drm_i915_file_private {
 	struct {
 		spinlock_t lock;
 		struct list_head request_list;
+/* 20ms is a fairly arbitrary limit (greater than the average frame time)
+ * chosen to prevent the CPU getting more than a frame ahead of the GPU
+ * (when using lax throttling for the frontbuffer). We also use it to
+ * offer free GPU waitboosts for severely congested workloads.
+ */
+#define DRM_I915_THROTTLE_JIFFIES msecs_to_jiffies(20)
 	} mm;
 	struct idr context_idr;
 

drivers/gpu/drm/i915/i915_gem.c

@@ -4239,7 +4239,7 @@ i915_gem_ring_throttle(struct drm_device *dev, struct drm_file *file)
 {
 	struct drm_i915_private *dev_priv = dev->dev_private;
 	struct drm_i915_file_private *file_priv = file->driver_priv;
-	unsigned long recent_enough = jiffies - msecs_to_jiffies(20);
+	unsigned long recent_enough = jiffies - DRM_I915_THROTTLE_JIFFIES;
 	struct drm_i915_gem_request *request, *target = NULL;
 	unsigned reset_counter;
 	int ret;

drivers/gpu/drm/i915/intel_pm.c

@@ -4158,7 +4158,7 @@ void gen6_rps_boost(struct drm_i915_private *dev_priv,
 	/* Force a RPS boost (and don't count it against the client) if
 	 * the GPU is severely congested.
 	 */
-	if (rps && time_after(jiffies, submitted + msecs_to_jiffies(20)))
+	if (rps && time_after(jiffies, submitted + DRM_I915_THROTTLE_JIFFIES))
 		rps = NULL;
 
 	mutex_lock(&dev_priv->rps.hw_lock);