drm: Measure Self Refresh Entry/Exit times to avoid thrashing

Currently the self refresh idle timer is a const set by the crtc. This
is fine if the self refresh entry/exit times are well-known for all
panels used on that crtc. However panels and workloads can vary quite a
bit, and a timeout which works well for one doesn't work well for
another.

In the extreme, if the timeout is too short we could get in a situation
where the self refresh exits are taking so long we queue up a self refresh
entry before the exit commit is even finished.

This patch changes the idle timeout to a moving average of the entry
times + a moving average of exit times + the crtc constant.

This patch was tested on rockchip, with a kevin CrOS panel the idle
delay averages out to about ~235ms (35 entry + 100 exit + 100 const). On
the same board, the bob panel idle delay lands around ~340ms (90 entry
+ 150 exit + 100 const).

WRT the dedicated mutex in self_refresh_data, it would be nice if we
could rely on drm_crtc.mutex to protect the average times, but there are
a few reasons why a separate lock is a better choice:
- We can't rely on drm_crtc.mutex being held if we're doing a nonblocking
  commit
- We can't grab drm_crtc.mutex since drm_modeset_lock() doesn't tell us
  whether the lock was already held in the acquire context (it eats
  -EALREADY), so we can't tell if we should drop it or not
- We don't need such a heavy-handed lock for what we're trying to do,
  commit ordering doesn't matter, so a point-of-use lock will be less
  contentious

Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Sean Paul <seanpaul@chromium.org>
Link to v1: https://patchwork.freedesktop.org/patch/msgid/20190917200443.64481-2-sean@poorly.run
Link: https://patchwork.freedesktop.org/patch/msgid/20190918200734.149876-2-sean@poorly.run

Changes in v2:
- Migrate locking explanation from comment to commit msg (Daniel)
- Turf constant entry delay and multiply the avg times by 2 (Daniel)

drivers/gpu/drm/drm_atomic_helper.c

@@ -26,6 +26,7 @@
  */
 
 #include <linux/dma-fence.h>
+#include <linux/ktime.h>
 
 #include <drm/drm_atomic.h>
 #include <drm/drm_atomic_helper.h>
@@ -1580,9 +1581,23 @@ static void commit_tail(struct drm_atomic_state *old_state)
 {
 	struct drm_device *dev = old_state->dev;
 	const struct drm_mode_config_helper_funcs *funcs;
+	ktime_t start;
+	s64 commit_time_ms;
 
 	funcs = dev->mode_config.helper_private;
 
+	/*
+	 * We're measuring the _entire_ commit, so the time will vary depending
+	 * on how many fences and objects are involved. For the purposes of self
+	 * refresh, this is desirable since it'll give us an idea of how
+	 * congested things are. This will inform our decision on how often we
+	 * should enter self refresh after idle.
+	 *
+	 * These times will be averaged out in the self refresh helpers to avoid
+	 * overreacting over one outlier frame
+	 */
+	start = ktime_get();
+
 	drm_atomic_helper_wait_for_fences(dev, old_state, false);
 
 	drm_atomic_helper_wait_for_dependencies(old_state);
@@ -1592,6 +1607,11 @@ static void commit_tail(struct drm_atomic_state *old_state)
 	else
 		drm_atomic_helper_commit_tail(old_state);
 
+	commit_time_ms = ktime_ms_delta(ktime_get(), start);
+	if (commit_time_ms > 0)
+		drm_self_refresh_helper_update_avg_times(old_state,
+						 (unsigned long)commit_time_ms);
+
 	drm_atomic_helper_commit_cleanup_done(old_state);
 
 	drm_atomic_state_put(old_state);