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 <[e-mail]>
Signed-off-by: Sean Paul <[e-mail]>
Link to v1: https://patchwork.freedesktop.org/patch/msgid/[e-mail]
Link: https://patchwork.freedesktop.org/patch/msgid/[e-mail]

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

···
  */
 
 #include <linux/dma-fence.h>
+#include <linux/ktime.h>
 
 #include <drm/drm_atomic.h>
 #include <drm/drm_atomic_helper.h>
···
 {
 	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);
 
···
 		funcs->atomic_commit_tail(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);
 

drivers/gpu/drm/drm_self_refresh_helper.c

···
  * Authors:
  * Sean Paul <[e-mail]>
  */
+#include <linux/average.h>
 #include <linux/bitops.h>
 #include <linux/slab.h>
 #include <linux/workqueue.h>
···
  * atomic_check when &drm_crtc_state.self_refresh_active is true.
  */
 
+#define SELF_REFRESH_AVG_SEED_MS 200
+
+DECLARE_EWMA(psr_time, 4, 4)
+
 struct drm_self_refresh_data {
 	struct drm_crtc *crtc;
 	struct delayed_work entry_work;
-	unsigned int entry_delay_ms;
+
+	struct mutex avg_mutex;
+	struct ewma_psr_time entry_avg_ms;
+	struct ewma_psr_time exit_avg_ms;
 };
 
 static void drm_self_refresh_helper_entry_work(struct work_struct *work)
···
 }
 
 /**
+ * drm_self_refresh_helper_update_avg_times - Updates a crtc's SR time averages
+ * @state: the state which has just been applied to hardware
+ * @commit_time_ms: the amount of time in ms that this commit took to complete
+ *
+ * Called after &drm_mode_config_funcs.atomic_commit_tail, this function will
+ * update the average entry/exit self refresh times on self refresh transitions.
+ * These averages will be used when calculating how long to delay before
+ * entering self refresh mode after activity.
+ */
+void drm_self_refresh_helper_update_avg_times(struct drm_atomic_state *state,
+					      unsigned int commit_time_ms)
+{
+	struct drm_crtc *crtc;
+	struct drm_crtc_state *old_crtc_state, *new_crtc_state;
+	int i;
+
+	for_each_oldnew_crtc_in_state(state, crtc, old_crtc_state,
+				      new_crtc_state, i) {
+		struct drm_self_refresh_data *sr_data = crtc->self_refresh_data;
+		struct ewma_psr_time *time;
+
+		if (old_crtc_state->self_refresh_active ==
+		    new_crtc_state->self_refresh_active)
+			continue;
+
+		if (new_crtc_state->self_refresh_active)
+			time = &sr_data->entry_avg_ms;
+		else
+			time = &sr_data->exit_avg_ms;
+
+		mutex_lock(&sr_data->avg_mutex);
+		ewma_psr_time_add(time, commit_time_ms);
+		mutex_unlock(&sr_data->avg_mutex);
+	}
+}
+EXPORT_SYMBOL(drm_self_refresh_helper_update_avg_times);
+
+/**
  * drm_self_refresh_helper_alter_state - Alters the atomic state for SR exit
  * @state: the state currently being checked
  *
···
 
 	for_each_new_crtc_in_state(state, crtc, crtc_state, i) {
 		struct drm_self_refresh_data *sr_data;
+		unsigned int delay;
 
 		/* Don't trigger the entry timer when we're already in SR */
 		if (crtc_state->self_refresh_active)
···
 		if (!sr_data)
 			continue;
 
+		mutex_lock(&sr_data->avg_mutex);
+		delay = (ewma_psr_time_read(&sr_data->entry_avg_ms) +
+			 ewma_psr_time_read(&sr_data->exit_avg_ms)) * 2;
+		mutex_unlock(&sr_data->avg_mutex);
+
 		mod_delayed_work(system_wq, &sr_data->entry_work,
-				 msecs_to_jiffies(sr_data->entry_delay_ms));
+				 msecs_to_jiffies(delay));
 	}
 }
 EXPORT_SYMBOL(drm_self_refresh_helper_alter_state);
···
 /**
  * drm_self_refresh_helper_init - Initializes self refresh helpers for a crtc
  * @crtc: the crtc which supports self refresh supported displays
- * @entry_delay_ms: amount of inactivity to wait before entering self refresh
  *
  * Returns zero if successful or -errno on failure
  */
-int drm_self_refresh_helper_init(struct drm_crtc *crtc,
-				 unsigned int entry_delay_ms)
+int drm_self_refresh_helper_init(struct drm_crtc *crtc)
 {
 	struct drm_self_refresh_data *sr_data = crtc->self_refresh_data;
 
···
 
 	INIT_DELAYED_WORK(&sr_data->entry_work,
 			  drm_self_refresh_helper_entry_work);
-	sr_data->entry_delay_ms = entry_delay_ms;
 	sr_data->crtc = crtc;
+	mutex_init(&sr_data->avg_mutex);
+	ewma_psr_time_init(&sr_data->entry_avg_ms);
+	ewma_psr_time_init(&sr_data->exit_avg_ms);
+
+	/*
+	 * Seed the averages so they're non-zero (and sufficiently large
+	 * for even poorly performing panels). As time goes on, this will be
+	 * averaged out and the values will trend to their true value.
+	 */
+	ewma_psr_time_add(&sr_data->entry_avg_ms, SELF_REFRESH_AVG_SEED_MS);
+	ewma_psr_time_add(&sr_data->exit_avg_ms, SELF_REFRESH_AVG_SEED_MS);
 
 	crtc->self_refresh_data = sr_data;
 	return 0;

drivers/gpu/drm/rockchip/rockchip_drm_vop.c

···
 #include "rockchip_drm_vop.h"
 #include "rockchip_rgb.h"
 
-#define VOP_SELF_REFRESH_ENTRY_DELAY_MS 100
-
 #define VOP_WIN_SET(vop, win, name, v) \
 		vop_reg_set(vop, &win->phy->name, win->base, ~0, v, #name)
 #define VOP_SCL_SET(vop, win, name, v) \
···
 	init_completion(&vop->line_flag_completion);
 	crtc->port = port;
 
-	ret = drm_self_refresh_helper_init(crtc,
-					   VOP_SELF_REFRESH_ENTRY_DELAY_MS);
+	ret = drm_self_refresh_helper_init(crtc);
 	if (ret)
 		DRM_DEV_DEBUG_KMS(vop->dev,
 			"Failed to init %s with SR helpers %d, ignoring\n",

include/drm/drm_self_refresh_helper.h

···
 struct drm_crtc;
 
 void drm_self_refresh_helper_alter_state(struct drm_atomic_state *state);
+void drm_self_refresh_helper_update_avg_times(struct drm_atomic_state *state,
+					      unsigned int commit_time_ms);
 
-int drm_self_refresh_helper_init(struct drm_crtc *crtc,
-				 unsigned int entry_delay_ms);
-
+int drm_self_refresh_helper_init(struct drm_crtc *crtc);
 void drm_self_refresh_helper_cleanup(struct drm_crtc *crtc);
 #endif