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)

1 files changed, 1 insertions, 4 deletions

diff --git a/drivers/gpu/drm/rockchip/rockchip_drm_vop.c b/drivers/gpu/drm/rockchip/rockchip_drm_vop.c
index 2f821c58007c..613404f86668 100644
--- a/drivers/gpu/drm/rockchip/rockchip_drm_vop.c
+++ b/drivers/gpu/drm/rockchip/rockchip_drm_vop.c
@@ -39,8 +39,6 @@
 #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) \
@@ -1563,8 +1561,7 @@ static int vop_create_crtc(struct vop *vop)
 	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",