drm/i915/display: Record the plane update times for debugging

Since we try and estimate how long we require to update the registers to
perform a plane update, it is of vital importance that we measure the
distribution of plane updates to better guide our estimate. If we
underestimate how long it takes to perform the plane update, we may
slip into the next scanout frame causing a tear. If we overestimate, we
may unnecessarily delay the update to the next frame, causing visible
jitter.

Replace the warning that we exceed some arbitrary threshold for the
vblank update with a histogram for debugfs.

v2: Add a per-crtc debugfs entry so that the information is easier to
extract when testing individual CRTC, and so that it can be reset before
a test.

v3: Flip the graph on its side; creates space to label the time axis.

Updates: 4684
       |
   1us |
       |
   4us |********
       |**********
  16us |***********
       |*****
  66us |
       |
 262us |
       |
   1ms |
       |
   4ms |
       |
  17ms |
       |
Min update: 5918ns
Max update: 54781ns
Average update: 16628ns
Overruns > 250us: 0

Closes: https://gitlab.freedesktop.org/drm/intel/-/issues/1982
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Jani Nikula <jani.nikula@linux.intel.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com> #v2
Link: https://patchwork.freedesktop.org/patch/msgid/20201202212814.26320-1-chris@chris-wilson.co.uk

1 files changed, 10 insertions, 0 deletions

diff --git a/drivers/gpu/drm/i915/display/intel_sprite.h b/drivers/gpu/drm/i915/display/intel_sprite.h
index cd2104ba1ca1..76126dd8d584 100644
--- a/drivers/gpu/drm/i915/display/intel_sprite.h
+++ b/drivers/gpu/drm/i915/display/intel_sprite.h
@@ -17,6 +17,16 @@ struct drm_i915_private;
 struct intel_crtc_state;
 struct intel_plane_state;
 
+/*
+ * FIXME: We should instead only take spinlocks once for the entire update
+ * instead of once per mmio.
+ */
+#if IS_ENABLED(CONFIG_PROVE_LOCKING)
+#define VBLANK_EVASION_TIME_US 250
+#else
+#define VBLANK_EVASION_TIME_US 100
+#endif
+
 int intel_usecs_to_scanlines(const struct drm_display_mode *adjusted_mode,
 			     int usecs);
 struct intel_plane *intel_sprite_plane_create(struct drm_i915_private *dev_priv,