[Intel-gfx] [PATCH] drm/i915/psr: Bump vblank evasion time for seamless updates

[Chris Wilson]

Each set of registers we need to rewrite during a pageflip/modeset
increases the required evasion window. Modesets with PSR enabled
empirically take up to 350us to complete the register programming, so
provide a corresponding boost to the evasion window.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=105870
Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
Cc: Jani Nikula <jani.nikula at intel.com>
Cc: Ville Syrjälä <ville.syrjala at linux.intel.com>
Cc: Paulo Zanoni <paulo.r.zanoni at intel.com>
Cc: Jose Roberto de Souza <jose.souza at intel.com>
---
 drivers/gpu/drm/i915/intel_sprite.c | 20 ++++++++++++++++++--
 1 file changed, 18 insertions(+), 2 deletions(-)

diff --git a/drivers/gpu/drm/i915/intel_sprite.c b/drivers/gpu/drm/i915/intel_sprite.c
index 610398607e8e..03cafb953538 100644
--- a/drivers/gpu/drm/i915/intel_sprite.c
+++ b/drivers/gpu/drm/i915/intel_sprite.c
@@ -84,14 +84,30 @@ void intel_pipe_update_start(const struct intel_crtc_state *new_crtc_state)
 		intel_crtc_has_type(new_crtc_state, INTEL_OUTPUT_DSI);
 	DEFINE_WAIT(wait);
 	u32 psr_status;
+	int evasion;
 
 	vblank_start = adjusted_mode->crtc_vblank_start;
 	if (adjusted_mode->flags & DRM_MODE_FLAG_INTERLACE)
 		vblank_start = DIV_ROUND_UP(vblank_start, 2);
 
+	/*
+	 * Estimate how long it will take to reprogram display registers.
+	 *
+	 * As we wish to avoid changing registers across the vblank to
+	 * avoid register write tearing, where the new frame uses an incomplete
+	 * mismash of state, we aim to complete our writes before the start
+	 * of the vblank. So we must delay starting our writes until after our
+	 * evasion window.
+	 *
+	 * Each additional bit of state adds a set of registers we need to
+	 * reprogram, increasing our required evasion window.
+	 */
+	evasion = VBLANK_EVASION_TIME_US;
+	if (dev_priv->psr.enabled || new_crtc_state->has_psr)
+		evasion += 300;
+
 	/* FIXME needs to be calibrated sensibly */
-	min = vblank_start - intel_usecs_to_scanlines(adjusted_mode,
-						      VBLANK_EVASION_TIME_US);
+	min = vblank_start - intel_usecs_to_scanlines(adjusted_mode, evasion);
 	max = vblank_start - 1;
 
 	if (min <= 0 || max <= 0)
-- 
2.20.1