[Intel-gfx] [PATCH v2] drm/i915: Bump up CDCLK to eliminate underruns on TGL

[Stanislav Lisovskiy]

There seems to be some undocumented bandwidth
bottleneck/dependency which scales with CDCLK,
causing FIFO underruns when CDCLK is too low,
even when it's correct from BSpec point of view.

Currently for TGL platforms we calculate
min_cdclk initially based on pixel_rate divided
by 2, accounting for also plane requirements,
however in some cases the lowest possible CDCLK
doesn't work and causing the underruns.

Explicitly stating here that this seems to be currently
rather a Hack, than final solution.

v2: Use clamp operation instead of min(Matt Roper)

Signed-off-by: Stanislav Lisovskiy <stanislav.lisovskiy at intel.com>
Bugzilla: https://gitlab.freedesktop.org/drm/intel/issues/402
---
 drivers/gpu/drm/i915/display/intel_cdclk.c | 14 ++++++++++++++
 1 file changed, 14 insertions(+)

diff --git a/drivers/gpu/drm/i915/display/intel_cdclk.c b/drivers/gpu/drm/i915/display/intel_cdclk.c
index 7d1ab1e5b7c3..23ef30175090 100644
--- a/drivers/gpu/drm/i915/display/intel_cdclk.c
+++ b/drivers/gpu/drm/i915/display/intel_cdclk.c
@@ -2004,6 +2004,20 @@ int intel_crtc_compute_min_cdclk(const struct intel_crtc_state *crtc_state)
 	/* Account for additional needs from the planes */
 	min_cdclk = max(intel_planes_min_cdclk(crtc_state), min_cdclk);
 
+	/*
+	 * HACK. Currently for TGL platforms we calculate
+	 * min_cdclk initially based on pixel_rate divided
+	 * by 2, accounting for also plane requirements,
+	 * however in some cases the lowest possible CDCLK
+	 * doesn't work and causing the underruns.
+	 * Explicitly stating here that this seems to be currently
+	 * rather a Hack, than final solution.
+	 */
+	if (IS_TIGERLAKE(dev_priv))
+		min_cdclk = clamp(min_cdclk,
+				  (int)crtc_state->pixel_rate,
+				  (int)dev_priv->max_cdclk_freq);
+
 	if (min_cdclk > dev_priv->max_cdclk_freq) {
 		DRM_DEBUG_KMS("required cdclk (%d kHz) exceeds max (%d kHz)\n",
 			      min_cdclk, dev_priv->max_cdclk_freq);
-- 
2.24.1.485.gad05a3d8e5