Revert "drm/i915: Remove unneeded hack now for CDCLK"

This reverts commit 82ea174.

Unfortunately according to our recent findings there is still some
unidentified factor, requiring CDCLK to be set higher - otherwise we
still get underruns on some multipipe configurations, despite CDCLK
being set according to BSpec formula. So getting again back into debug
mode to indentify the cause, meanwhile setting CDCLK=Pixel rate back in
order to remove regression in 10% of the cases due to FIFO underruns.

Signed-off-by: Stanislav Lisovskiy <stanislav.lisovskiy@intel.com>
Fixes: cd19154 ("drm/i915: Adjust CDCLK accordingly to our DBuf bw needs")
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200608065552.21728-1-stanislav.lisovskiy@intel.com

Author: StanFox1984

drivers/gpu/drm/i915/display/intel_cdclk.c

@@ -2071,6 +2071,18 @@ 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 = max(min_cdclk, (int)crtc_state->pixel_rate);
+
 	if (min_cdclk > dev_priv->max_cdclk_freq) {
 		drm_dbg_kms(&dev_priv->drm,
 			    "required cdclk (%d kHz) exceeds max (%d kHz)\n",