drm/i915/dp: Use slow and wide link training for everything

Screen flickers on Innolux eDP 1.3 panel when clock rate 540000 is in use.

According to the panel vendor, though clock rate 540000 is advertised,
but the max clock rate it really supports is 270000.

Ville Syrjälä mentioned that fast and narrow also breaks some eDP 1.4
panel, so use slow and wide training for all panels to resolve the
issue.

User also confirmed that the new strategy doesn't introduce any
regression on XPS 9380.

v2:
 - Use slow and wide for everything.

Closes: https://gitlab.freedesktop.org/drm/intel/-/issues/3384
References: https://gitlab.freedesktop.org/drm/intel/-/issues/272
Signed-off-by: Kai-Heng Feng <kai.heng.feng@canonical.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210421052054.1434718-1-kai.heng.feng@canonical.com
(cherry picked from commit acca776)
Fixes: 2bbd6db ("drm/i915: Try to use fast+narrow link on eDP again and fall back to the old max strategy on failure")
Cc: <stable@vger.kernel.org> # v5.12+
Signed-off-by: Jani Nikula <jani.nikula@intel.com>

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

@@ -1095,44 +1095,6 @@ intel_dp_compute_link_config_wide(struct intel_dp *intel_dp,
 	return -EINVAL;
 }
 
-/* Optimize link config in order: max bpp, min lanes, min clock */
-static int
-intel_dp_compute_link_config_fast(struct intel_dp *intel_dp,
-				  struct intel_crtc_state *pipe_config,
-				  const struct link_config_limits *limits)
-{
-	const struct drm_display_mode *adjusted_mode = &pipe_config->hw.adjusted_mode;
-	int bpp, clock, lane_count;
-	int mode_rate, link_clock, link_avail;
-
-	for (bpp = limits->max_bpp; bpp >= limits->min_bpp; bpp -= 2 * 3) {
-		int output_bpp = intel_dp_output_bpp(pipe_config->output_format, bpp);
-
-		mode_rate = intel_dp_link_required(adjusted_mode->crtc_clock,
-						   output_bpp);
-
-		for (lane_count = limits->min_lane_count;
-		     lane_count <= limits->max_lane_count;
-		     lane_count <<= 1) {
-			for (clock = limits->min_clock; clock <= limits->max_clock; clock++) {
-				link_clock = intel_dp->common_rates[clock];
-				link_avail = intel_dp_max_data_rate(link_clock,
-								    lane_count);
-
-				if (mode_rate <= link_avail) {
-					pipe_config->lane_count = lane_count;
-					pipe_config->pipe_bpp = bpp;
-					pipe_config->port_clock = link_clock;
-
-					return 0;
-				}
-			}
-		}
-	}
-
-	return -EINVAL;
-}
-
 static int intel_dp_dsc_compute_bpp(struct intel_dp *intel_dp, u8 dsc_max_bpc)
 {
 	int i, num_bpc;
@@ -1382,22 +1344,11 @@ intel_dp_compute_link_config(struct intel_encoder *encoder,
 	    intel_dp_can_bigjoiner(intel_dp))
 		pipe_config->bigjoiner = true;
 
-	if (intel_dp_is_edp(intel_dp))
-		/*
-		 * Optimize for fast and narrow. eDP 1.3 section 3.3 and eDP 1.4
-		 * section A.1: "It is recommended that the minimum number of
-		 * lanes be used, using the minimum link rate allowed for that
-		 * lane configuration."
-		 *
-		 * Note that we fall back to the max clock and lane count for eDP
-		 * panels that fail with the fast optimal settings (see
-		 * intel_dp->use_max_params), in which case the fast vs. wide
-		 * choice doesn't matter.
-		 */
-		ret = intel_dp_compute_link_config_fast(intel_dp, pipe_config, &limits);
-	else
-		/* Optimize for slow and wide. */
-		ret = intel_dp_compute_link_config_wide(intel_dp, pipe_config, &limits);
+	/*
+	 * Optimize for slow and wide for everything, because there are some
+	 * eDP 1.3 and 1.4 panels don't work well with fast and narrow.
+	 */
+	ret = intel_dp_compute_link_config_wide(intel_dp, pipe_config, &limits);
 
 	/* enable compression if the mode doesn't fit available BW */
 	drm_dbg_kms(&i915->drm, "Force DSC en = %d\n", intel_dp->force_dsc_en);