drm/i915: Try to use fast+narrow link on eDP again and fall back to the old max strategy on failure

Some new eDP panels don't like to operate at the max parameters, and
instead we need to go for an optimal confiugration. That unfortunately
doesn't work with older eDP panels which are generally only guaranteed
to work at the max parameters.

To solve these two conflicting requirements let's start with the optimal
setup, and if that fails we start again with the max parameters. The
downside is probably an extra modeset when we switch strategies but
I don't see a good way to avoid that.

For a bit of history we first tried to go for the fast+narrow in
commit 7769db58 ("drm/i915/dp: optimize eDP 1.4+ link config
fast and narrow"). but that had to be reverted due to regression
on older panels in commit f11cb1c1 ("drm/i915/dp: revert back
to max link rate and lane count on eDP"). So now we try to get
the best of both worlds by using both strategies.

v2: Deal with output_bpp and uapi vs. hw state split
    Reword some comments
v3: Rebase

Cc: Jani Nikula <jani.nikula@intel.com>
Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Cc: Albert Astals Cid <aacid@kde.org> # v5.0 backport
Cc: Emanuele Panigati <ilpanich@gmail.com> # v5.0 backport
Cc: Matteo Iervasi <matteoiervasi@gmail.com> # v5.0 backport
Cc: Timo Aaltonen <tjaalton@ubuntu.com>
Cc: Kai-Heng Feng <kai.heng.feng@canonical.com>
Reviewed-by: Manasi Navare <manasi.d.navare@intel.com>
References: https://bugs.freedesktop.org/show_bug.cgi?id=105267
References: https://bugs.freedesktop.org/show_bug.cgi?id=109959
References: https://gitlab.freedesktop.org/drm/intel/issues/272Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210107182026.24848-1-ville.syrjala@linux.intel.com

drivers/gpu/drm/i915/display/intel_display_types.h

@@ -1354,6 +1354,7 @@ struct intel_dp {
 	bool has_hdmi_sink;
 	bool has_audio;
 	bool reset_link_params;
+	bool use_max_params;
 	u8 dpcd[DP_RECEIVER_CAP_SIZE];
 	u8 psr_dpcd[EDP_PSR_RECEIVER_CAP_SIZE];
 	u8 downstream_ports[DP_MAX_DOWNSTREAM_PORTS];

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

@@ -480,6 +480,13 @@ int intel_dp_get_link_train_fallback_values(struct intel_dp *intel_dp,
 		return -1;
 	}
 
+	if (intel_dp_is_edp(intel_dp) && !intel_dp->use_max_params) {
+		drm_dbg_kms(&i915->drm,
+			    "Retrying Link training for eDP with max parameters\n");
+		intel_dp->use_max_params = true;
+		return 0;
+	}
+
 	index = intel_dp_rate_index(intel_dp->common_rates,
 				    intel_dp->num_common_rates,
 				    link_rate);
@@ -2286,6 +2293,44 @@ 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;
@@ -2509,13 +2554,14 @@ intel_dp_compute_link_config(struct intel_encoder *encoder,
 	limits.min_bpp = intel_dp_min_bpp(pipe_config->output_format);
 	limits.max_bpp = intel_dp_max_bpp(intel_dp, pipe_config);
 
-	if (intel_dp_is_edp(intel_dp)) {
+	if (intel_dp->use_max_params) {
 		/*
 		 * Use the maximum clock and number of lanes the eDP panel
-		 * advertizes being capable of. The panels are generally
+		 * advertizes being capable of in case the initial fast
+		 * optimal params failed us. The panels are generally
 		 * designed to support only a single clock and lane
-		 * configuration, and typically these values correspond to the
-		 * native resolution of the panel.
+		 * configuration, and typically on older panels these
+		 * values correspond to the native resolution of the panel.
 		 */
 		limits.min_lane_count = limits.max_lane_count;
 		limits.min_clock = limits.max_clock;
@@ -2534,11 +2580,22 @@ intel_dp_compute_link_config(struct intel_encoder *encoder,
 	    intel_dp_can_bigjoiner(intel_dp))
 		pipe_config->bigjoiner = true;
 
-	/*
-	 * Optimize for slow and wide. This is the place to add alternative
-	 * optimization policy.
-	 */
-	ret = intel_dp_compute_link_config_wide(intel_dp, pipe_config, &limits);
+	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);
 
 	/* 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);