drm/tegra: dp: Read AUX read interval from DPCD

Store the AUX read interval from DPCD, so that it can be used to wait
for the durations given in the specification during link training.
Signed-off-by: Thierry Reding <treding@nvidia.com>

drivers/gpu/drm/tegra/dp.c

@@ -40,6 +40,8 @@ static void drm_dp_link_reset(struct drm_dp_link *link)
 	link->max_lanes = 0;
 
 	drm_dp_link_caps_reset(&link->caps);
+	link->aux_rd_interval.cr = 0;
+	link->aux_rd_interval.ce = 0;
 	link->edp = 0;
 
 	link->rate = 0;
@@ -60,6 +62,7 @@ static void drm_dp_link_reset(struct drm_dp_link *link)
 int drm_dp_link_probe(struct drm_dp_aux *aux, struct drm_dp_link *link)
 {
 	u8 dpcd[DP_RECEIVER_CAP_SIZE], value;
+	unsigned int rd_interval;
 	int err;
 
 	drm_dp_link_reset(link);
@@ -90,6 +93,34 @@ int drm_dp_link_probe(struct drm_dp_aux *aux, struct drm_dp_link *link)
 			link->edp = drm_dp_edp_revisions[value];
 	}
 
+	/*
+	 * The DPCD stores the AUX read interval in units of 4 ms. There are
+	 * two special cases:
+	 *
+	 *   1) if the TRAINING_AUX_RD_INTERVAL field is 0, the clock recovery
+	 *      and channel equalization should use 100 us or 400 us AUX read
+	 *      intervals, respectively
+	 *
+	 *   2) for DP v1.4 and above, clock recovery should always use 100 us
+	 *      AUX read intervals
+	 */
+	rd_interval = dpcd[DP_TRAINING_AUX_RD_INTERVAL] &
+			   DP_TRAINING_AUX_RD_MASK;
+
+	if (rd_interval > 4) {
+		DRM_DEBUG_KMS("AUX interval %u out of range (max. 4)\n",
+			      rd_interval);
+		rd_interval = 4;
+	}
+
+	rd_interval *= 4 * USEC_PER_MSEC;
+
+	if (rd_interval == 0 || link->revision >= DP_DPCD_REV_14)
+		link->aux_rd_interval.cr = 100;
+
+	if (rd_interval == 0)
+		link->aux_rd_interval.ce = 400;
+
 	link->rate = link->max_rate;
 	link->lanes = link->max_lanes;
 

drivers/gpu/drm/tegra/dp.h

@@ -60,6 +60,7 @@ void drm_dp_link_caps_copy(struct drm_dp_link_caps *dest,
  * @max_rate: maximum clock rate supported on the link
  * @max_lanes: maximum number of lanes supported on the link
  * @caps: capabilities supported on the link (see &drm_dp_link_caps)
+ * @aux_rd_interval: AUX read interval to use for training (in microseconds)
  * @edp: eDP revision (0x11: eDP 1.1, 0x12: eDP 1.2, ...)
  * @rate: currently configured link rate
  * @lanes: currently configured number of lanes
@@ -70,6 +71,16 @@ struct drm_dp_link {
 	unsigned int max_lanes;
 
 	struct drm_dp_link_caps caps;
+
+	/**
+	 * @cr: clock recovery read interval
+	 * @ce: channel equalization read interval
+	 */
+	struct {
+		unsigned int cr;
+		unsigned int ce;
+	} aux_rd_interval;
+
 	unsigned char edp;
 
 	unsigned int rate;