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>

1 files changed, 31 insertions, 0 deletions

diff --git a/drivers/gpu/drm/tegra/dp.c b/drivers/gpu/drm/tegra/dp.c
index 2be0a47ecbec..757a0256592f 100644
--- a/drivers/gpu/drm/tegra/dp.c
+++ b/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;