[Intel-gfx] [PATCH] drm/i915: Reading DPRX caps in LTTPR transparent mode after LTTPR detection

[William Tseng]

In some cases, the MAX_LANE_COUNT in the register at DCPD Address 0002h
may be updated by LTTPR in non-transparent mode while reading DPRX Caps
registers, e.g., the lane count is changed from 2 to 4. This may cause
Link Training failure because of the updated lane count, which might not
be supported by the DPRX.

This change may work around the problem, which LTTPR updates the DCPD
register not liseted in the table, i.e., Table 3-65, mentioned in the
DP standard, Section 3.6.3, Version 2.0.

""
Upon discovering its location between the DPTX and DPRX, the LTTPR
replies to AUX request transactions to its DPCD address range within the
LTTPR field. The LTTPR passes through all other AUX request transactions
with one exception – an LTTPR shall snoop AUX request transactions to
those DPCD Addresses listed in Table 3-65 and take necessary actions as
specified in the table. (For complete register descriptions,
see Table 2-184.)
""

Cs : Khaled Almahallawy <khaled.almahallawy at intel.com>
Cc : Imre Deak <imre.deak at intel.com>
Cc : Tejas Upadhyay <tejaskumarx.surendrakumar.upadhyay at intel.com>
Cc : Ville Syrjälä <ville.syrjala at linux.intel.com>
Signed-off-by: William Tseng <william.tseng at intel.com>
---
 .../drm/i915/display/intel_dp_link_training.c | 44 +++++++++----------
 1 file changed, 22 insertions(+), 22 deletions(-)

diff --git a/drivers/gpu/drm/i915/display/intel_dp_link_training.c b/drivers/gpu/drm/i915/display/intel_dp_link_training.c
index 50cae0198a3d..3658deb9da1c 100644
--- a/drivers/gpu/drm/i915/display/intel_dp_link_training.c
+++ b/drivers/gpu/drm/i915/display/intel_dp_link_training.c
@@ -131,7 +131,6 @@ intel_dp_set_lttpr_transparent_mode(struct intel_dp *intel_dp, bool enable)
 static int intel_dp_init_lttpr(struct intel_dp *intel_dp)
 {
 	int lttpr_count;
-	int i;
 
 	if (!intel_dp_read_lttpr_common_caps(intel_dp))
 		return 0;
@@ -152,27 +151,6 @@ static int intel_dp_init_lttpr(struct intel_dp *intel_dp)
 	 */
 	intel_dp_set_lttpr_transparent_mode(intel_dp, true);
 
-	/*
-	 * In case of unsupported number of LTTPRs or failing to switch to
-	 * non-transparent mode fall-back to transparent link training mode,
-	 * still taking into account any LTTPR common lane- rate/count limits.
-	 */
-	if (lttpr_count < 0)
-		return 0;
-
-	if (!intel_dp_set_lttpr_transparent_mode(intel_dp, false)) {
-		drm_dbg_kms(&dp_to_i915(intel_dp)->drm,
-			    "Switching to LTTPR non-transparent LT mode failed, fall-back to transparent mode\n");
-
-		intel_dp_set_lttpr_transparent_mode(intel_dp, true);
-		intel_dp_reset_lttpr_count(intel_dp);
-
-		return 0;
-	}
-
-	for (i = 0; i < lttpr_count; i++)
-		intel_dp_read_lttpr_phy_caps(intel_dp, DP_PHY_LTTPR(i));
-
 	return lttpr_count;
 }
 
@@ -197,6 +175,7 @@ static int intel_dp_init_lttpr(struct intel_dp *intel_dp)
 int intel_dp_init_lttpr_and_dprx_caps(struct intel_dp *intel_dp)
 {
 	int lttpr_count = intel_dp_init_lttpr(intel_dp);
+	int i;
 
 	/* The DPTX shall read the DPRX caps after LTTPR detection. */
 	if (drm_dp_read_dpcd_caps(&intel_dp->aux, intel_dp->dpcd)) {
@@ -204,6 +183,27 @@ int intel_dp_init_lttpr_and_dprx_caps(struct intel_dp *intel_dp)
 		return -EIO;
 	}
 
+	/*
+	 * In case of unsupported number of LTTPRs or failing to switch to
+	 * non-transparent mode fall-back to transparent link training mode,
+	 * still taking into account any LTTPR common lane- rate/count limits.
+	 */
+	if (lttpr_count <= 0)
+		return 0;
+
+	if (!intel_dp_set_lttpr_transparent_mode(intel_dp, false)) {
+		drm_dbg_kms(&dp_to_i915(intel_dp)->drm,
+		"Switching to LTTPR non-transparent LT mode failed, fall-back to transparent mode\n");
+
+		intel_dp_set_lttpr_transparent_mode(intel_dp, true);
+		intel_dp_reset_lttpr_count(intel_dp);
+
+		return 0;
+	}
+
+	for (i = 0; i < lttpr_count; i++)
+		intel_dp_read_lttpr_phy_caps(intel_dp, DP_PHY_LTTPR(i));
+
 	return lttpr_count;
 }
 EXPORT_SYMBOL(intel_dp_init_lttpr_and_dprx_caps);
-- 
2.17.1