drm/amd/display: Avoid reading LTTPR caps in non-LTTPR mode.

[Why]
LTTPR capable devices on the DisplayPort path may assume that
extended LTTPR AUX timeouts will be used after LTTPR capabilities
are read.

When DPTX operates in non-LTTPR mode, AUX timeouts are not
extended and this can result in AUX transactions timing out.

[How]
Use shared helper function to determine LTTPR mode and do not
read LTTPR capabilities in non-LTTPR mode.
Reviewed-by: Mustapha Ghaddar <Mustapha.Ghaddar@amd.com>
Reviewed-by: Meenakshikumar Somasundaram <Meenakshikumar.Somasundaram@amd.com>
Reviewed-by: Jun Lei <Jun.Lei@amd.com>
Acked-by: Hamza Mahfooz <hamza.mahfooz@amd.com>
Signed-off-by: Jimmy Kizito <Jimmy.Kizito@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/amd/display/dc/core/dc_link_dp.c

@@ -5129,16 +5129,13 @@ static bool dpcd_read_sink_ext_caps(struct dc_link *link)
 	return true;
 }
 
-bool dp_retrieve_lttpr_cap(struct dc_link *link)
+/* Logic to determine LTTPR mode */
+static void determine_lttpr_mode(struct dc_link *link)
 {
-	uint8_t lttpr_dpcd_data[8];
 	bool allow_lttpr_non_transparent_mode = 0;
 	bool vbios_lttpr_enable = link->dc->caps.vbios_lttpr_enable;
 	bool vbios_lttpr_interop = link->dc->caps.vbios_lttpr_aware;
-	enum dc_status status = DC_ERROR_UNEXPECTED;
-	bool is_lttpr_present = false;
 
-	memset(lttpr_dpcd_data, '\0', sizeof(lttpr_dpcd_data));
 
 	if ((link->dc->config.allow_lttpr_non_transparent_mode.bits.DP2_0 &&
 			link->dpcd_caps.channel_coding_cap.bits.DP_128b_132b_SUPPORTED)) {
@@ -5148,9 +5145,6 @@ bool dp_retrieve_lttpr_cap(struct dc_link *link)
 		allow_lttpr_non_transparent_mode = 1;
 	}
 
-	/*
-	 * Logic to determine LTTPR mode
-	 */
 	link->lttpr_mode = LTTPR_MODE_NON_LTTPR;
 	if (vbios_lttpr_enable && vbios_lttpr_interop)
 		link->lttpr_mode = LTTPR_MODE_NON_TRANSPARENT;
@@ -5172,6 +5166,18 @@ bool dp_retrieve_lttpr_cap(struct dc_link *link)
 	    link->dc->debug.dpia_debug.bits.force_non_lttpr)
 		link->lttpr_mode = LTTPR_MODE_NON_LTTPR;
 #endif
+}
+
+bool dp_retrieve_lttpr_cap(struct dc_link *link)
+{
+	uint8_t lttpr_dpcd_data[8];
+	enum dc_status status = DC_ERROR_UNEXPECTED;
+	bool is_lttpr_present = false;
+
+	memset(lttpr_dpcd_data, '\0', sizeof(lttpr_dpcd_data));
+
+	/* Logic to determine LTTPR mode*/
+	determine_lttpr_mode(link);
 
 	if (link->lttpr_mode == LTTPR_MODE_NON_TRANSPARENT || link->lttpr_mode == LTTPR_MODE_TRANSPARENT) {
 		/* By reading LTTPR capability, RX assumes that we will enable
@@ -5287,11 +5293,23 @@ static enum dc_status wa_try_to_wake_dprx(struct dc_link *link, uint64_t timeout
 	uint64_t time_taken_ms = 0;
 	enum dc_connection_type type = dc_connection_none;
 
-	status = core_link_read_dpcd(
-			link,
-			DP_LT_TUNABLE_PHY_REPEATER_FIELD_DATA_STRUCTURE_REV,
-			&dpcd_data,
-			sizeof(dpcd_data));
+	determine_lttpr_mode(link);
+
+	/* Issue an AUX read to test DPRX responsiveness. If LTTPR is supported the first read is expected to
+	 * be to determine LTTPR capabilities. Otherwise trying to read power state should be an innocuous AUX read.
+	 */
+	if (link->lttpr_mode == LTTPR_MODE_NON_TRANSPARENT || link->lttpr_mode == LTTPR_MODE_TRANSPARENT)
+		status = core_link_read_dpcd(
+				link,
+				DP_LT_TUNABLE_PHY_REPEATER_FIELD_DATA_STRUCTURE_REV,
+				&dpcd_data,
+				sizeof(dpcd_data));
+	else
+		status = core_link_read_dpcd(
+				link,
+				DP_SET_POWER,
+				&dpcd_data,
+				sizeof(dpcd_data));
 
 	if (status != DC_OK) {
 		DC_LOG_WARNING("%s: Read DPCD LTTPR_CAP failed - try to toggle DPCD SET_POWER for %lld ms.",