drm/amd/display: Send DP_TOTAL_LTTPR_CNT during detection if LTTPR is present

[WHY]
New register field added in DP2.1 SCR, needed for auxless ALPM

[HOW]
Echo value read from 0xF0007 back to sink

Reviewed-by: Wenjing Liu <[e-mail]>
Cc: Mario Limonciello <[e-mail]>
Cc: Alex Deucher <[e-mail]>
Cc: [e-mail]
Signed-off-by: Alex Hung <[e-mail]>
Signed-off-by: Michael Strauss <[e-mail]>
Tested-by: Daniel Wheeler <[e-mail]>
Signed-off-by: Alex Deucher <[e-mail]>

drivers/gpu/drm/amd/display/dc/link/protocols/link_dp_capability.c

···
 			return false;
 	}
 
-	if (dp_is_lttpr_present(link))
+	if (dp_is_lttpr_present(link)) {
 		configure_lttpr_mode_transparent(link);
+
+		// Echo TOTAL_LTTPR_CNT back downstream
+		core_link_write_dpcd(
+				link,
+				DP_TOTAL_LTTPR_CNT,
+				&link->dpcd_caps.lttpr_caps.phy_repeater_cnt,
+				sizeof(link->dpcd_caps.lttpr_caps.phy_repeater_cnt));
+	}
 
 	/* Read DP tunneling information. */
 	status = dpcd_get_tunneling_device_data(link);

drivers/gpu/drm/amd/display/include/dpcd_defs.h

···
 #define DP_SINK_PR_PIXEL_DEVIATION_PER_LINE     0x379
 #define DP_SINK_PR_MAX_NUMBER_OF_DEVIATION_LINE 0x37A
 
+/* Remove once drm_dp_helper.h is updated upstream */
+#ifndef DP_TOTAL_LTTPR_CNT
+#define DP_TOTAL_LTTPR_CNT                                  0xF000A /* 2.1 */
+#endif
+
 #endif /* __DAL_DPCD_DEFS_H__ */