drm/amd/display: Fix MST link encoder availability check.

[Why]
MST streams share the same link and should share the same encoder.
The current availability check may erroneously determine that an
encoder is unavailable for MST streams.

[How]
When checking for link encoder availability, check if an encoder
in use shares a link with the stream for which the availability
check is being conducted. If the link is shared, then the link
encoder should be shared too and will be deemed available.
Reviewed-by: NJun Lei <Jun.Lei@amd.com>
Acked-by: NAnson Jacob <Anson.Jacob@amd.com>
Signed-off-by: NJimmy Kizito <Jimmy.Kizito@amd.com>
Tested-by: NDaniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: NAlex Deucher <alexander.deucher@amd.com>

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

-/*
- * Copyright 2021 Advanced Micro Devices, Inc.
+/* Copyright 2021 Advanced Micro Devices, Inc. All rights reserved.
  *
  * Permission is hereby granted, free of charge, to any person obtaining a
  * copy of this software and associated documentation files (the "Software"),
@@ -177,13 +176,27 @@ static enum engine_id find_first_avail_link_enc(
 	return eng_id;
 }
 
-static bool is_avail_link_enc(struct dc_state *state, enum engine_id eng_id)
+/* Check for availability of link encoder eng_id. */
+static bool is_avail_link_enc(struct dc_state *state, enum engine_id eng_id, struct dc_stream_state *stream)
 {
 	bool is_avail = false;
 	int eng_idx = eng_id - ENGINE_ID_DIGA;
 
-	if (eng_id != ENGINE_ID_UNKNOWN && state->res_ctx.link_enc_cfg_ctx.link_enc_avail[eng_idx] != ENGINE_ID_UNKNOWN)
+	/* An encoder is available if it is still in the availability pool. */
+	if (eng_id != ENGINE_ID_UNKNOWN && state->res_ctx.link_enc_cfg_ctx.link_enc_avail[eng_idx] != ENGINE_ID_UNKNOWN) {
 		is_avail = true;
+	} else {
+		struct dc_stream_state *stream_assigned = NULL;
+
+		/* MST streams share the same link and should share the same encoder.
+		 * If a stream that has already been assigned a link encoder uses as the
+		 * same link as the stream checking for availability, it is an MST stream
+		 * and should use the same link encoder.
+		 */
+		stream_assigned = get_stream_using_link_enc(state, eng_id);
+		if (stream_assigned && stream != stream_assigned && stream->link == stream_assigned->link)
+			is_avail = true;
+	}
 
 	return is_avail;
 }
@@ -296,7 +309,7 @@ void link_enc_cfg_link_encs_assign(
 				if (stream == prev_stream && stream->link == prev_stream->link &&
 						prev_state->res_ctx.link_enc_cfg_ctx.link_enc_assignments[j].valid) {
 					eng_id = prev_state->res_ctx.link_enc_cfg_ctx.link_enc_assignments[j].eng_id;
-					if (is_avail_link_enc(state, eng_id))
+					if (is_avail_link_enc(state, eng_id, stream))
 						add_link_enc_assignment(state, stream, eng_id);
 				}
 			}