drm/amd/display: Prefer currently used OTG master when acquiring free pipe

[WHY & HOW]
When acquiring an OTG master pipe we should prefer currently enabled OTG
master pipes first. If there are no free pipes used as current OTG
master pipe then we will try to acquire a currently unused free pipe as
new OTG master instead of tearing down current secondary pipes from ODM
or MPC combine.
Reviewed-by: Alvin Lee <alvin.lee2@amd.com>
Reviewed-by: Dillon Varone <dillon.varone@amd.com>
Acked-by: Alex Hung <alex.hung@amd.com>
Signed-off-by: Wenjing Liu <wenjing.liu@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_resource.c

@@ -1764,6 +1764,29 @@ int recource_find_free_pipe_not_used_in_cur_res_ctx(
 	return free_pipe_idx;
 }
 
+int recource_find_free_pipe_used_as_otg_master_in_cur_res_ctx(
+		const struct resource_context *cur_res_ctx,
+		struct resource_context *new_res_ctx,
+		const struct resource_pool *pool)
+{
+	int free_pipe_idx = FREE_PIPE_INDEX_NOT_FOUND;
+	const struct pipe_ctx *new_pipe, *cur_pipe;
+	int i;
+
+	for (i = 0; i < pool->pipe_count; i++) {
+		cur_pipe = &cur_res_ctx->pipe_ctx[i];
+		new_pipe = &new_res_ctx->pipe_ctx[i];
+
+		if (resource_is_pipe_type(cur_pipe, OTG_MASTER) &&
+				resource_is_pipe_type(new_pipe, FREE_PIPE)) {
+			free_pipe_idx = i;
+			break;
+		}
+	}
+
+	return free_pipe_idx;
+}
+
 int resource_find_free_pipe_used_as_cur_sec_dpp_in_mpcc_combine(
 		const struct resource_context *cur_res_ctx,
 		struct resource_context *new_res_ctx,
@@ -3600,8 +3623,20 @@ static bool acquire_otg_master_pipe_for_stream(
 	int pipe_idx;
 	struct pipe_ctx *pipe_ctx = NULL;
 
-	pipe_idx = recource_find_free_pipe_not_used_in_cur_res_ctx(
+	/*
+	 * Upper level code is responsible to optimize unnecessary addition and
+	 * removal for unchanged streams. So unchanged stream will keep the same
+	 * OTG master instance allocated. When current stream is removed and a
+	 * new stream is added, we want to reuse the OTG instance made available
+	 * by the removed stream first. If not found, we try to avoid of using
+	 * any free pipes already used in current context as this could tear
+	 * down exiting ODM/MPC/MPO configuration unnecessarily.
+	 */
+	pipe_idx = recource_find_free_pipe_used_as_otg_master_in_cur_res_ctx(
 			&cur_ctx->res_ctx, &new_ctx->res_ctx, pool);
+	if (pipe_idx == FREE_PIPE_INDEX_NOT_FOUND)
+		pipe_idx = recource_find_free_pipe_not_used_in_cur_res_ctx(
+				&cur_ctx->res_ctx, &new_ctx->res_ctx, pool);
 	if (pipe_idx == FREE_PIPE_INDEX_NOT_FOUND)
 		pipe_idx = resource_find_any_free_pipe(&new_ctx->res_ctx, pool);
 	if (pipe_idx != FREE_PIPE_INDEX_NOT_FOUND) {

drivers/gpu/drm/amd/display/dc/inc/resource.h

@@ -500,6 +500,18 @@ int recource_find_free_pipe_not_used_in_cur_res_ctx(
 		struct resource_context *new_res_ctx,
 		const struct resource_pool *pool);
 
+/*
+ * Look for a free pipe in new resource context that is used in current resource
+ * context as an OTG master pipe.
+ *
+ * return - FREE_PIPE_INDEX_NOT_FOUND if free pipe is not found, otherwise
+ * pipe idx of the free pipe
+ */
+int recource_find_free_pipe_used_as_otg_master_in_cur_res_ctx(
+		const struct resource_context *cur_res_ctx,
+		struct resource_context *new_res_ctx,
+		const struct resource_pool *pool);
+
 /*
  * Look for a free pipe in new resource context that is used as a secondary DPP
  * pipe in any MPCC combine in current resource context.