drm/amd/display: add preferred pipe split logic

[why]
existing logic finds "first free pipe from 5 -> 0" to split
this will cause certain sequences to require DC to move
an MPCC from one tree to another, which is unsupported
this leads to blackscreen

to mitigate this problem, we will always try to acquire the
"preferred" pipe, and each pipe has a unique preferred pipe
this means we avoid most of the scenarios where
pipe splitting leads to moving MPCC from one tree
to another
Signed-off-by: Jun Lei <Jun.Lei@amd.com>
Reviewed-by: Tony Cheng <Tony.Cheng@amd.com>
Acked-by: Bhawanpreet Lakha <Bhawanpreet.Lakha@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/amd/display/dc/calcs/dcn_calcs.c

@@ -1141,7 +1141,7 @@ bool dcn_validate_bandwidth(
 						hsplit_pipe->pipe_dlg_param.vblank_end = pipe->pipe_dlg_param.vblank_end;
 					} else {
 						/* pipe not split previously needs split */
-						hsplit_pipe = find_idle_secondary_pipe(&context->res_ctx, pool);
+						hsplit_pipe = find_idle_secondary_pipe(&context->res_ctx, pool, pipe);
 						ASSERT(hsplit_pipe);
 						split_stream_across_pipes(
 							&context->res_ctx, pool,

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

@@ -1038,24 +1038,60 @@ enum dc_status resource_build_scaling_params_for_context(
 
 struct pipe_ctx *find_idle_secondary_pipe(
 		struct resource_context *res_ctx,
-		const struct resource_pool *pool)
+		const struct resource_pool *pool,
+		const struct pipe_ctx *primary_pipe)
 {
 	int i;
 	struct pipe_ctx *secondary_pipe = NULL;
 
 	/*
-	 * search backwards for the second pipe to keep pipe
-	 * assignment more consistent
+	 * We add a preferred pipe mapping to avoid the chance that
+	 * MPCCs already in use will need to be reassigned to other trees.
+	 * For example, if we went with the strict, assign backwards logic:
+	 *
+	 * (State 1)
+	 * Display A on, no surface, top pipe = 0
+	 * Display B on, no surface, top pipe = 1
+	 *
+	 * (State 2)
+	 * Display A on, no surface, top pipe = 0
+	 * Display B on, surface enable, top pipe = 1, bottom pipe = 5
+	 *
+	 * (State 3)
+	 * Display A on, surface enable, top pipe = 0, bottom pipe = 5
+	 * Display B on, surface enable, top pipe = 1, bottom pipe = 4
+	 *
+	 * The state 2->3 transition requires remapping MPCC 5 from display B
+	 * to display A.
+	 *
+	 * However, with the preferred pipe logic, state 2 would look like:
+	 *
+	 * (State 2)
+	 * Display A on, no surface, top pipe = 0
+	 * Display B on, surface enable, top pipe = 1, bottom pipe = 4
+	 *
+	 * This would then cause 2->3 to not require remapping any MPCCs.
 	 */
-
-	for (i = pool->pipe_count - 1; i >= 0; i--) {
-		if (res_ctx->pipe_ctx[i].stream == NULL) {
-			secondary_pipe = &res_ctx->pipe_ctx[i];
-			secondary_pipe->pipe_idx = i;
-			break;
+	if (primary_pipe) {
+		int preferred_pipe_idx = (pool->pipe_count - 1) - primary_pipe->pipe_idx;
+		if (res_ctx->pipe_ctx[preferred_pipe_idx].stream == NULL) {
+			secondary_pipe = &res_ctx->pipe_ctx[preferred_pipe_idx];
+			secondary_pipe->pipe_idx = preferred_pipe_idx;
 		}
 	}
 
+	/*
+	 * search backwards for the second pipe to keep pipe
+	 * assignment more consistent
+	 */
+	if (!secondary_pipe)
+		for (i = pool->pipe_count - 1; i >= 0; i--) {
+			if (res_ctx->pipe_ctx[i].stream == NULL) {
+				secondary_pipe = &res_ctx->pipe_ctx[i];
+				secondary_pipe->pipe_idx = i;
+				break;
+			}
+		}
 
 	return secondary_pipe;
 }

drivers/gpu/drm/amd/display/dc/dcn10/dcn10_resource.c

@@ -1081,7 +1081,7 @@ static struct pipe_ctx *dcn10_acquire_idle_pipe_for_layer(
 {
 	struct resource_context *res_ctx = &context->res_ctx;
 	struct pipe_ctx *head_pipe = resource_get_head_pipe_for_stream(res_ctx, stream);
-	struct pipe_ctx *idle_pipe = find_idle_secondary_pipe(res_ctx, pool);
+	struct pipe_ctx *idle_pipe = find_idle_secondary_pipe(res_ctx, pool, head_pipe);
 
 	if (!head_pipe) {
 		ASSERT(0);

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

@@ -131,7 +131,8 @@ bool resource_attach_surfaces_to_context(
 
 struct pipe_ctx *find_idle_secondary_pipe(
 		struct resource_context *res_ctx,
-		const struct resource_pool *pool);
+		const struct resource_pool *pool,
+		const struct pipe_ctx *primary_pipe);
 
 bool resource_is_stream_unchanged(
 	struct dc_state *old_context, struct dc_stream_state *stream);