drm/amd/display: Move PRIMARY plane zpos higher

[Why]

Compositors have different ways of assigning surfaces to DRM planes for
render offloading. It may decide between various strategies: overlay,
underlay, or a mix of both (see here for more info:
https://gitlab.freedesktop.org/emersion/libliftoff/-/issues/76)

One way for compositors to implement the underlay strategy is to assign
a higher zpos to the DRM_PRIMARY plane than the DRM_OVERLAY planes,
effectively turning the DRM_OVERLAY plane into an underlay plane.

Today, amdgpu attaches an immutable zpos of 0 to the DRM_PRIMARY plane.
This however, is an arbitrary restriction. DCN pipes are general
purpose, and can be arranged in any z-order. To support compositors
using this allocation scheme, we can set a non-zero immutable zpos for
the PRIMARY, allowing the placement of OVERLAYS (mutable zpos range
0-254) beneath the PRIMARY.

[How]

Assign a zpos = #no of OVERLAY planes to the PRIMARY plane. Then, clean
up any assumptions in the driver of PRIMARY plane having the lowest
zpos.

v2: Fix typo s/decending/descending/
Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Acked-by: Zaeem Mohamed <zaeem.mohamed@amd.com>
Signed-off-by: Leo Li <sunpeng.li@amd.com>
Acked-by: Pekka Paalanen <pekka.paalanen@collabora.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c

@@ -81,6 +81,7 @@
 #include <linux/firmware.h>
 #include <linux/component.h>
 #include <linux/dmi.h>
+#include <linux/sort.h>
 
 #include <drm/display/drm_dp_mst_helper.h>
 #include <drm/display/drm_hdmi_helper.h>
@@ -376,6 +377,20 @@ static inline void reverse_planes_order(struct dc_surface_update *array_of_surfa
 		swap(array_of_surface_update[i], array_of_surface_update[j]);
 }
 
+/*
+ * DC will program planes with their z-order determined by their ordering
+ * in the dc_surface_updates array. This comparator is used to sort them
+ * by descending zpos.
+ */
+static int dm_plane_layer_index_cmp(const void *a, const void *b)
+{
+	const struct dc_surface_update *sa = (struct dc_surface_update *)a;
+	const struct dc_surface_update *sb = (struct dc_surface_update *)b;
+
+	/* Sort by descending dc_plane layer_index (i.e. normalized_zpos) */
+	return sb->surface->layer_index - sa->surface->layer_index;
+}
+
 /**
  * update_planes_and_stream_adapter() - Send planes to be updated in DC
  *
@@ -400,7 +415,8 @@ static inline bool update_planes_and_stream_adapter(struct dc *dc,
 						    struct dc_stream_update *stream_update,
 						    struct dc_surface_update *array_of_surface_update)
 {
-	reverse_planes_order(array_of_surface_update, planes_count);
+	sort(array_of_surface_update, planes_count,
+	     sizeof(*array_of_surface_update), dm_plane_layer_index_cmp, NULL);
 
 	/*
 	 * Previous frame finished and HW is ready for optimization.
@@ -9775,6 +9791,8 @@ static void amdgpu_dm_atomic_commit_tail(struct drm_atomic_state *state)
 		for (j = 0; j < status->plane_count; j++)
 			dummy_updates[j].surface = status->plane_states[0];
 
+		sort(dummy_updates, status->plane_count,
+		     sizeof(*dummy_updates), dm_plane_layer_index_cmp, NULL);
 
 		mutex_lock(&dm->dc_lock);
 		dc_exit_ips_for_hw_access(dm->dc);
@@ -10510,6 +10528,16 @@ static bool should_reset_plane(struct drm_atomic_state *state,
 	if (new_crtc_state->color_mgmt_changed)
 		return true;
 
+	/*
+	 * On zpos change, planes need to be reordered by removing and re-adding
+	 * them one by one to the dc state, in order of descending zpos.
+	 *
+	 * TODO: We can likely skip bandwidth validation if the only thing that
+	 * changed about the plane was it'z z-ordering.
+	 */
+	if (new_crtc_state->zpos_changed)
+		return true;
+
 	if (drm_atomic_crtc_needs_modeset(new_crtc_state))
 		return true;
 
@@ -11352,7 +11380,7 @@ static int amdgpu_dm_atomic_check(struct drm_device *dev,
 	}
 
 	/* Remove exiting planes if they are modified */
-	for_each_oldnew_plane_in_state_reverse(state, plane, old_plane_state, new_plane_state, i) {
+	for_each_oldnew_plane_in_descending_zpos(state, plane, old_plane_state, new_plane_state) {
 		if (old_plane_state->fb && new_plane_state->fb &&
 		    get_mem_type(old_plane_state->fb) !=
 		    get_mem_type(new_plane_state->fb))
@@ -11397,7 +11425,7 @@ static int amdgpu_dm_atomic_check(struct drm_device *dev,
 	}
 
 	/* Add new/modified planes */
-	for_each_oldnew_plane_in_state_reverse(state, plane, old_plane_state, new_plane_state, i) {
+	for_each_oldnew_plane_in_descending_zpos(state, plane, old_plane_state, new_plane_state) {
 		ret = dm_update_plane_state(dc, state, plane,
 					    old_plane_state,
 					    new_plane_state,

drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm_plane.c

@@ -104,8 +104,6 @@ void amdgpu_dm_plane_fill_blending_from_plane_state(const struct drm_plane_state
 	*global_alpha = false;
 	*global_alpha_value = 0xff;
 
-	if (plane_state->plane->type == DRM_PLANE_TYPE_PRIMARY)
-		return;
 
 	if (plane_state->pixel_blend_mode == DRM_MODE_BLEND_PREMULTI ||
 		plane_state->pixel_blend_mode == DRM_MODE_BLEND_COVERAGE) {
@@ -1701,6 +1699,7 @@ int amdgpu_dm_plane_init(struct amdgpu_display_manager *dm,
 	int res = -EPERM;
 	unsigned int supported_rotations;
 	uint64_t *modifiers = NULL;
+	unsigned int primary_zpos = dm->dc->caps.max_slave_planes;
 
 	num_formats = amdgpu_dm_plane_get_plane_formats(plane, plane_cap, formats,
 							ARRAY_SIZE(formats));
@@ -1730,10 +1729,19 @@ int amdgpu_dm_plane_init(struct amdgpu_display_manager *dm,
 	}
 
 	if (plane->type == DRM_PLANE_TYPE_PRIMARY) {
-		drm_plane_create_zpos_immutable_property(plane, 0);
+		/*
+		 * Allow OVERLAY planes to be used as underlays by assigning an
+		 * immutable zpos = # of OVERLAY planes to the PRIMARY plane.
+		 */
+		drm_plane_create_zpos_immutable_property(plane, primary_zpos);
 	} else if (plane->type == DRM_PLANE_TYPE_OVERLAY) {
-		unsigned int zpos = 1 + drm_plane_index(plane);
-		drm_plane_create_zpos_property(plane, zpos, 1, 254);
+		/*
+		 * OVERLAY planes can be below or above the PRIMARY, but cannot
+		 * be above the CURSOR plane.
+		 */
+		unsigned int zpos = primary_zpos + 1 + drm_plane_index(plane);
+
+		drm_plane_create_zpos_property(plane, zpos, 0, 254);
 	} else if (plane->type == DRM_PLANE_TYPE_CURSOR) {
 		drm_plane_create_zpos_immutable_property(plane, 255);
 	}