drm: apple: respect drm_plane_state zpos

The for_each_oldnew_plane_in_state iterator is nondeterministic
in terms of the order of planes. DCP expects surfaces to be
fed to it in the correct order. Relying on the iterator to
lazily increment the index into surf[] means we cannot meet
this expectation. The constant reordering of planes in
the surf[] array seems to cause DCP to crash under certain
circumstances. Cursors will also often be drawn under the main
plane, which is less than ideal.

Populate surf[] in the order everyone expects us to. This fixes
a whole host of odd behaviour when wiring up multiple DRM universal
planes.

Signed-off-by: James Calligeros <[email protected]>

Author: chadmed

drivers/gpu/drm/apple/iomfb_template.c

@@ -1295,8 +1295,6 @@ void DCP_FW_NAME(iomfb_flush)(struct apple_dcp *dcp, struct drm_crtc *crtc, stru
 		dcp->surfaces_cleared = true;
 	}
 
-	// Surface 0 has limitations at least on t600x.
-	l = 1;
 	for_each_oldnew_plane_in_state(state, plane, old_state, new_state, plane_idx) {
 		struct drm_framebuffer *fb = new_state->fb;
 		struct drm_gem_dma_object *obj;
@@ -1307,6 +1305,17 @@ void DCP_FW_NAME(iomfb_flush)(struct apple_dcp *dcp, struct drm_crtc *crtc, stru
 		if (old_state->crtc != crtc && new_state->crtc != crtc)
 			continue;
 
+		/*
+		 * Plane order is nondeterministic for this iterator. DCP will
+		 * almost always crash at some point if the z order of planes
+		 * flip-flops around. Make sure we are always blending them
+		 * in the correct order.
+		 *
+		 * Despite having 4 surfaces, we can only blend two. Surface 0 is
+		 * also unusable on some machines, so ignore it.
+		 */
+		l = 2 - new_state->zpos;
+
 		WARN_ON(l >= SWAP_SURFACES);
 
 		req->swap.swap_enabled |= BIT(l);
@@ -1333,7 +1342,6 @@ void DCP_FW_NAME(iomfb_flush)(struct apple_dcp *dcp, struct drm_crtc *crtc, stru
 		}
 
 		if (!new_state->fb) {
-			l += 1;
 			continue;
 		}
 		req->surf_null[l] = false;
@@ -1383,7 +1391,6 @@ void DCP_FW_NAME(iomfb_flush)(struct apple_dcp *dcp, struct drm_crtc *crtc, stru
 			.has_planes = 1,
 		};
 
-		l += 1;
 	}
 
 	if (!has_surface && !crtc_state->color_mgmt_changed) {