diff --git a/drivers/gpu/drm/i915/display/skl_universal_plane.c b/drivers/gpu/drm/i915/display/skl_universal_plane.c
index ad4bfff6903d..164b7d61c9a3 100644
--- a/drivers/gpu/drm/i915/display/skl_universal_plane.c
+++ b/drivers/gpu/drm/i915/display/skl_universal_plane.c
@@ -2126,19 +2126,6 @@ static int skl_check_main_surface(struct intel_plane_state *plane_state)
 	return 0;
 }
 
-
-/* Divide a U16.16 fixed-point value by 2, staying in fixed-point domain */
-static inline u32 fp_16_16_div2(u32 fp)
-{
-	return fp >> 1;
-}
-
-/* Convert a U16.16 fixed-point value to integer, rounding up */
-static inline int fp_16_16_to_int_ceil(u32 fp)
-{
-	return DIV_ROUND_UP(fp, 1 << 16);
-}
-
 static int skl_check_nv12_aux_surface(struct intel_plane_state *plane_state)
 {
 	struct intel_display *display = to_intel_display(plane_state);
@@ -2154,14 +2141,20 @@ static int skl_check_nv12_aux_surface(struct intel_plane_state *plane_state)
 	int max_height = intel_plane_max_height(plane, fb, uv_plane, rotation);
 
 	/*
-	 * LNL+ UV surface start/size =
-	 * ceiling(half of Y plane start/size). Use ceiling division
-	 * unconditionally; it is a no-op for even values.
+	 * UV (chroma) start/size = ceiling(half of the *integer* Y plane
+	 * start/size), i.e. the value the luma surface programs (src >> 16),
+	 * not the raw U16.16. A bigjoiner seam mapped through the scaler can
+	 * give a fractional luma src; ceiling that directly would round the
+	 * chroma one column too far and read past the chroma surface.
 	 */
-	int x = fp_16_16_to_int_ceil(fp_16_16_div2(plane_state->uapi.src.x1));
-	int y = fp_16_16_to_int_ceil(fp_16_16_div2(plane_state->uapi.src.y1));
-	int w = fp_16_16_to_int_ceil(fp_16_16_div2(drm_rect_width(&plane_state->uapi.src)));
-	int h = fp_16_16_to_int_ceil(fp_16_16_div2(drm_rect_height(&plane_state->uapi.src)));
+	int luma_x = plane_state->uapi.src.x1 >> 16;
+	int luma_y = plane_state->uapi.src.y1 >> 16;
+	int luma_w = drm_rect_width(&plane_state->uapi.src) >> 16;
+	int luma_h = drm_rect_height(&plane_state->uapi.src) >> 16;
+	int x = DIV_ROUND_UP(luma_x, 2);
+	int y = DIV_ROUND_UP(luma_y, 2);
+	int w = DIV_ROUND_UP(luma_x + luma_w, 2) - x;
+	int h = DIV_ROUND_UP(luma_y + luma_h, 2) - y;
 	u32 offset;
 
 	/* FIXME not quite sure how/if these apply to the chroma plane */