Merge tag 'drm-intel-next-fixes-2026-01-29' of https://gitlab.freedesktop.org/drm/i915/kernel into drm-next

- Prevent u64 underflow in intel_fbc_stolen_end

Signed-off-by: Dave Airlie <airlied@redhat.com>
From: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: https://patch.msgid.link/aXsWGWjacEJ03rTs@jlahtine-mobl

Author: airlied

drivers/gpu/drm/i915/display/intel_fbc.c

@@ -809,19 +809,32 @@ static u64 intel_fbc_cfb_base_max(struct intel_display *display)
 
 static u64 intel_fbc_stolen_end(struct intel_display *display)
 {
-	u64 end;
+	u64 end = intel_fbc_cfb_base_max(display);
 
-	/* The FBC hardware for BDW/SKL doesn't have access to the stolen
+	/*
+	 * The FBC hardware for BDW/SKL doesn't have access to the stolen
 	 * reserved range size, so it always assumes the maximum (8mb) is used.
 	 * If we enable FBC using a CFB on that memory range we'll get FIFO
-	 * underruns, even if that range is not reserved by the BIOS. */
+	 * underruns, even if that range is not reserved by the BIOS.
+	 */
 	if (display->platform.broadwell ||
-	    (DISPLAY_VER(display) == 9 && !display->platform.broxton))
-		end = intel_parent_stolen_area_size(display) - 8 * 1024 * 1024;
-	else
-		end = U64_MAX;
+	    (DISPLAY_VER(display) == 9 && !display->platform.broxton)) {
+		u64 stolen_area_size = intel_parent_stolen_area_size(display);
+
+		/*
+		 * If stolen_area_size is less than SZ_8M, use
+		 * intel_fbc_cfb_base_max instead.  This should not happen,
+		 * so warn if it does.
+		 */
+		if (drm_WARN_ON(display->drm,
+				check_sub_overflow(stolen_area_size,
+						   SZ_8M, &stolen_area_size)))
+			return end;
+
+		return min(end, stolen_area_size);
+	}
 
-	return min(end, intel_fbc_cfb_base_max(display));
+	return end;
 }
 
 static int intel_fbc_min_limit(const struct intel_plane_state *plane_state)