diff --git a/common/hw-gfx-gma-config_helpers.adb b/common/hw-gfx-gma-config_helpers.adb
index c95bebc..162c9c6 100644
--- a/common/hw-gfx-gma-config_helpers.adb
+++ b/common/hw-gfx-gma-config_helpers.adb
@@ -1,5 +1,5 @@
 --
--- Copyright (C) 2015-2016 secunet Security Networks AG
+-- Copyright (C) 2015-2016, 2019 secunet Security Networks AG
 --
 -- This program is free software; you can redistribute it and/or modify
 -- it under the terms of the GNU General Public License as published by
@@ -80,6 +80,33 @@
             when DP1 .. DP3      => DP);
    end To_Display_Type;
 
+   function Highest_Dotclock (Configs : Pipe_Configs) return Frequency_Type
+   is
+      Max : Frequency_Type := Frequency_Type'First;
+   begin
+      for I in Pipe_Index loop
+         if Configs (I).Port /= Disabled and
+            Max < Configs (I).Mode.Dotclock
+         then
+            Max := Configs (I).Mode.Dotclock;
+         end if;
+      end loop;
+      return Max;
+   end Highest_Dotclock;
+
+   procedure Limit_Dotclocks
+     (Configs  : in out Pipe_Configs;
+      Max      : in     Frequency_Type) is
+   begin
+      for I in Pipe_Index loop
+         if Configs (I).Port /= Disabled and
+            Max < Configs (I).Mode.Dotclock
+         then
+            Configs (I).Mode.Dotclock := Max;
+         end if;
+      end loop;
+   end Limit_Dotclocks;
+
    ----------------------------------------------------------------------------
 
    -- Prepares link rate and lane count settings for an FDI connection.
diff --git a/common/hw-gfx-gma-config_helpers.ads b/common/hw-gfx-gma-config_helpers.ads
index 7bbf423..f02b6e5 100644
--- a/common/hw-gfx-gma-config_helpers.ads
+++ b/common/hw-gfx-gma-config_helpers.ads
@@ -60,4 +60,25 @@
    with
       Post => (if Validate_Config'Result then Valid_FB (FB, Mode));
 
+   -- For still active pipes, ensure only timings
+   -- changed that don't affect FB validity.
+   function Stable_FB (Old_C, New_C : Pipe_Configs) return Boolean is
+     (for all P in Pipe_Index =>
+         New_C (P).Port = Disabled or
+           (New_C (P).Port = Old_C (P).Port and
+            New_C (P).Framebuffer = Old_C (P).Framebuffer and
+            New_C (P).Cursor = Old_C (P).Cursor and
+            New_C (P).Mode.H_Visible = Old_C (P).Mode.H_Visible and
+            New_C (P).Mode.V_Visible = Old_C (P).Mode.V_Visible));
+
+   ----------------------------------------------------------------------------
+
+   function Highest_Dotclock (Configs : Pipe_Configs) return Frequency_Type;
+
+   procedure Limit_Dotclocks
+     (Configs  : in out Pipe_Configs;
+      Max      : in     Frequency_Type)
+   with
+      Post => Stable_FB (Configs'Old, Configs);
+
 end HW.GFX.GMA.Config_Helpers;