diff --git a/common/hw-gfx-gma-config_helpers.adb b/common/hw-gfx-gma-config_helpers.adb
index 162c9c6..ede140c 100644
--- a/common/hw-gfx-gma-config_helpers.adb
+++ b/common/hw-gfx-gma-config_helpers.adb
@@ -113,13 +113,20 @@
    procedure Configure_FDI_Link
      (Port_Cfg : in out Port_Config;
       Success  :    out Boolean)
-   with Pre => True
+   with
+      Post =>
+         Port_Cfg.Mode.H_Visible = Port_Cfg'Old.Mode.H_Visible and
+         Port_Cfg.Mode.V_Visible = Port_Cfg'Old.Mode.V_Visible
    is
-      procedure Limit_Lane_Count
-      is
-         FDI_TX_CTL_FDI_TX_ENABLE : constant := 1 * 2 ** 31;
-         Enabled : Boolean;
-      begin
+      FDI_TX_CTL_FDI_TX_ENABLE : constant := 1 * 2 ** 31;
+      Enabled : Boolean;
+   begin
+      Port_Cfg.FDI.Receiver_Caps.Max_Link_Rate    := DP_Bandwidth_2_7;
+      Port_Cfg.FDI.Receiver_Caps.Max_Lane_Count   :=
+         Config.FDI_Lane_Count (Port_Cfg.Port);
+      Port_Cfg.FDI.Receiver_Caps.Enhanced_Framing := True;
+
+      if Config.Has_FDI_C and then Port_Cfg.Port = DIGI_C then
          -- if DIGI_D enabled: (FDI names are off by one)
          Registers.Is_Set_Mask
            (Register => Registers.FDI_TX_CTL_C,
@@ -128,15 +135,8 @@
          if Enabled then
             Port_Cfg.FDI.Receiver_Caps.Max_Lane_Count := DP_Lane_Count_2;
          end if;
-      end Limit_Lane_Count;
-   begin
-      Port_Cfg.FDI.Receiver_Caps.Max_Link_Rate    := DP_Bandwidth_2_7;
-      Port_Cfg.FDI.Receiver_Caps.Max_Lane_Count   :=
-         Config.FDI_Lane_Count (Port_Cfg.Port);
-      Port_Cfg.FDI.Receiver_Caps.Enhanced_Framing := True;
-      if Config.Has_FDI_C and then Port_Cfg.Port = DIGI_C then
-         Limit_Lane_Count;
       end if;
+
       DP_Info.Preferred_Link_Setting (Port_Cfg.FDI, Port_Cfg.Mode, Success);
    end Configure_FDI_Link;
 
@@ -216,8 +216,7 @@
    function Validate_Config
      (FB                : Framebuffer_Type;
       Mode              : Mode_Type;
-      Pipe              : Pipe_Index;
-      Scaler_Available  : Boolean)
+      Pipe              : Pipe_Index)
       return Boolean
    is
    begin
@@ -233,8 +232,7 @@
       return
          ((Rotated_Width (FB) = Mode.H_Visible and
            Rotated_Height (FB) = Mode.V_Visible) or
-          (Scaler_Available and
-           Rotated_Width (FB) <= Config.Maximum_Scalable_Width (Pipe) and
+          (Rotated_Width (FB) <= Config.Maximum_Scalable_Width (Pipe) and
            Rotated_Width (FB) <= Mode.H_Visible and
            Rotated_Height (FB) <= Mode.V_Visible)) and
          (FB.Offset /= VGA_PLANE_FRAMEBUFFER_OFFSET or Pipe = Primary) and
diff --git a/common/hw-gfx-gma-config_helpers.ads b/common/hw-gfx-gma-config_helpers.ads
index f02b6e5..9d689c7 100644
--- a/common/hw-gfx-gma-config_helpers.ads
+++ b/common/hw-gfx-gma-config_helpers.ads
@@ -31,7 +31,12 @@
       Pipe     : in     Pipe_Index;
       Port     : in     Port_Type;
       Mode     : in     Mode_Type;
-      Success  :    out Boolean);
+      Success  :    out Boolean)
+   with
+      Post =>
+        (if Success then
+            Port_Cfg.Mode.H_Visible = Mode.H_Visible and
+            Port_Cfg.Mode.V_Visible = Mode.V_Visible);
 
    ----------------------------------------------------------------------------
 
@@ -54,8 +59,7 @@
    function Validate_Config
      (FB                : Framebuffer_Type;
       Mode              : Mode_Type;
-      Pipe              : Pipe_Index;
-      Scaler_Available  : Boolean)
+      Pipe              : Pipe_Index)
       return Boolean
    with
       Post => (if Validate_Config'Result then Valid_FB (FB, Mode));
diff --git a/common/hw-gfx-gma-pipe_setup.adb b/common/hw-gfx-gma-pipe_setup.adb
index 000c536..82f5222 100644
--- a/common/hw-gfx-gma-pipe_setup.adb
+++ b/common/hw-gfx-gma-pipe_setup.adb
@@ -635,12 +635,20 @@
       end if;
    end Setup_Scaling;
 
-   procedure Scaler_Available (Available : out Boolean; Pipe : Pipe_Index)
+   procedure Reserve_Scaler
+     (Success     :    out Boolean;
+      Reservation : in out Scaler_Reservation;
+      Pipe        : in     Pipe_Index)
    is
       Pipe_Using_PF : Pipe_Index := Pipe_Index'First;
       PF_Enabled : Boolean;
    begin
       if Config.Has_GMCH_PFIT_CONTROL then
+         if Reservation.Reserved then
+            Success := Reservation.Pipe = Pipe;
+            return;
+         end if;
+
          Registers.Is_Set_Mask
            (Register => Registers.GMCH_PFIT_CONTROL,
             Mask     => PF_CTRL_ENABLE,
@@ -649,11 +657,15 @@
             Gmch_Panel_Fitter_Pipe (Pipe_Using_PF);
          end if;
 
-         Available := not PF_Enabled or Pipe_Using_PF = Pipe;
+         Success := not PF_Enabled or Pipe_Using_PF = Pipe;
+         if Success then
+            Reservation.Reserved := True;
+            Reservation.Pipe     := Pipe;
+         end if;
       else
-         Available := True;
+         Success := True;
       end if;
-   end Scaler_Available;
+   end Reserve_Scaler;
 
    ----------------------------------------------------------------------------
 
diff --git a/common/hw-gfx-gma-pipe_setup.ads b/common/hw-gfx-gma-pipe_setup.ads
index 3ecb6dd..960e70f 100644
--- a/common/hw-gfx-gma-pipe_setup.ads
+++ b/common/hw-gfx-gma-pipe_setup.ads
@@ -51,10 +51,22 @@
       FB       : Framebuffer_Type;
       Cursor   : Cursor_Type);
 
-   procedure Scaler_Available (Available : out Boolean; Pipe : Pipe_Index);
+   type Scaler_Reservation is private;
+   Null_Scaler_Reservation : constant Scaler_Reservation;
+   procedure Reserve_Scaler
+     (Success     :    out Boolean;
+      Reservation : in out Scaler_Reservation;
+      Pipe        : in     Pipe_Index);
 
 private
 
+   type Scaler_Reservation is record
+      Reserved : Boolean;
+      Pipe     : Pipe_Index;
+   end record;
+   Null_Scaler_Reservation : constant Scaler_Reservation :=
+     (Reserved => False, Pipe => Pipe_Index'First);
+
    subtype WM_Levels is Natural range 0 .. 7;
    type PLANE_WM_Type is array (WM_Levels) of Registers.Registers_Index;
 
diff --git a/common/hw-gfx-gma.adb b/common/hw-gfx-gma.adb
index dbdee5c..269feec 100644
--- a/common/hw-gfx-gma.adb
+++ b/common/hw-gfx-gma.adb
@@ -1,5 +1,5 @@
 --
--- Copyright (C) 2014-2018 secunet Security Networks AG
+-- Copyright (C) 2014-2019 secunet Security Networks AG
 -- Copyright (C) 2017 Nico Huber <nico.h@gmx.de>
 --
 -- This program is free software; you can redistribute it and/or modify
@@ -98,10 +98,11 @@
       Pipe_Cfg : in     Pipe_Config;
       Success  :    out Boolean)
    with
-      Pre => Pipe_Cfg.Port in Active_Port_Type
+      Pre =>
+         Pipe_Cfg.Port in Active_Port_Type and
+         Config_Helpers.Valid_FB (Pipe_Cfg.Framebuffer, Pipe_Cfg.Mode)
    is
       Port_Cfg : Port_Config;
-      Scaler_Available : Boolean;
    begin
       pragma Debug (Debug.New_Line);
       pragma Debug (Debug.Put_Line
@@ -111,12 +112,6 @@
         (Port_Cfg, Pipe, Pipe_Cfg.Port, Pipe_Cfg.Mode, Success);
 
       if Success then
-         Display_Controller.Scaler_Available (Scaler_Available, Pipe);
-         Success := Config_Helpers.Validate_Config
-           (Pipe_Cfg.Framebuffer, Port_Cfg.Mode, Pipe, Scaler_Available);
-      end if;
-
-      if Success then
          Connector_Info.Preferred_Link_Setting (Port_Cfg, Success);
       end if;
 
@@ -223,19 +218,12 @@
          end if;
       end Check_HPD;
 
-      Power_Changed  : Boolean := False;
-      Old_Configs    : Pipe_Configs;
+      Scaler_Reservation : Display_Controller.Scaler_Reservation :=
+         Display_Controller.Null_Scaler_Reservation;
 
-      -- Only called when we actually tried to change something
-      -- so we don't congest the log with unnecessary messages.
-      procedure Update_Power
-      is
-      begin
-         if not Power_Changed then
-            Power_And_Clocks.Power_Up (Old_Configs, Configs);
-            Power_Changed := True;
-         end if;
-      end Update_Power;
+      Update_Power   : Boolean := False;
+      Old_Configs,
+      New_Configs    : Pipe_Configs;
 
       function Full_Update (Cur_Config, New_Config : Pipe_Config) return Boolean
       is
@@ -255,13 +243,48 @@
       end Full_Update;
    begin
       Old_Configs := Cur_Configs;
+      New_Configs := Configs;
+
+      -- validate new configs, filter invalid configs and those waiting for HPD
+      for Pipe in Pipe_Index loop
+         declare
+            Success : Boolean := True;
+            Cur_Config : Pipe_Config renames Cur_Configs (Pipe);
+            New_Config : Pipe_Config renames New_Configs (Pipe);
+         begin
+            if New_Config.Port /= Disabled then
+               if Wait_For_HPD (New_Config.Port) then
+                  Check_HPD (New_Config.Port, Success);
+                  Wait_For_HPD (New_Config.Port) := not Success;
+               end if;
+
+               Success := Success and then
+                           Config_Helpers.Validate_Config
+                             (New_Config.Framebuffer, New_Config.Mode, Pipe);
+
+               if Success and then Requires_Scaling (New_Config) then
+                  Display_Controller.Reserve_Scaler
+                    (Success, Scaler_Reservation, Pipe);
+               end if;
+
+               if not Success then
+                  New_Config.Port := Disabled;
+               end if;
+            end if;
+         end;
+         pragma Loop_Invariant
+           (for all P in Pipe_Index'First .. Pipe =>
+               New_Configs (P).Port = Disabled or
+               Config_Helpers.Valid_FB
+                 (New_Configs (P).Framebuffer, New_Configs (P).Mode));
+      end loop;
 
       -- disable all pipes that changed or had a hot-plug event
       for Pipe in Pipe_Index loop
          declare
             Unplug_Detected : Boolean;
             Cur_Config : Pipe_Config renames Cur_Configs (Pipe);
-            New_Config : Pipe_Config renames Configs (Pipe);
+            New_Config : Pipe_Config renames New_Configs (Pipe);
          begin
             if Cur_Config.Port /= Disabled then
                Check_HPD (Cur_Config.Port, Unplug_Detected);
@@ -269,7 +292,7 @@
                if Full_Update (Cur_Config, New_Config) or Unplug_Detected then
                   Disable_Output (Pipe, Cur_Config);
                   Cur_Config.Port := Disabled;
-                  Update_Power;
+                  Update_Power := True;
                end if;
             end if;
          end;
@@ -279,25 +302,17 @@
       for Pipe in Pipe_Index loop
          declare
             Success : Boolean;
-            Scaler_Available : Boolean;
             Cur_Config : Pipe_Config renames Cur_Configs (Pipe);
-            New_Config : Pipe_Config renames Configs (Pipe);
+            New_Config : Pipe_Config renames New_Configs (Pipe);
          begin
+            -- full update
             if New_Config.Port /= Disabled and
                Full_Update (Cur_Config, New_Config)
             then
-               if Wait_For_HPD (New_Config.Port) then
-                  Check_HPD (New_Config.Port, Success);
-                  Wait_For_HPD (New_Config.Port) := not Success;
-               else
-                  Success := True;
-               end if;
+               Power_And_Clocks.Power_Up (Old_Configs, New_Configs);
+               Update_Power := True;
 
-               if Success then
-                  Update_Power;
-                  Enable_Output (Pipe, New_Config, Success);
-               end if;
-
+               Enable_Output (Pipe, New_Config, Success);
                if Success then
                   Cur_Config := New_Config;
                end if;
@@ -306,23 +321,17 @@
             elsif New_Config.Port /= Disabled and
                   Cur_Config.Framebuffer /= New_Config.Framebuffer
             then
-               Display_Controller.Scaler_Available (Scaler_Available, Pipe);
-               if Config_Helpers.Validate_Config
-                    (New_Config.Framebuffer, New_Config.Mode,
-                     Pipe, Scaler_Available)
-               then
-                  Display_Controller.Setup_FB
-                    (Pipe, New_Config.Mode, New_Config.Framebuffer);
-                  Display_Controller.Update_Cursor
-                    (Pipe, New_Config.Framebuffer, New_Config.Cursor);
-                  Cur_Config := New_Config;
-               end if;
+               Display_Controller.Setup_FB
+                 (Pipe, New_Config.Mode, New_Config.Framebuffer);
+               Display_Controller.Update_Cursor
+                 (Pipe, New_Config.Framebuffer, New_Config.Cursor);
+               Cur_Config := New_Config;
             end if;
          end;
       end loop;
 
-      if Power_Changed then
-         Power_And_Clocks.Power_Down (Old_Configs, Configs, Cur_Configs);
+      if Update_Power then
+         Power_And_Clocks.Power_Down (Old_Configs, New_Configs, Cur_Configs);
       end if;
    end Update_Outputs;