при использовании -f генерится конфиг не перевариваемый X-ами.
Ошибка следующая:
Parse error on line 50 of section Screen in file /tmp/xconfig                   
        "EndSection" is not a valid keyword in this section.

Речь идёь о EndSection для SubSection Display.
Сам конфиг такой:
Section "Files"                                                                
                                                                            
        FontPath   "unix/:-1"                                                  
                                                                            
EndSection                                                                     
                                                                            
                                                                               
                                                                            
Section "Module"                                                               
                                                                            
        Load        "dbe"                                                      
                                                                            
        Load        "/usr/X11R6/lib/modules/extensions/libglx.a"               
                                                                            
        Load        "dri"                                                      
                                                                            
        Load        "type1"                                                    
                                                                            
        Load        "freetype"                                                 
                                                                            
EndSection                                                                     
                                                                            
                                                                               
                                                                            
Section "DRI"                                                                  
                                                                            
        Mode        0666                                                       
                                                                            
EndSection                                                                     
                                                                            
                                                                               
                                                                            
Section "InputDevice"                                                          
                                                                            
        Identifier "Keyboard1"                                                 
                                                                            
        Driver      "kbd"                                                      
                                                                            
        Option "AutoRepeat"  "250 30"                                          
                                                                            
        EndSection                                                             
                                                                            
                                                                               
                                                                            
Section "InputDevice"                                                          
                                                                            
        Identifier  "mouse0"                                                   
                                                                            
        Option  "Device"        "/dev/input/mice"                              
                                                                            
        #"Logitech N48"                                                        
                                                                            
        Driver  "mouse"                                                        
                                                                            
        Option "Protocol"    "IMPS/2"                                          
                                                                            
        Option "ZAxisMapping" "4 5"                                            
                                                                            
EndSection                                                                     
                                                                            
                                                                               
                                                                            
Section "Device"                                                               
                                                                            
        Identifier  "Device0"                                                  
                                                                            
        Driver  "r128"                                                         
                                                                            
EndSection                                                                     
                                                                            
                                                                               
                                                                            
Section "Monitor"                                                              
                                                                            
        Identifier  "Monitor at Device0"                                       
                                                                            
EndSection                                                                     
                                                                            
                                                                               
                                                                            
Section "Screen"                                                               
                                                                            
        Identifier  "Screen at Device0"                                        
                                                                            
        Device      "Device0"                                                  
                                                                            
        Monitor     "Monitor at Device0"                                       
                                                                            
        DefaultColorDepth       8                                              
                                                                            
        Subsection "Display"                                                   
                                                                            
                Depth       8                                                  
                                                                            
                Modes      "640x480"                                           
                                                                            
                ViewPort    0 0                                                
                                                                            
        EndSection                                                             
                                                                            
EndSection                                                                     
                                                                            
                                                                               
                                                                            
Section "ServerLayout"                                                         
                                                                            
        Identifier      "Default Layout"                                       
                                                                            
        Screen  "Screen at Device0"                                            
                                                                            
        InputDevice  "mouse0"     "CorePointer"                                
                                                                            
        InputDevice "Keyboard1" "CoreKeyboard"                                 
                                                                            
                                                                               
                                                                            
EndSection