The xf86InitFBManager function in the  
xc/programs/Xserver/hw/xfree86/common/xf86fbman.c file contains a sanity check  
for input parameters that is always false.  This is found in the following  
line:  
  
    if (FullBox->x2 < FullBox->x2) return FALSE;  
  
As you can see, FullBox->x2 will always be equal to itself, so this check will  
never evaluate to true and do the 'return FALSE'. 
 
I think that the second FullBox->x2 in the conditional should be FullBox->x1 
similar to what the previous line in the file does for the Y coordinate.  See 
the attached patch.