这两天经过无数次的尝试，终于让双头显示正常工作了，可以实现桌面扩展，我的芯片是945GT集成显卡，主板上提供了VGA TVOUT 和DVI接口，两个显示器，一个接在VGA上，一个通过DVI->VGA转接头接在DVI上，下面是我的xorg.conf
# Xorg configuration created by system-config-display
#这里配置屏幕布局
Section "ServerLayout"
Identifier "Multihead layout"
Screen 0 "Screen0" 0 0
Screen 1 "Screen1" RightOf "Screen0"
InputDevice "Keyboard0" "CoreKeyboard"
Option "Xinerama" "on"
Option "Clone" "off"
EndSection
Section "InputDevice"
Identifier "Keyboard0"
Driver "kbd"
Option "XkbModel" "pc105"
Option "XkbLayout" "us"
EndSection
Section "Monitor"
Identifier "Monitor0"
VendorName "Monitor Vendor"
ModelName "Monitor 1024x768"
HorizSync 31.5 - 57.0
VertRefresh 50.0 - 70.0
Option "dpms"
EndSection
Section "Monitor"
Identifier "Monitor1"
VendorName "Monitor Vendor"
ModelName "Monitor 1024x768"
### Comment all HorizSync and VertSync values to use DDC:
HorizSync 31.5 - 57.0
VertRefresh 50.0 - 70.0
Option "dpms"
EndSection
Section "Device"
Identifier "Videocard0"
Driver "i810"
#参见 man i810，似乎一定要有这个选项，分别配置PipeA和PipeB的选项
Option "MonitorLayout" "CRT,DFP"
Option "Clone" "off"
BusID "PCI:0:2:0"
EndSection
Section "Device"
Identifier "Videocard1"
Driver "vesa`"
VendorName "Videocard Vendor"
BoardName "Intel Corporation Mobile 945GM/GMS/940GML Express Integrated Graphics Controller"
Option "MonitorLayout" "CRT,DFP"
Option "Clone" "off"
BusID "PCI:0:2:0"
Screen 1
EndSection
Section "Screen"
Identifier "Screen0"
Device "Videocard0"
Monitor "Monitor0"
DefaultDepth 24
SubSection "Display"
Viewport 0 0
Depth 24
EndSubSection
EndSection
Section "Screen"
Identifier "Screen1"
Device "Videocard1"
Monitor "Monitor1"
DefaultDepth 24
SubSection "Display"
Viewport 1 0 #这个1不知道能不能换成0，还没试验
Depth 24
Modes "800x600"
EndSubSection
EndSection
刚开始是配置文件不知道怎么写，在网上搜索，各有各的说法，而且还很不完整，要么是不能启动XServer，要么启动后桌面还是扩展不了，当把配置文件写成这个样后，发现有了希望，根据桌面背景来看，桌面应该是扩展了的，而且下面的工作区指示器上也可以看出，桌面明显变宽了，可是我的两个显示器还是显示的内容一样，而且都是桌面的左半边。就是在这里，让我绕了好大一个弯，原来我的DVI接口上里面也有一组模拟信号，和VGA是并联在一起的，通过转接头就是使用的这一组模拟信号，这样无论如何是不能扩展的，后来，我通过TVOUT来接第二个显示器，将
"MonitorLayout" "CRT,DFP"
改为：
"MonitorLayout" "CRT,TV"
终于成功实现了桌面扩展！
下面还要试从HDMI接口输出进行桌面扩展，不过暂时没有设备，先放着吧。