Question about dual monitors

[Antony Gelberg]

Daniel Carrera wrote:
> Karl Auer wrote:
> 
>>> How is X.org or Xinerama supposed to know which one is supposed to be 
>>> the LCD screen on my laptop and which one is the CRT connected to the 
>>> VGA output?
>>
>>
>> Because in the screen section you say what monitor to use and what
>> device it is attached to. A screen is a monitor attached to a device.
>> And in the device section you number the screens.
> 
> 
> This just shifts the question to the devices. Given that in reality 
> there is only one device in the PCI bus, how does X know which one is 
> supposed to be the external monitor. Here is another snip from the 
> config file:
> 
> Section "Device"
>         Screen          0
>         Identifier      "Intel Corporation ..."
>         Driver          "i810"
>         BusID           "PCI:0:2:0"
> EndSection
> 
> Section "Device"
>         Screen          1
>         Identifier      "Intel Corporation ..."
>         Driver          "i810"
>         BusID           "PCI:0:2:0"
> EndSection
> 
> 
> Notice that the devices have the same BusID and identifier. What is to 
> tell X that Screen 1 is supposed to mean "VGA out"? The same is true for 
> your config file. Let's take a look at what you posted:
> 
>> Section "Device"
>>     Identifier    "ATI_0"
>>     Driver        "radeon"
>>     BusID        "PCI:1:0:0"
>>     Screen        0
>> EndSection
>>
>> Section "Device"
>>     Identifier    "ATI_1"
>>     Driver        "radeon"
>>     BusID        "PCI:1:0:0"
>>     Screen        1
>> EndSection
> 
> 
> You see that those are actually the same device? Okay, you've chosen 
> different identifiers. But the identifier is just a user-chosen string. 
> There is nothing to tell X that "ATI_1" means "VGA out". These two 
> devices are the same.

The Devices are not the same.  They are different due to the differing 
Screen parameter.