[Xorg] The big multiconsole nasty

Egbert Eich eich at pdx.freedesktop.org
Wed Jul 7 02:19:14 PDT 2004

John Dennis writes:
 > You should assume the kernel in conjunction with other low level system
 > services such as ACPI have correctly enumerated and configured the PCI
 > bus topology and that VGA routing is correct. Unlike the current

 > implementation you should not write new values into PCI bridge registers
 > or in any way alter the hardware subsystem behind the kernel's back. You
 > should trust the kernel and cooperate with it. In the past the X server
 > has been a rude user mode guest who thought it knew more than the kernel
 > and as a consequence has machine checked (crashed) entire systems :-(


you try to make people believe that the Xserver is constantly reconfiguring
PCI config space. In fact it does this only under very rare circumstances 
- and we had to implement this as at the time we added multi head support 
many BIOSes were incapable to handle multiple graphics devices correctly 
and there was no Linux around that was able to validate the PCI config space.

In fact Im sure you will have to dig around a lot to come up with an example
where this actually happens. The only cases where I have seen this happening 
over the last two or three years was with old and broken PCI cards or other
rather strange configurations.

Oh, you've raised a second issue: VGA routing: VGA routing is *required* 
with a lot of graphics cards if you want to offer multihead support. If 
you have an AGP and a PCI card and want to do multihead using both cards 
there  is no way around VGA routing on a lot of chipsets. You may say this 
is not a very likely configuration today...


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