[coreboot] how to prevent legacy resource conflict with multiple VGA cards

Scott Duplichan scott at notabs.org
Thu Oct 28 19:07:04 CEST 2010

]> I found and tried CONFIG_CONSOLE_VGA_MULTI. I see a couple of problems
]> with it. First, it seems to apply only to AMD K8 and family 10h processors
]> (in my case this is OK). A bigger problem is that it seems to control
]> VGA on an HT link by HT link basis. One problem there is that an HT link
]> could have multiple graphics cards. The CONFIG_CONSOLE_VGA_MULTI doesn't
]> work for me for a different reason. The graphics that needs to be disabled
]> is not on an HT link at all, it is the internal UMA graphics.

]Internal to where?  Is it internal to the processor?

Hello Myles,

My mistake with the word 'internal', I really meant UMA. 

]I think in the past it has been enough to set the VGA bits on the bridges.
]I guess there could have been a problem where there were two VGA devices on
]the same bus.
]So you want to implement the logic to disable UMA if there's an external VGA
]device added, right?  It seems like the place to add that is in the code for
]the device that implements it.  Do you want to disable allocation of the UMA
]area if there's an external card too?

I should explain the sequence of events that lead to my question. I have Win7
running well on this AMD RS780/SB700 board with the exception of a couple of
problems. One problem is that the in-box ATI driver for the UMA graphics is
unstable. So to get Win7 installed, I must add a PCI video card. When Win7 is
installed with the PCI video card present, device manager reports that the
UMA graphics device cannot find enough free resources.

When a reference BIOS is used, the device manager warnings are not present
because the BIOS disables the UMA. One reference BIOS disables UMA only by
clearing the PCI command register bits for memory and I/O decode. A different
reference BIOS skips UMA initialization altogether. I think for now, the 
command register method might be easiest for coreboot.

The problem with the existing coreboot CONFIG_CONSOLE_VGA_MULTI feature is
that it can only disable graphics devices that are on an HT link. In the
case of UMA, non-AMD or non-HT systems, it doesn't work.


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