[coreboot] PciExpressGraphics on Intel Sandybridge

[Timothy Pearson]

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On 08/11/2015 02:05 AM, Patrick Rudolph wrote:
> Hello,
> I'm currently working on enabling PEG and run into some trouble.
> With pending patches [1] applied, I was able to activate the Nvidia GPU.
> The system hangs at Seabios/Grub2 with 2 GPUs enabled at the same time.
> As I'm using the EHCI Debug Port I don't have any Seabios console
> output.
> Is there EHCI Debug Port support available ?
> My first guess is some VGA IO ranges are overlapping.
> Any ideas whats wrong ?

Legacy VGA I/O ranges cannot overlap by design.  All legacy VGA I/O and
memory accesses will be routed to one and only one device (the "primary"
display controller).

It is possible that coreboot is becoming confused as to which GPU to
route legacy access to.  Look in your coreboot log for lines similar to
these:

found VGA at PCI: 01:00.0
Setting up VGA for PCI: 01:00.0
Setting PCI_BRIDGE_CTL_VGA for bridge PCI: 00:02.0
Setting PCI_BRIDGE_CTL_VGA for bridge PCI: 00:18.0
Setting PCI_BRIDGE_CTL_VGA for bridge DOMAIN: 0000
Setting PCI_BRIDGE_CTL_VGA for bridge Root Device

That will give more information as to what is actually happening.

> Do I need to provide two VBIOS blobs ? At the moment it is only possible
> to provide one (using make menuconfig).
> The ACPI code "parses" the IGD VBIOS, do I need to add similar code for
> PEG ?

Yes, you need two blobs if the GPUs require different ROMs.  However,
only one VBIOS is actually used -- one of the option ROMs is only used
to initialize the secondary GPU, while the other initializes the primary
GPU and sets up the VBIOS.

There's more to this topic to be sure, but hopefully this will point you
in the right direction.  I have tested multiple nVidia cards under
coreboot at this point (discrete cards on AMD boards) without too many
issues, but watch your overall BAR allocations as with two GPUs active
you may exceed coreboot's allocator limits (coreboot does not have
support for 64-bit prefetchable BARs at this time).

- -- 
Timothy Pearson
Raptor Engineering
+1 (415) 727-8645
http://www.raptorengineeringinc.com
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