[PATCH 0/2] x86, ia64: Move EFI_FB vga_default_device() initialization to pci_vga_fixup()

Bjorn Helgaas bhelgaas at google.com
Thu Aug 21 21:39:31 PDT 2014

On Thu, Aug 21, 2014 at 4:34 PM, Bruno Prémont
<bonbons at linux-vserver.org> wrote:

> A second step would then be to tune vgaarb's initial selection.
> Bjorn, is it possible to verify which I/O ports are decoded by a PCI
> device at the time of adding it to vgaarb? If so, how? I would like to
> check for legacy VGA I/O range (0x03B0-0x03DF) and only let vgaarb set
> a device as default if that I/O range is decoded by the device.

I don't know of a way.  I'm pretty sure VGA devices are allowed to
respond to those legacy addresses even if there's no BAR for them, but
I haven't found a spec reference for this.  There is the VGA Enable
bit in bridges, of course (PCI Bridge spec, sec 12.1.1.  If the VGA
device is behind a bridge that doesn't have the VGA Enable bit set, it
probably isn't the default device.


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