[SeaBIOS] RFC: Primary VGA
Michael S. Tsirkin
mst at redhat.com
Thu Feb 28 07:50:20 CET 2013
On Wed, Feb 27, 2013 at 02:16:23PM -0700, Alex Williamson wrote:
>
> When we start adding root ports and bridges to systems we need some
> concept of a primary VGA device.The differentiation of the primary
> device is that it's the default one that responds to the Legacy VGA
> address ranges. PCs often have a BIOS selection for this.
>
> Seabios already seems to have some concept of this and looks for a VGA
> class device for which the parent devices all have VGA routing enabled.
> This seems to work today if QEMU initializes VGA routing for the path it
> considers the primary.
>
> The first question is whether this bridge path pre-configuration is what
> we want to keep as the way QEMU communicates the primary VGA device to
> Seabios? Obviously we could switch to some kind of fwcfg interface, but
> I tend to think what we have is sufficient.
>
> If it is sufficient, then I think we need to rebuild that path on system
> reset and we need some way to specify which device to use. One option
> would be some kind of per PCIDevice property, perhaps "primary_vga". A
> downside is that users can abuse it by trying to set it for more than
> one device. Maybe a better approach would be to add a machine property
> for it, -machine primary_vga=$id.
Yes. And a command to change it when we support hotplug in the future?
> We'll also need some reasonable way
> to pick a default if unspecified.
>
> Does anyone have any thoughts on managing this? Thanks,
>
> Alex
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