make fbdev/fbcon switchable per driver?

[Alex Williamson]

On Mon, 30 Jan 2017 09:15:50 +0100
Gerd Hoffmann <kraxel at redhat.com> wrote:

>   Hi,
> 
> > The vgaarb code has a concept of a vga_default_device(), it's rather
> > PCI-centric, but maybe better than nothing.  This is typically the
> > first VGA class code device found with I/O and MMIO enabled.  If fbcon
> > defaulted to running on the vga_default_device(), a user could select
> > which to use by re-ordering the VM hardware.  
> 
> The qemu drivers don't register as vgaarb clients though.  Which easily
> explains why igd always wins the primary selection, no matter how you
> order your hardware.
> 
> So, should they register?  The drivers don't need access to the vga
> registers[1][2].

The VGA arbiter sets up a notifier on the PCI bus and will add any VGA
class code devices it finds.  So even if the driver doesn't
participate, it'll still be tracked and might be marked as primary.  If
a graphics driver claims a VGA device that does not depend on VGA
region access then the driver should configure the device not to claim
VGA accesses (maybe only relevant for integrated graphics - i915 gets
this wrong), and register with the arbiter to opt-out of VGA
arbitration.  Picking a "primary" can be done w/o any of that latter if
we agree on the arbiter algorithm of picking the first device with VGA
routing to it (or it can be overridden by arch/platform code).  Thanks,

Alex