[PATCH v2 1/1] Split VGA default device handler out of VGA arbiter

[Bjorn Helgaas]

On Wed, Aug 23, 2017 at 02:48:42PM +0100, Ard Biesheuvel wrote:
> On 22 August 2017 at 23:19, Bjorn Helgaas <helgaas at kernel.org> wrote:
> > On Mon, Aug 21, 2017 at 11:53:01AM +0100, Lorenzo Pieralisi wrote:
> >> On Thu, Aug 17, 2017 at 09:30:28PM +1000, Daniel Axtens wrote:
> >> > A system without PCI legacy resources (e.g. ARM64) may find that no
> >> > default/boot VGA device has been marked, because the VGA arbiter
> >> > checks for legacy resource decoding before marking a card as default.
> >>
> >> I do not understand this paragraph, in particular:
> >>
> >> - "A system without PCI legacy resources (e.g. ARM64)". What does this
> >>   mean ? I take this as "ARM64 does not support IO space"; if a PCI host
> >>   bridge supports IO space, there is nothing from an architectural
> >>   point of view that prevents an MMIO based IO space implementation to
> >>   work on ARM64. It is PCI bridge specific, not arch specific.
> >
> > This reference to ARM64 is the same thing I stumbled over.  Maybe it
> > could be written along the lines of:
> >
> >   The VGA arbiter selects a default VGA device that is enabled and
> >   reachable via the legacy VGA resources (mem 0xa0000-0xbffff, io
> >   0x3b0-0x3bb, io 0x3c0-0x3df, etc).
> >
> >   If there is no such device, e.g., because there's no enabled VGA
> >   device, the host bridge doesn't support access to those legacy
> >   resources, or a PCI-PCI bridge doesn't have VGA Enable set, a
> >   platform may select an arbitrary device by calling
> >   vga_set_default_device().
> >
> > Then I think this patch changes the previous behavior by allowing the
> > arbiter to select a device that is enabled and has a driver but may
> > not be reachable via the legacy VGA resources.
> >
> > I don't know what all the consequences of that would be, but it does
> > muddy the VGA arbiter waters a bit.  AIUI, the main reason for the
> > arbiter is to allow multiple legacy VGA devices by manipulating bridge
> > VGA Enable bits so only one receives the legacy resources at a time.
> >
> > These devices that do not need the legacy resources don't need that
> > special treatment because they don't care about the bridge VGA Enable
> > bits and several can coexist in the system with no need for VGA
> > arbitration.
> >
> > I guess the powerpc fixup_vga() already selects a device that doesn't
> > need the VGA resources, so we already have that situation.
> >
> 
> Perhaps it is unclear that the whole point of tinkering with the VGA
> arbiter is that X may refuse to use a GFX card if it is not tagged as
> the default VGA device by the arbiter. I have suggested before that
> fixing X may be more appropriate in this case, given that ownership of
> the legacy VGA resources may not correlate at all with being the
> primary display device on non-x86 architectures.

Yeah, maybe it's time to disconnect the "default display device" idea
from the VGA arbiter.  I have no idea what (if any) dependencies X has
on the legacy VGA resources.  I assume X works fine on power, where it
sounds like those resources are rarely or never available.