[PATCH] vgaarb: Call vga_arb_device_init() after PCI enumeration

[Daniel Vetter]

On Wed, Jun 02, 2021 at 06:36:03PM +0800, Huacai Chen wrote:
> Hi, Daniel,
> 
> On Wed, Jun 2, 2021 at 2:03 AM Daniel Vetter <daniel at ffwll.ch> wrote:
> >
> > On Tue, Jun 01, 2021 at 07:12:27PM +0200, Greg KH wrote:
> > > On Tue, Jun 01, 2021 at 05:56:40PM +0200, Daniel Vetter wrote:
> > > > On Fri, May 28, 2021 at 04:26:07PM +0800, Huacai Chen wrote:
> > > > > We should call vga_arb_device_init() after PCI enumeration, otherwise it
> > > > > may fail to select the default VGA device. Since vga_arb_device_init()
> > > > > and PCI enumeration function (i.e., pcibios_init() or acpi_init()) are
> > > > > both wrapped by subsys_initcall(), their sequence is not assured. So, we
> > > > > use subsys_initcall_sync() instead of subsys_initcall() to wrap vga_arb_
> > > > > device_init().
> > >
> > > Trying to juggle levels like this always fails if you build the code as
> > > a module.
> > >
> > > Why not fix it properly and handle the out-of-order loading by returning
> > > a "deferred" error if you do not have your resources yet?
> >
> > It's not a driver, it's kinda a bolted-on-the-side subsytem of pci. So not
> > something you can -EPROBE_DEFER I think, without potentially upsetting the
> > drivers that need this.
> >
> > Which might mean we should move this into pci subsystem proper perhaps?
> > Then adding the init call at the right time becomes trivial since we just
> > plug it in at the end of pci init.
> >
> > Also maybe that's how distros avoid this pain, pci is built-in, vgaarb is
> > generally a module, problem solved.
> >
> > Bjorn, would you take this entire vgaarb.c thing? From a quick look I
> > don't think it has a drm-ism in it (unlike vga_switcheroo, but that works
> > a bit differently and doesn't have this init order issue).
> Emmm, this patch cannot handle the hotplug case and module case, it
> just handles the case that vgaarb, drm driver and pci all built-in.
> But I think this is enough, because the original problem only happens
> on very few BMC-based VGA cards (BMC doesn't set the VGA Enable bit on
> the bridge, which breaks vgaarb).

I'm not talking aout hotplug, just ordering the various pieces correctly.
That vgaarb isn't really a driver and also can't really handle hotplug is
my point. I guess that got lost a bit?

Anyway my proposal is essentially to do a

$ git move drivers/gpu/vga/vgaarb.c drivers/pci

But I just realized that vgaarb is a bool option, so module isn't possible
anyway, and we could fix this by calling vgaarb from pcibios init (with an
empty static inline in the header if vgaarb is disabled). That makes the
dependency very explicit and guarantees it works correctly.

Whether we move vgaarb into drivers/pci or not is then kinda orthogonal.

Thoughts on that?
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch