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

[Bjorn Helgaas]

On Fri, Jun 04, 2021 at 12:50:03PM +0800, Huacai Chen wrote:
> On Thu, Jun 3, 2021 at 2:31 AM Bjorn Helgaas <bhelgaas at google.com> wrote:
> >
> > [+cc linux-pci]
> >
> > On Wed, Jun 2, 2021 at 11:22 AM Daniel Vetter <daniel at ffwll.ch> wrote:
> > > On Wed, Jun 02, 2021 at 06:36:03PM +0800, Huacai Chen wrote:
> > > > 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.
> >
> > pcibios_init() is also an initcall and is implemented by every arch.
> > I agree that calling vga_arb_device_init() directly from
> > pcibios_init() would probably fix this problem, and it would be really
> > nice to have it not be an initcall.  But it's also kind of a pain to
> > have to update all those copies of pcibios_init(), and I would be
> > looking for a way to unify it since it's not really an arch-specific
> > thing.
> >
> > I think the simplest solution, which I suggested earlier [1], would be
> > to explicitly call vga_arbiter_add_pci_device() directly from the PCI
> > core when it enumerates a VGA device.  Then there's no initcall and no
> > need for the BUS_NOTIFY_ADD/DEL_DEVICE stuff.
> > vga_arbiter_add_pci_device() could set the default VGA device when it
> > is enumerated, and change the default device if we enumerate a
> > "better" one.  And hotplug VGA devices would work automatically.
> Emm, It seems that your solution has some difficulties to remove the
> whole initcall(vga_arb_device_init): we call
> vga_arbiter_add_pci_device() in pci_bus_add_device(), the
> list_for_each_entry() loop can be moved to
> vga_arbiter_check_bridge_sharing(), vga_arb_select_default_device()
> can be renamed to vga_arb_update_default_device() and be called in
> vga_arbiter_add_pci_device(), but how to handle
> misc_register(&vga_arb_device)?

Might need to keep vga_arb_device_init() as an initcall, but remove
everything from it except the misc_register().