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

[Bjorn Helgaas <helgaas@kernel.org>]

On Sat, Jun 05, 2021 at 10:02:05AM +0800, Huacai Chen wrote:
> On Sat, Jun 5, 2021 at 3:56 AM Bjorn Helgaas <helgaas@kernel.org> wrote:
> > 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@google.com> wrote:

> > > > 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().
>
> OK, let me try. But I think call  vga_arbiter_add_pci_device() in
> pci core is nearly the same as notifier.  Anyway, I will send a new
> patch later.

Notifiers are useful in some situations, for example, if a loadable
module needs to be called when a device is added or removed.

But when possible, I will always choose a direct call instead because
it's much less complicated.  The VGA arbiter cannot be a loadable
module, so I don't think there's any reason to use a notifier in this
case.

Bjorn