Re: [PATCH] ACPI / hotplug / PCI: Allocate resources directly under the non-hotplug bridge

Bjorn Helgaas
2019-11-07T14:41:01Z

On Thu, Nov 07, 2019 at 04:02:59PM +0200, Mika Westerberg wrote:
> On Thu, Nov 07, 2019 at 07:52:46AM -0600, Bjorn Helgaas wrote:
> > > > What would happen if a device below one of the non-hotplug
> > > > bridges, e.g., 3a:00.0, had an I/O BAR? Would this patch
> > > > still work?
> > >
> > > I think it would still work because now we call
> > > pci_bus_size_bridges() only for non-hotplug bridge which do not
> > > have I/O window open so pbus_size_io() fails to find the "free"
> > > I/O resource on that bus and the kernel then fails to assign
> > > that I/O resource for the device.
> >
> > Not sure I understand; are you saying that we wouldn't have the
> > EC/GPE issue, but we'd be unable to use a device below 3a:00.0
> > that happened to have an I/O BAR?
>
> Yes.
>
> > That doesn't sound optimal because there is I/O space available
> > that could be routed to 3a:00.0
>
> If the none of the upstream bridges up to the PCIe root port does
> not have I/O window open, I don't think we can do much about it.
> Unless I'm missing something of course.

The path to this hypothetical 3a:00.0 device is:

PCI host bridge to bus 0000:00
pci_bus 0000:00: root bus resource [io 0x0000-0x0cf7 window]
pci_bus 0000:00: root bus resource [io 0x0d00-0xffff window]
00:1b.0: Root Port to [bus 02-3a]
02:00.0: Switch Upstream Port to [bus 03-3a]
03:02.0: Switch Downstream Port to [bus 3a]
3a:00.0: reg 0x10: [io 0x????-0x????]

None of the bridges (00:1b.0, 02:00.0, 03:02.0) currently has an open
I/O window, but there's space available on bus 00 and windows *could*
be opened.

I guess it comes down to that ordering problem: this Notify() and
acpiphp_native_scan_bridge() happens before pnp/system.c reserves
things, so we don't yet know what space is actually available.

If firmware had configured I/O windows for these bridges, 3a:00.0
would probably work. But it doesn't seem right that we would depend
on that firmware configuration.

Bjorn