Re: [RFC PATCH] Revert "arm64: PCI: Exclude ACPI "consumer" resources from host bridge windows"

[Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>]

On Thu, May 27, 2021 at 11:34:52AM -0500, Bjorn Helgaas wrote:

[...]

> > > https://lore.kernel.org/r/20210510234020.1330087-1-luzmaximilian@gmail.com
> > 
> > Sigh. We can't apply this patch since it would trigger regressions on
> > other platforms (IIUC the root complex registers would end up in the
> > host bridge memory windows).
> > 
> > I am not keen on reverting commit 8fd4391ee717 because it does the
> > right thing.
> > 
> > I think this requires a quirk and immediate reporting to Microsoft.
> > 
> > Bjorn, what are your thoughts on this ?
> 
> In retrospect, I think 8fd4391ee717 (which I wrote), was probably a
> mistake.
> 
> Sure, it's a nice idea to have PNP0A03 _CRS methods that work nicely
> as designed, by describing host bridge registers as "consumer"
> resources and host bridge windows as "producer" registers, instead of
> having the bridge registers in _CRS of an unrelated PNP0C02 device.
> 
> But realistically, the PNP0A03/PNP0C02 issue is a solved problem, even
> though it's ugly, and I'm not sure why I thought Microsoft would see
> value in doing this differently on arm64 than on x86 and ia64.

We hoped we could comply with the specs, given that we were starting
from a clean slate (and not from ACPI tables cut and paste)

> What would break if we reverted 8fd4391ee717?  I guess any arm64
> platforms that described host bridge register space in PNP0A03 _CRS
> "consumer" resources ?

Yes. We would end up with that register space in the host bridge memory
windows - this does not sound right.

> And Windows probably doesn't work or isn't supported on those
> platforms?

By the look of it the answer is yes, Windows was not bootstrapped on
those platforms given that I *assume* Windows does not discriminate
between producer and consumer resources at all.

Lorenzo