Re: [PATCH 0/2] arm64: acpi/pci: allow the firmware BAR configuration to be preserved

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

On Thu, May 18, 2017 at 05:51:44PM +0100, Ard Biesheuvel wrote:
> On 18 May 2017 at 16:47, Lorenzo Pieralisi <lorenzo.pieralisi@arm.com> wrote:
> > On Thu, May 18, 2017 at 04:10:28PM +0100, Ard Biesheuvel wrote:
> >
> > [...]
> >
> >> >> Re _DSM: I think it makes sense to honour it, because it puts the
> >> >> allocation under the control of the firmware, which completely removes
> >> >> the burden of having to reason about a policy in the kernel. That
> >> >> leaves the question which will be the default, but that is of minor
> >> >> importance IMO.
> >> >
> >> > I agree; we should try to follow the spec unless we have a good reason
> >> > not to, which argues for honoring the _DSM, so I think it's worth a
> >> > try.  Booting with "pci=realloc" could override the _DSM and taint the
> >> > kernel (because we don't know the effect of reassigning something the
> >> > firmware told us not to touch).
> >> >
> >>
> >> I'd like to hear Lorenzo's view on this first, but I can certainly
> >> respin my _DSM patch to take pci=realloc into account, and move the
> >> handling to generic code as well.
> >
> > I agree with both of you on _DSM implementation and interpretation.
> >
> > Now, if we use it correctly (ie by the FW standard) on ARM64 systems we
> > are going to trigger regressions, that's certain (ie we can then boot
> > with pci=realloc - still, we are breaking systems), that's the reason
> > why for patch(2) I'd like to create a branch and send a CFT for ARM64
> > ACPI testing before queuing it (either I can set-up a testing branch
> > or we ask Bjorn to do it - as you guys prefer - as long as we have
> > a branch for people to test patch(2) on ARM64 ACPI systems).
> >
> > You still need to assign resources that could not be claimed though
> > so patch(2) still needs updating:
> >
> > PCI FW spec 3.1 - 4.6.5
> >
> > "...However, the operating system is free to configure the devices in this
> > hierarchy that have not been configured by the firmware."
> >
> > Which in kernel-speak it means that you have to assign resources that
> > could not be claimed.
> >
> 
> Right. AFAICT this is the part that is typically handled by
> pcibios_resource_survey() et al, whose default __weak implementations
> are empty functions. Shall I override those for arm64 to host this
> logic?

I think it makes sense yes unless Bjorn spots something wrong with that
but you should also call it in ARM64 pci_acpi_scan_root() since it is
not called by PCI core on non-hot-added bridges, I reckon you figured
that out already though.

Thanks a lot !
Lorenzo