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

[Ard Biesheuvel]

On 18 May 2017 at 18:46, Lorenzo Pieralisi <lorenzo.pieralisi at arm.com> wrote:
> On Thu, May 18, 2017 at 05:51:44PM +0100, Ard Biesheuvel wrote:
>> On 18 May 2017 at 16:47, Lorenzo Pieralisi <lorenzo.pieralisi at 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.
>

Another data point for this discussion: currently, when booting arm64
via DT, we set PCI_REASSIGN_ALL_RSRC | PCI_REASSIGN_ALL_BUS (unless
PCI_PROBE_ONLY is requested), which forces not only resource
allocations but also the bus numbering to be reconfigured from
scratch.

On arm64/ACPI, we never set those flags, which will cause
pci_scan_bridge() to preserve the secondary and subordinate bridge
numbers if they are non-zero. This actually prevents log messages like

pci_bus 0000:02: busn_res: can not insert [bus 02-ff] under [bus
00-7f] (conflicts with (null) [bus 00-7f])

which I see on AMD Seattle as well as QEMU when booting via DT (and I
suspect on any DT PCI root whose bus range != {0x0 0xff>). However, it
also means that we already have different behavior between ACPI and DT
boot on arm64, which makes it ambiguous what the behavior should be if
_DSM indicates that the configuration should not be preserved. IOW,
'reconfigure everything' currently means different things between DT
and ACPI boot.

Thoughts?

-- 
Ard.