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

[Lorenzo Pieralisi]

Hi Ard,

On Thu, Jun 01, 2017 at 04:18:54PM +0000, Ard Biesheuvel wrote:
> On 1 June 2017 at 16:15, Lorenzo Pieralisi <lorenzo.pieralisi at arm.com> wrote:
> > On Thu, Jun 01, 2017 at 03:04:09PM +0000, Ard Biesheuvel wrote:
> >> 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.
> >
> > IMO they should mean the same thing which implies setting those flags
> > (ie PCI_REASSIGN_ALL_RSRC has no effect though on ARM64) on ARM64
> > ACPI systems as a starting point and then changes in this thread can
> > be applied on top.
> >
> 
> OK
> 
> > Having said that, I am not sure the message you get above is really
> > effective (not sure I undestand the net effect of that resource
> > conflict) so I should look into this.
> >
> 
> It appears to be behavior that is known to be incorrect but is
> preserved for historical reasons.
> 
> Please refer to
> 12d8706963f0 Revert "PCI: Make sure bus number resources stay within
> their parents bounds"
> 1820ffdccb9b PCI: Make sure bus number resources stay within their
> parents bounds

Do you want me to create a branch out of these patches (inclusive of
another patch to fix this bus reallocation policy discrepancy) for
ARM64 folks to test ? Let me know, thanks !

Lorenzo