Re: [PATCH V1 11/11] arm64, pci, acpi: Support for ACPI based PCI hostbridge init

From: Arnd Bergmann
Date: Tue Nov 03 2015 - 11:56:26 EST

On Tuesday 03 November 2015 11:33:18 Sinan Kaya wrote:
> On 11/3/2015 10:59 AM, Arnd Bergmann wrote:
> > On Tuesday 03 November 2015 10:10:21 Sinan Kaya wrote:
> >>
> >> I don't see anywhere in the SBSA spec addendum that the PCI
> >> configuration space section that unaligned accesses *MUST* be supported.
> >>
> >> If this is required, please have this info added to the spec. I can work
> >> with the designers for the next chip.
> >>
> >> Unaligned access on the current hardware returns incomplete values or
> >> can cause bus faults. The behavior is undefined.
> >
> > Unaligned accesses are not allowed, but any PCI compliant device must
> > support aligned 1, 2 or 4 byte accesses on its configuration space,
> > though the byte-enable mechanism. In an ECAM host bridge, those are
> > mapped to load/store accesses from the CPU with the respective width
> > and natural alignment.
> As far as I see, the endpoints do not have any problems with unaligned
> accesses. It is the host bridge itself (stuff that doesn't get on the
> PCIe bus and uses traditional AXI kind bus internally) has problems with
> alignment.
> If Linux is expecting all HW vendors to implement alignment support,
> this needs to be put in the SBSA spec as a hard requirement.

As I said, it's not unaligned accesses at all, just 1-byte and aligned
2-byte accesses, and it's not Linux mandating this but the PCI
spec. Please read Russell's email again, it is not possible for PCI
to work according to the specification unless the host bridge allows
sub-32-bit accesses.

You can probably work around this by using the legacy I/O port method
rather than ECAM, if the PCI host bridge itself is functional and just
the host bus it is connected to is buggy.

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