[PATCH] PCI: Add information about describing PCI in ACPI

Bjorn Helgaas helgaas at kernel.org
Wed Nov 23 07:06:33 PST 2016

On Wed, Nov 23, 2016 at 07:28:12AM +0000, Ard Biesheuvel wrote:
> On 23 November 2016 at 01:06, Bjorn Helgaas <helgaas at kernel.org> wrote:
> > On Tue, Nov 22, 2016 at 10:09:50AM +0000, Ard Biesheuvel wrote:
> >> On 17 November 2016 at 17:59, Bjorn Helgaas <bhelgaas at google.com> wrote:
> >
> >> > +PCI host bridges are PNP0A03 or PNP0A08 devices.  Their _CRS should
> >> > +describe all the address space they consume.  In principle, this would
> >> > +be all the windows they forward down to the PCI bus, as well as the
> >> > +bridge registers themselves.  The bridge registers include things like
> >> > +secondary/subordinate bus registers that determine the bus range below
> >> > +the bridge, window registers that describe the apertures, etc.  These
> >> > +are all device-specific, non-architected things, so the only way a
> >> > +PNP0A03/PNP0A08 driver can manage them is via _PRS/_CRS/_SRS, which
> >> > +contain the device-specific details.  These bridge registers also
> >> > +include ECAM space, since it is consumed by the bridge.
> >> > +
> >> > +ACPI defined a Producer/Consumer bit that was intended to distinguish
> >> > +the bridge apertures from the bridge registers [4, 5].  However,
> >> > +BIOSes didn't use that bit correctly, and the result is that OSes have
> >> > +to assume that everything in a PCI host bridge _CRS is a window.  That
> >> > +leaves no way to describe the bridge registers in the PNP0A03/PNP0A08
> >> > +device itself.
> >>
> >> Is that universally true? Or is it still possible to do the right
> >> thing here on new ACPI architectures such as arm64?
> >
> > That's a very good question.  I had thought that the ACPI spec had
> > given up on Consumer/Producer completely, but I was wrong.  In the 6.0
> > spec, the Consumer/Producer bit is still documented in the Extended
> > Address Space Descriptor (sec  It is documented as
> > "ignored" in the QWord, DWord, and Word descriptors (sec,2,3).
> >
> > Linux looks at the producer_consumer bit in acpi_decode_space(), which
> > I think is used for all these descriptors (QWord, DWord, Word, and
> > Extended).  This doesn't quite follow the spec -- we probably should
> > ignore it except for Extended.  In any event, acpi_decode_space() sets
> > IORESOURCE_WINDOW for Producer descriptors, but we don't test
> > IORESOURCE_WINDOW in the PCI host bridge code.
> >
> > x86 and ia64 supply their own pci_acpi_root_prepare_resources()
> > functions that call acpi_pci_probe_root_resources(), which parses _CRS
> > and looks at producer_consumer.  Then they do a little arch-specific
> > stuff on the result.
> >
> > On arm64 we use acpi_pci_probe_root_resources() directly, with no
> > arch-specific stuff.
> >
> > On all three arches, we ignore the Consumer/Producer bit, so all the
> > resources are treated as Producers, e.g., as bridge windows.
> >
> > I think we *could* implement an arm64 version of
> > pci_acpi_root_prepare_resources() that would pay attention to the
> > Consumer/Producer bit by checking IORESOURCE_WINDOW.  To be spec
> > compliant, we would have to use Extended descriptors for all bridge
> > windows, even if they would fit in a DWord or QWord.
> >
> > Should we do that?  I dunno.  I'd like to hear your opinion(s).
> >
> Yes, I think we should. If the spec allows for a way for a PNP0A03
> device to describe all of its resources unambiguously, we should not
> be relying on workarounds that were designed for another architecture
> in another decade (for, presumably, another OS)
> Just for my understanding, we will need to use extended descriptors
> for all consumed *and* produced regions, even though dword/qword are
> implicitly produced-only, due to the fact that the bit is ignored?

>From an ACPI spec point of view, I would say QWord/DWord/Word
descriptors are implicitly *consumer*-only because ResourceConsumer
is the default and they don't have a bit to indicate otherwise.

The current code assumes all PNP0A03 resources are producers.  If we
implement an arm64 pci_acpi_root_prepare_resources() that pays
attention to the Consumer/Producer bit, we would have to:

  - Reserve all producer regions in the iomem/ioport trees.  This is
    already done via pci_acpi_root_add_resources(), but we might need
    a new check to handle consumers differently.

  - Reserve all consumer regions.  This corresponds to what
    pnp/system.c does for PNP0C02 devices.  This is similar to the
    producer regions, but I think the consumer ones should be marked

  - Use every producer (IORESOURCE_WINDOW) as a host bridge window.

I think it's a bug that acpi_decode_space() looks at producer_consumer
for QWord/DWord/Word descriptors, but I think QWord/DWord/Word
descriptors for consumed regions should be safe, as long as they don't
set the Consumer/Producer bit.

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