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

[Lorenzo Pieralisi]

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 6.4.3.5.4).  It is documented as
> > "ignored" in the QWord, DWord, and Word descriptors (sec 6.4.3.5.1,2,3).
> >
> > 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)

That was the idea I floated at LPC16. We can override the
acpi_pci_root_ops prepare_resources() function pointer with a function
that checks IORESOURCE_WINDOW and filters resources accordingly (and
specific quirk "drivers" may know how to intepret resources that aren't
IORESOURCE_WINDOW - ie they can use it to describe the PCI ECAM config
space quirk region in their _CRS).

In a way that's something that makes sense anyway because given
that we are starting from a clean slate on ARM64 considering resources
that are not IORESOURCE_WINDOW as host bridge windows is just something
we are inheriting from x86, it is not really ACPI specs compliant (is
it ?).

> 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?

That's something that has to be clarified within the ASWG ie why the
consumer bit is ignored for *some* descriptors and not for others.

As things stand unfortunately the answer seems yes (I do not know
why).

> > It *would* be nice to have bridge registers in the bridge _CRS.  That
> > would eliminate the need for looking up the HISI0081/PNP0C02 devices
> > to find the bridge registers.  Avoiding that lookup is only a
> > temporary advantage -- the next round of bridges are supposed to fully
> > implement ECAM, and then we won't need to know where the registers
> > are.
> >
> > Apart from the lookup, there's still some advantage in describing the
> > registers in the PNP0A03 device instead of an unrelated PNP0C02
> > device, because it makes /proc/iomem more accurate and potentially
> > makes host bridge hotplug cleaner.  We would have to enhance the host
> > bridge driver to do the reservations currently done by pnp/system.c.
> >
> > There's some value in doing it the same way as on x86, even though
> > that way is somewhat broken.
> >
> > Whatever we decide, I think it's very important to get it figured out
> > ASAP because it affects the ECAM quirks that we're trying to merge in
> > v4.10.
> >
> 
> I agree. What exactly is the impact for the quirks mechanism as proposed?
The impact is that we could just use the PNP0A03 _CRS to report the PCI
ECAM config space quirk region through a consumer resource keeping in
mind what I say above (actually I think that's what was done on APM
firmware initially, for the records).

Lorenzo