[RFC PATCH 1/2] PCI/ACPI: Add ACPI support for non ECAM Host Bridge Controllers

Arnd Bergmann arnd at arndb.de
Fri Dec 4 05:57:21 PST 2015

On Friday 04 December 2015 12:04:04 Lorenzo Pieralisi wrote:
> On Thu, Dec 03, 2015 at 09:58:14PM +0100, Arnd Bergmann wrote:

> > pci-host-generic.c is just for standard PCI implementations, and it has
> > zero code that would be shared with ACPI: Most of the implementation
> > deals with parsing DT properties, and all that code is entirely differnet
> > for ACPI and already exists in drivers/acpi. The one thing that could be
> > shared is the ECAM config space access, but ACPI already needs something
> > else here because it requires access to the config space at early boot
> > time, way before we even load that driver, see raw_pci_read/raw_pci_write.
> Yes, I agree, basically ACPI has already a concept of "host generic"
> layer, there is not much point in "merging" it with the pci-host-generic.c
> driver. One thing is for certain: nothing in this and Tomasz patchsets is
> arm64 specific, and should not live in arch/arm64.
> Side note: for the time being raw_pci_read/write will stay empty on
> arm64 till someone explains to me what they are used for, we are not
> adding them just because they are there for x86, I enquired within
> the ACPI spec working group and frankly I do not see a usage for those
> on arm64.

I think this is mainly so AML can poke into PCI config space to
reconfigure things even during early boot, if necessary. You can
have PCI devices that are owned by ACPI and not to be touched by
the OS.

> > > I will put together a proposal to define the way we specify HID and
> > > related DSD properties for PCI host controllers and send it to
> > > the ACPI working group for review.
> > 
> > That also requires a change to SBSA, right? Today, SBSA assumes that
> > we have a standard PCI host that will work with any hardware independent
> > PCI implementation in an OS. We either have to give up on SBSA saying
> > much about how PCI hosts are implemented, or stop assuming that hardware
> > is SBSA compliant.
> It is not even a SBSA change, ECAM is a PCIe standard. I am fine with
> NAK'ing all code that is not ECAM compliant, problem is, we are dealing
> with HW quirks here, it is not something we can fix in FW either.
> I do not think SBSA can rule out HW bugs (call them quirks if you wish),
> because that's what we are dealing with here, the line between HW bugs
> and designs that deliberately deviate from ECAM is thin.

Right, and some are further away from the standard than others.

> > > Second, I am against merging _any_ ACPI/PCI code for arm64 before we
> > > define a way for the kernel to detect if resources should be reassigned
> > > or just claimed as they are, as set-up by BIOS.
> > 
> > Why would it ever reassign anything that has been set up by the BIOS?
> > We are still talking about server systems, right?
> Do not ask me I agree 100% with you here :), but I can bet some systems
> currently shipping with ACPI/PCI on ARM (not upstream) tend to be inherited
> from DT where resources are _always_ reassigned and if we start claiming
> them they till break in a spectacular way, someone has to update that
> FW.
> Does "booting with ACPI" implies "FW set-up resources - do not reassign" ?

I think that should be true on any server regardless of ACPI: if we
have a BIOS, we can expect it to do the job right. The reason we tend
to completely ignore any PCI setup on most embedded systems is that
we don't trust u-boot to do that right (or at all).

> That's an optimistic assumption IMHO. We either need a FW flag, or we just
> force resource claiming on ACPI, and reassign the resources that could not
> be claimed. We have to do it for ACPI only, on DT due to legacy we can't
> do that anymore, we would break the world.

Hmm, but having a flag in the ACPI tables for "BIOS is broken" won't
work if we require the BIOS to set that flag. In that case, we could
just fix the BIOS. ;-)

> I am quite happy to force resource claiming when booting with ACPI,
> since that will force FW developers to toe the line, what I am saying
> is that it is not well defined, at all.
> I rest my case: I am against merging _any_ ACPI/PCI code before we
> define in stone when/if the kernel should reassign resources (answer
> can be "never"), as soon as we merge a platform that requires resources
> assignment to work we are stuck with it forever (see DT host controllers).

Fair enough.


More information about the linux-arm-kernel mailing list