Re: [PATCH 05/11] x86, pci, acpi: Move arch-agnostic MMCONFIG (aka ECAM) and ACPI code out of arch/x86/ directory

From: Tomasz Nowicki
Date: Mon Sep 07 2015 - 05:59:56 EST

On 31.08.2015 13:01, Tomasz Nowicki wrote:
On 08.06.2015 17:14, Lorenzo Pieralisi wrote:
On Mon, Jun 08, 2015 at 03:57:38AM +0100, Hanjun Guo wrote:


Why can't we make use of the ECAM implementation used by
and drivers/pci/access.c?

We had that question when I had posted MMCFG patch set separately,
please see:

Yes, but the real question is, why do we need to have PCI config
up and running before a bus struct is even created ? I think the
reason is
the PCI configuration address space format (ACPI 6.0, Table 5-27,

"PCI Configuration space addresses must be confined to devices on
PCI Segment Group 0, bus 0. This restriction exists to accommodate
access to fixed hardware prior to PCI bus enumeration".

On HW reduced platforms I do not even think this is required at all,
we have to look into this to avoid code duplication that might well
turn out useless.

This is only for the fixed hardware, which will be not available for
ARM64 (reduced hardware mode), but in Generic Hardware Programming
Model, we using OEM-provided ACPI Machine Language (AML) code to
generic hardware registers, this will be available for reduced

So in ACPI spec, it says: (ACPI 6.0 page 66, last paragraph)

ACPI defines eight address spaces that may be accessed by generic
hardware implementations. These include:
* System I/O space
* System memory space
* PCI configuration space
* Embedded controller space
* System Management Bus (SMBus) space
* PCI BAR Target
* IPMI space

So if any device using the PCI address space for control, such
as a system reset control device, its address space can be reside
in PCI configuration space (who can prevent a OEM do that crazy
thing? :) ), and it should be accessible before the PCI bus is

Us, by changing attitude and questioning features whose usefulness
is questionable. I will look into this and raise the point, I am not
thrilled by the idea of adding another set of PCI accessor functions
and drivers because we have to access a register through PCI before
enumerating the bus (and on arm64 this is totally useless since
we are not meant to support fixed HW anyway). Maybe we can make acpica
code use a "special" stub (ACPI specific, PCI configuration space
space has restrictions anyway), I have to review this set in its
entirety to see how to do that (and I would kindly ask you to do
it too, before saying it is not possible to implement it).

I'm willing to do that, actually, if we don't need a mechanism to
access PCI config space before the bus is created, the code can be
simplified a lot.

After more investigation on the spec and the ACPI core code, I'm
still not convinced that accessing to PCI config space before PCI
bus creating is impossible, also there is no enough ARM64 hardware
to prove that too. But I think we can go in this way, reuse the
ECAM implementation by pci-host-generic for now, and implement the PCI
accessor functions before enumerating PCI bus when needed in the
future, does it make sense?

You mean we rewrite the patch to make sure we can use the PCI host
driver with MCFG and we leave the acpica PCI config call empty stubs on
arm64 (as they are now) ?

Hi Bjorn, Rafael,

Lorenzo pointed out very important problem we are having with PCI config
space access for ARM64. Please refer to the above discussion and add
your 2 cents. Can we forget about accessing PCI config space (for
Hardware Reduced profile) before PCI bus creation? If not, do you see a
way to use drivers/pci/access.c accessors here, like acpica change? Any
opinion is very appreciated.

Kindly remainder.

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