linux-kernel - Re: [PATCH] PCI / ACPI: Rework ACPI device nodes lookup for the PCI bus type

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Message-ID: <CAErSpo43PYdDLiRFeqjADiZ5w76jNXxJ8dNcffvw7=6dYZ9UHg@mail.gmail.com>
Date:	Thu, 3 Jan 2013 16:00:55 -0700
From:	Bjorn Helgaas <bhelgaas@...gle.com>
To:	"Rafael J. Wysocki" <rjw@...k.pl>
Cc:	ACPI Devel Maling List <linux-acpi@...r.kernel.org>,
	Len Brown <lenb@...nel.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Peter Wu <lekensteyn@...il.com>
Subject: Re: [PATCH] PCI / ACPI: Rework ACPI device nodes lookup for the PCI
 bus type

On Thu, Jan 3, 2013 at 3:38 PM, Rafael J. Wysocki <rjw@...k.pl> wrote:
> On Thursday, January 03, 2013 02:44:32 PM Bjorn Helgaas wrote:
>> On Thu, Jan 3, 2013 at 1:17 PM, Rafael J. Wysocki <rjw@...k.pl> wrote:
>> > On Thursday, January 03, 2013 08:16:26 AM Bjorn Helgaas wrote:
>> >> On Fri, Dec 28, 2012 at 2:29 PM, Rafael J. Wysocki <rjw@...k.pl> wrote:
>> >> > From: Rafael J. Wysocki <rafael.j.wysocki@...el.com>
>> >> >
>> >> > As the kernel Bugzilla report #42696 indicates, it generally is not
>> >> > sufficient to use _ADR to get an ACPI device node corresponding to
>> >> > the given PCI device, because there may be multiple objects with
>> >> > matching _ADR in the ACPI namespace (this probably is against the
>> >> > spec, but it evidently happens in practice).
>> >>
>> >> I don't see anything in sec 6.1.1 (_ADR) that precludes having
>> >> multiple objects that contain the same _ADR. Do you have any other
>> >> pointers?
>> >
>> > Section 6.1 implicitly means that.  It says that for PCI devices _ADR
>> > must be present to identify which device is represented by the given
>> > ACPI node.  Next, Section 6.1.1 says that the parent bus should be inferred
>> > from the location of the _ADR object's device package in the ACPI namespace,
>> > so clearly, if that's under the PCI root bridge ACPI node, the _ADR
>> > corresponds to a PCI device's bus address.
>>
>> I agree that for namespace Devices below a PCI host bridge, the _ADR
>> value and its position in the hierarchy is required to be sufficient
>> to identify a PCI device and function (or the set of all functions on
>> a device #).
>>
>> > Then, Table 6-139 specifies the format of _ADR for PCI devices as being
>> > euqivalent to devfn, which means that if two nodes with the same _ADR are
>> > present in one scope (under one parent), then it is impossible to distinguish
>> > between them and that's against Section 6.1.
>>
>> This is the bit I don't understand.  Where's the requirement that we
>> be able to distinguish between two namespace nodes with the same _ADR?
>
> According to the spec we can't (if they are under the same parent) and that's
> the whole problem.

It's only a problem if you make the assumptions Linux does.  I can
imagine a system with different assumptions.  For example, an OS could
start with PCI device X and ask "please run any _PS0 method that
matches X."  In that case, you don't care how many objects have an
_ADR that matches X; you merely find *any* matching object that
contains _PS0.

>> Linux assumes we can start from a PCI device and identify a single
>> related ACPI namespace node, e.g., in acpi_pci_find_device().  But all
>> I see in the spec is a requirement that we can start from an ACPI
>> namespace node and find a PCI device.  So I'm not sure
>> acpi_pci_find_device() is based on a valid assumption.
>
> I think it is.
>
> Suppose that we have two namespace nodes with the same _ADR under one parent
> (PCI bridge ACPI node) and they both contain things like _PS0 and _PS3.  Which
> one of these are we supposed to use for the power management of the
> corresponding PCI device?  Because they both would point to the same device,
> right?

That's a good question.  It's more complicated if two objects supply
the same method.

>> Let's say we want to provide _SUN and _UID.  _SUN is a slot number
>> that may apply to several PCI functions, while _UID probably refers to
>> a single PCI function.  Is it legal to provide two namespace objects,
>> one with _ADR 0x0003ffff and _SUN, and another with _ADR 0x00030000
>> and _UID?
>
> I don't think it is valid to do that.

Is there something in the spec that says you can't?  I can imagine a
BIOS writer doing that, and I don't know how I could convince him that
it's illegal.

It would be really interesting to try some of these scenarios on
Windows with qemu.

Bjorn
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