linux-kernel - Re: [Bugfix v3] x86/PCI/ACPI: Fix regression caused by commit 63f1789ec716

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Date:	Fri, 10 Apr 2015 00:37:17 +0200
From:	"Rafael J. Wysocki" <rjw@...ysocki.net>
To:	Bjorn Helgaas <bhelgaas@...gle.com>
Cc:	Jiang Liu <jiang.liu@...ux.intel.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>,
	"x86@...nel.org" <x86@...nel.org>, Len Brown <lenb@...nel.org>,
	Lv Zheng <lv.zheng@...el.com>,
	LKML <linux-kernel@...r.kernel.org>,
	"linux-pci@...r.kernel.org" <linux-pci@...r.kernel.org>,
	"linux-acpi@...r.kernel.org" <linux-acpi@...r.kernel.org>
Subject: Re: [Bugfix v3] x86/PCI/ACPI: Fix regression caused by commit 63f1789ec716

On Thursday, April 09, 2015 05:00:08 PM Bjorn Helgaas wrote:
> On Thu, Apr 9, 2015 at 4:37 PM, Rafael J. Wysocki <rjw@...ysocki.net> wrote:
> > On Thursday, April 09, 2015 10:50:02 AM Jiang Liu wrote:
> >> On 2015/4/9 7:44, Rafael J. Wysocki wrote:
> >> > On Wednesday, April 08, 2015 01:48:46 PM Jiang Liu wrote:
> >> >> On 2015/4/7 8:28, Rafael J. Wysocki wrote:
> >> >>> On Friday, April 03, 2015 10:04:11 PM Bjorn Helgaas wrote:
> >> >>>> Hi Jiang,
> >> >> <snip>
> >> >>>>> Currently acpi_dev_filter_resource_type() is only used by ACPI pci
> >> >>>>> host bridge and IOAPIC driver, so it shouldn't affect other drivers.
> >> >>>>
> >> >>>> We should assume it will eventually be used for all ACPI devices,
> >> >>>> shouldn't we?
> >> >>>
> >> >>> I'm not sure about that, really.  In fact, I'd restrict its use to devices
> >> >>> types that actually can "produce" resources (ie. do not require the resources
> >> >>> to be provided by their ancestors or to be available from a global pool).
> >> >>>
> >> >>> Otherwise we're pretty much guaranteed to get into trouble.
> >> >>>
> >> >>> And all of the above rules need to be documented in the kernel source tree
> >> >>> or people will get confused.
> >> >> Hi Rafael,
> >> >>     How about following comments for acpi_dev_filter_resource_type()?
> >> >> Thanks!
> >> >> Gerry
> >> >> --------------------------------------------------------------------
> >> >> /**
> >> >>  * According to ACPI specifications, Consumer/Producer flag in ACPI resource
> >> >>  * descriptor means:
> >> >>  *      1(CONSUMER): This device consumes this resource
> >> >>  *      0(PRODUCER): This device produces and consumes this resource
> >> >>  * But for ACPI PCI host bridge, it is interpreted in another way:
> >> >
> >> > So first of all, this leads to a question: Why is it interpreted for ACPI PCI
> >> > host bridges differently?
> >> >
> >> > Is it something we've figured out from experience, or is there a standard
> >> > mandating that?
> >> Hi Rafael,
> >>       I think we got this knowledge by experiences. PCI FW spec only
> >> states _CRS reports resources assigned to the host bridge by firmware.
> >> There's no statement about whether the resource is consumed by host
> >> bridge itself or provided to it's child bus/devices. On x86/IA64 side,
> >> the main resource consumed by PCI host bridge is IOPORT 0xCF8-0xCFF,
> >> but not sure about ARM64 side. So how about:
> >
> > This:
> >
> >> PCI Firmware specification states that _CRS reports resources
> >> assigned to the host bridge, but there's no way to tell whether
> >> the resource is consumed by host bridge itself or provided to
> >> its child bus/devices.
> >
> > looks OK to me, but I'd replace the below with something like:
> >
> > "However, experience shows, that in the PCI host bridge case firmware writers
> >  expect the resource to be provided to devices on the bus (below the bridge) for
> >  consumption entirely if its Consumer/Producer flag is clear.  That expectation
> >  is reflected by the code in this routine as follows:".
> 
> What a mess.  The spec is regrettably lacking in Consumer/Producer
> specifics.  But I don't see what's confusing about the descriptors
> that have Consumer/Producer bits.
> 
> QWord, DWord, and Word descriptors don't seem to have a
> Consumer/Producer bit, but they do contain _TRA, so they must be
> intended for bridge windows.  Can they also be used for device
> registers?  I don't know.
> 
> The Extended Address descriptor has a Consumer/Producer bit, and I
> would interpret the spec to mean that if the flag is clear (the device
> produces and consumes this resource), the resource is available on the
> bus below the bridge, i.e., the bridge consumes the resource on its
> upstream side and produces it on its downstream side.

OK, that makes sense for bridges.

> If the bit were
> set (the device only consumes the resource), I would expect that to
> mean the descriptor is for bridge registers like 0xcf8/0xcfc that
> never appear on the downstream side.

That part is clear.  What is not clear is whether or not we can *always*
expect the resources with Consumer/Producer *clear* to be produced on the
downstram side.  That appears to be the case for host bridges if my
understanding of things is correct, but is it the case in general?

> Maybe I'm reading the spec too naively, but this doesn't seem a matter
> of experience.

Well, the specification is not clear enough in that respect, at least as
far as I can say, or maybe it is, but then does firmware always follow that
interpretation?

Rafael

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