linux-kernel - Re: [PATCH V2 22/23] pci, acpi: Match PCI config space accessors against platfrom specific quirks.

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Date:	Fri, 8 Jan 2016 15:12:17 +0000
From:	Mark Rutland <mark.rutland@....com>
To:	Mark Salter <msalter@...hat.com>
Cc:	linaro-acpi@...ts.linaro.org, linux-pci@...r.kernel.org,
	will.deacon@....com, okaya@...eaurora.org, wangyijing@...wei.com,
	Lorenzo.Pieralisi@....com, Tomasz Nowicki <tn@...ihalf.com>,
	ddaney@...iumnetworks.com, linux-acpi@...r.kernel.org,
	robert.richter@...iumnetworks.com,
	linux-arm-kernel@...ts.infradead.org, catalin.marinas@....com,
	arnd@...db.de, Stefano.Stabellini@...citrix.com,
	Liviu.Dudau@....com, bhelgaas@...gle.com, tglx@...utronix.de,
	mw@...ihalf.com, jcm@...hat.com, jchandra@...adcom.com,
	rjw@...ysocki.net, linux-kernel@...r.kernel.org,
	hanjun.guo@...aro.org, Suravee.Suthikulpanit@....com,
	jiang.liu@...ux.intel.com
Subject: Re: [PATCH V2 22/23] pci, acpi: Match PCI config space accessors
 against platfrom specific quirks.

On Fri, Jan 08, 2016 at 03:01:37PM +0000, Mark Rutland wrote:
> On Fri, Jan 08, 2016 at 09:16:21AM -0500, Mark Salter wrote:
> > On Wed, 2015-12-16 at 16:16 +0100, Tomasz Nowicki wrote:
> > > Some platforms may not be fully compliant with generic set of PCI config
> > > accessors. For these cases we implement the way to overwrite accessors
> > > set before PCI buses enumeration. Algorithm that overwrite accessors
> > > matches against platform ID (DMI), domain and bus number, hopefully
> > > enough for all cases. All quirks can be defined using:
> > > DECLARE_ACPI_MCFG_FIXUP() and keep self contained.
> > > 
> > > example:
> > > 
> > > static const struct dmi_system_id yyy[] = {
> > >         {
> > >                 .ident = "<Platform ident string>",
> > >                 .callback = <handler>,
> > >                 .matches = {
> > >                         DMI_MATCH(DMI_SYS_VENDOR, "<system vendor>"),
> > >                         DMI_MATCH(DMI_PRODUCT_NAME, "<product name>"),
> > >                         DMI_MATCH(DMI_PRODUCT_VERSION, "product version"),
> > >                 },
> > >         },
> > >         { }
> > > };
> > > 
> > 
> > This seems awkward to me in the case where the quirk is SoC-based and there
> > may be multiple platforms affected. Needing a DECLARE_ACPI_MCFG_FIXUP for
> > each platform using such a SoC (i.e. Mustang and Moonshot) doesn't seem
> > right. In that case, I think it'd be better to check CPUID and possibly
> > some SoC register to cover all platforms affected.
> 
> CPUs get reused across SoCs, so as you've implicitly noted, the CPUID
> alone is insufficient.
> 
> Given that IP blocks get moved around between SoC variants, I don't
> think you can check "some SoC register" based on the CPU ID -- you can
> end up bringing the board down at that point.
> 
> If the CPU ID alone is insufficient to tell you about a component, it
> cannot give you enough information about a component you can use to
> query more information from.
> 
> If your platform requires a quirk, it's always going to be painful (and
> to some extent, rightfulyl so). We should aim for correctness here with
> explicit matching.

Further, if there is going to be an ever-expanding set of platforms
requring quirks, then we need a standard mechanism in ACPI to enable the
platform to tell us explicitly either which specific PCI implementation
is used, or which common quirk is necessary.

Thanks,
Mark.