linux-kernel - Re: [PATCH 6/6] pci, acpi: Share ACPI PCI config space accessors.

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Date:	Fri, 21 Nov 2014 13:24:52 +0100
From:	Arnd Bergmann <arnd@...db.de>
To:	linux-arm-kernel@...ts.infradead.org
Cc:	Myron Stowe <myron.stowe@...il.com>,
	Bjorn Helgaas <bhelgaas@...gle.com>, rjw@...ysocki.net,
	lorenzo.pieralisi@....com, linaro-acpi@...ts.linaro.org,
	linux-pci <linux-pci@...r.kernel.org>, catalin.marinas@....com,
	x86 <x86@...nel.org>, will.deacon@....com,
	LKML <linux-kernel@...r.kernel.org>,
	Tomasz Nowicki <tomasz.nowicki@...aro.org>,
	"linux-acpi@...r.kernel.org" <linux-acpi@...r.kernel.org>,
	Ingo Molnar <mingo@...hat.com>, hanjun.guo@...aro.org,
	"H. Peter Anvin" <hpa@...or.com>, wangyijing@...wei.com,
	Liviu.Dudau@....com, Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH 6/6] pci, acpi: Share ACPI PCI config space accessors.

On Thursday 20 November 2014 21:00:17 Myron Stowe wrote:
> On Thu, Nov 20, 2014 at 3:26 PM, Bjorn Helgaas <bhelgaas@...gle.com> wrote:
> > On Wed, Nov 19, 2014 at 05:19:20PM +0100, Arnd Bergmann wrote:
> >> On Wednesday 19 November 2014 17:04:51 Tomasz Nowicki wrote:
> >> > +/*
> >> > + * raw_pci_read/write - ACPI PCI config space accessors.
> >> > + *
> >> > + * ACPI spec defines MMCFG as the way we can access PCI config space,
> >> > + * so let MMCFG be default (__weak).
> >> > + *
> >> > + * If platform needs more fancy stuff, should provides its own implementation.
> >> > + */
> >> > +int __weak raw_pci_read(unsigned int domain, unsigned int bus,
> >> > +                       unsigned int devfn, int reg, int len, u32 *val)
> >> > +{
> >> > +       return pci_mmcfg_read(domain, bus, devfn, reg, len, val);
> >> > +}
> >> > +
> >> > +int __weak raw_pci_write(unsigned int domain, unsigned int bus,
> >> > +                        unsigned int devfn, int reg, int len, u32 val)
> >> > +{
> >> > +       return pci_mmcfg_write(domain, bus, devfn, reg, len, val);
> >> > +}
> >> > +
> >> >
> >>
> >> I think it would be better to avoid __weak functions here, as they tend
> >> to be hard to follow when trying to understand the code.
> >
> > That's interesting.  I would have said exactly the opposite -- I think the
> > extra Kconfiggery is harder to follow than weak/strong functions 
> >
> > But consistency is better than my personal opinion.  Is there a consensus
> > that we should use the Kconfig strategy instead of __weak?
> 
> I too find weak/strong functions easier to follow than "Kconfiggery" (nice term
> invention there).

I don't think there is a universal consensus, but the majority of
maintainers seems to avoid them for the same reasons that I think
__weak is problematic.

We have some uses of __weak in the core kernel, but there is
basically none in drivers outside of PCI, and the most common
uses are all providing an empty __weak function that can be
overridden with a function that actually does something, unlike
the code above.

My pragmatic approach so far has been to advocate __weak for
drivers/pci patches but discourage it elsewhere when I review
patches, in order to maintain consistency. I also think it
would be nice to change the way that PCI handles architecture
specific overrides in the process of unifying the host bridge
handling.

I wouldn't use Kconfig symbols in most cases though. My preferred
choice would be to turn a lot of the __weak symbols into function
pointers within a per-hostbridge structure. As an example, we could
replace pcibios_add_device() with a pointer in pci_host_bridge->ops
that gets set by all the architectures and host drivers that currently
override it, and replace the one caller with

	if (pci_host_bridge->ops->add_device)
		pci_host_bridge->ops->add_device(dev);

	Arnd
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