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

[Tomasz Nowicki]

W dniu 24.11.2014 o 11:41, Arnd Bergmann pisze:
> On Friday 21 November 2014 11:08:25 Bjorn Helgaas wrote:
> > On Fri, Nov 21, 2014 at 01:24:52PM +0100, Arnd Bergmann wrote:
> > > On Thursday 20 November 2014 21:00:17 Myron Stowe wrote:
> > > > On Thu, Nov 20, 2014 at 3:26 PM, Bjorn Helgaas <bhelgaas@xxxxxxxxxx> wrote:
> >
> > > > > 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.
> >
> > One thing I like better about __weak (when used correctly) is that you have
> > exactly one declaration, and the role of each definition (weak default
> > implementation or strong override) is obvious from looking at it.
>
> Right.
>
> > In your #ifdef example, the extern declaration and the inline definition
> > are never compiled together, so you have to repeat the signature and the
> > compiler doesn't enforce that they match.  So you end up with the extern
> > and the inline in one file, a #define in an arch header file or Kconfig,
> > and an arch definition in a third file.
> >
> > But it's certainly true that everybody knows how #ifdef works, and the fact
> > that __weak on a declaration affects all in-scope definitions is definitely
> > a land mine (multiple weak definitions with no strong one is a disaster).
> >
> > > 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);
> >
> > I definitely agree with this part, but I think it's orthogonal to the
> > __weak question.  In this case, we'd like to support multiple host bridges,
> > each with a different flavor of add_device().  We can't do that at all with
> > either __weak or #ifdef.
>
> What we currently have though is a a __weak definition of add_device,
> which some architectures override, and some of them (ARM in particular)
> by implementing their own abstraction. I suspect for the majority of
> what we currently define as __weak functions, we could use a similar
> approach and kill off the global symbols entirely.

What would be next steps regarding this patch set? I am not sure we have 
reached a consensus on weak vs #ifdef choice.

Regards,
Tomasz

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