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

Arnd Bergmann
2014-11-24T10:43:44Z

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@google.com> 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.

Arnd