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. Arnd -- To unsubscribe from this list: send the line "unsubscribe linux-acpi" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
