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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