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:
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