Re: [patch V2 34/46] PCI/MSI: Make arch_.*_msi_irq[s] fallbacks selectable

[Thomas Gleixner <tglx@xxxxxxxxxxxxx>]

On Thu, Aug 27 2020 at 13:20, Bjorn Helgaas wrote:
> On Wed, Aug 26, 2020 at 01:17:02PM +0200, Thomas Gleixner wrote:
>> Make the architectures and drivers which rely on them select them in Kconfig
>> and if not selected replace them by stub functions which emit a warning and
>> fail the PCI/MSI interrupt allocation.
>
> Sorry, I really don't understand this, so these are probably stupid
> questions.
>
> If CONFIG_PCI_MSI_ARCH_FALLBACKS is defined, we will supply
> implementations of:
>
>   arch_setup_msi_irq
>   arch_teardown_msi_irq
>   arch_setup_msi_irqs
>   arch_teardown_msi_irqs
>   default_teardown_msi_irqs    # non-weak
>
> You select CONFIG_PCI_MSI_ARCH_FALLBACKS for ia64, mips, powerpc,
> s390, sparc, and x86.  I see that all of those arches implement at
> least one of the functions above.  But x86 doesn't and I can't figure
> out why it needs to select CONFIG_PCI_MSI_ARCH_FALLBACKS.

X86 still has them at that point in the series and the next patch
removes them. I wanted to have the warnings in place before doing so.

> I assume there's a way to convert these arches to hierarchical irq
> domains so they wouldn't need this at all?  Is there a sample
> conversion to look at?

For a quick and dirty step it's pretty much the wrapper I used for XEN
and then make sure that the msi_domain pointer is populated is
pci_device::device.

> And I can't figure out what's special about tegra, rcar, and xilinx
> that makes them need it as well.

Those are old drivers from the time where ARM did not use hierarchical
irq domains and nobody cared to fix them up.

> Is there something I could grep for
> to identify them?

git grep arch_setup_msi_irq
git grep arch_teardown_msi_irq

> Is there a way to convert them so they don't need it?

Sure, it just needs some work and probably hardware to test.

Thanks,

        tglx