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

[Bjorn Helgaas <helgaas@kernel.org>]

[+cc Rob,
cover https://lore.kernel.org/r/20200826111628.794979401@linutronix.de/
this  https://lore.kernel.org/r/20200826112333.992429909@linutronix.de/]

On Wed, Aug 26, 2020 at 01:17:02PM +0200, Thomas Gleixner wrote:
> From: Thomas Gleixner <tglx@linutronix.de>
> 
> The arch_.*_msi_irq[s] fallbacks are compiled in whether an architecture
> requires them or not. Architectures which are fully utilizing hierarchical
> irq domains should never call into that code.
> 
> It's not only architectures which depend on that by implementing one or
> more of the weak functions, there is also a bunch of drivers which relies
> on the weak functions which invoke msi_controller::setup_irq[s] and
> msi_controller::teardown_irq.
> 
> 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.

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?

And I can't figure out what's special about tegra, rcar, and xilinx
that makes them need it as well.  Is there something I could grep for
to identify them?  Is there a way to convert them so they don't need
it?

> --- a/include/linux/msi.h
> +++ b/include/linux/msi.h
> @@ -193,17 +193,38 @@ void pci_msi_mask_irq(struct irq_data *d
>  void pci_msi_unmask_irq(struct irq_data *data);
>  
>  /*
> - * The arch hooks to setup up msi irqs. Those functions are
> - * implemented as weak symbols so that they /can/ be overriden by
> - * architecture specific code if needed.
> + * The arch hooks to setup up msi irqs. Default functions are implemented

s/msi/MSI/ to match the one below.

> + * as weak symbols so that they /can/ be overriden by architecture specific
> + * code if needed. These hooks must be enabled by the architecture or by
> + * drivers which depend on them via msi_controller based MSI handling.