Re: [RFC 1/2] arm64: PCI: Allow use arch-specific pci sysdata

On Sat, Mar 20, 2021 at 1:54 PM Marc Zyngier [off-list ref] wrote:

On Fri, 19 Mar 2021 21:12:46 +0000,

quoted

Ugh.  pci_root_bus_fwnode() is another callback to find the
irq_domain.  Only one call, from pci_host_bridge_msi_domain(), which
itself is only called from pci_set_bus_msi_domain().  This feels like
another case where we could simplify things by having the host bridge
driver figure out the irq_domain explicitly when it creates the
pci_host_bridge.  It seems like that's where we have the most
information about how to find the irq_domain.

Urgh. This is a perfect copy paste of the x86 horror, warts and all.
I can't say I'm thrilled (another way to say "Gawd, Noes! Never!").

One thing I am sure of is that I do not want to add more custom
indirection to build the MSI topology. We barely got rid of the
msi_controller structure, and this is the same thing by another
name. Probably worse, actually.

In this case, I don't see the point in going via a fwnode indirection
given that there is no firmware tables the first place.

As for finding the irq domain from the host bridge, that's not doable
in most cases on arm64, as it is pretty likely that the host bridge
knows nothing about MSIs when they are implemented in the GIC (see my
recent msi_controller removal series that has a few patches about
that).

Having an optional callback to host bridges to obtain the MSI domain
may be possible in some cases though (there might be a chicken/egg
problem for some drivers though...).

I would expect that the host bridge driver can find the MSI domain
at probe time and just add a pointer into the pci_host_bridge
structure.

        Arnd