Re: [PATCH v2 2/2] s390/pci: Migrate s390 IRQ logic to IRQ domain API

[Niklas Schnelle <schnelle@linux.ibm.com>]

On Tue, 2025-11-18 at 08:14 +0100, Tobias Schumacher wrote:
> Am 2025-11-17 23:46, schrieb Farhan Ali:
> > On 11/17/2025 12:59 AM, Tobias Schumacher wrote:
> --- snip ---
> 
> > > diff --git a/arch/s390/pci/pci_bus.c b/arch/s390/pci/pci_bus.c
> > > index 
> > > be8c697fea0cc755cfdb4fb0a9e3b95183bec0dc..2be33cfb8970409db4fcb75ea73543f49b583a5c 
> > > 100644
> > > --- a/arch/s390/pci/pci_bus.c
> > > +++ b/arch/s390/pci/pci_bus.c
> > > @@ -210,6 +210,7 @@ static int zpci_bus_create_pci_bus(struct zpci_bus 
> > > *zbus, struct zpci_dev *fr, s
> > >   	}
> > >     	zbus->bus = bus;
> > > +	zpci_set_msi_parent_domain(zbus);
> > 
> > Why are we setting the zpci_set_msi_parent_domain per root device
> > instead of per zpci device?
> 
> On other architectures the parent domain is set once for the root bus. 
> During bus scanning, pci_device_add() then sets the domain in the device 
> by calling pci_set_msi_domain(). By adding the parent domain to the root 
> busses, we are using the same mechanisms and are as close as possible to 
> other architectures.
> 

I didn't think about this enough, but reading your argument I wonder if
that points to having a parent domain per PCI domain vs having a single
global one like you have at the moment. And since we're limited to one
struct zpci_bus per PCI domain in the current code with struct
zpci_bus::bus being a root bus and the zpci_bus also carrying the
domain number, I think that would more closely match the parent domain
per PCI domain model.

As for Farhan's comment as I understand it the common code still
creates child MSI domains for the individual zpci devices. So both a
global parent domain as in this version or a per zpci bus domain will
work we just want to keep things as similar to non virtual PCI
topologies as possible I think.

Thanks,
Niklas