linux-kernel - Re: [PATCH v4 2/2] s390/pci: Migrate s390 IRQ logic to IRQ domain API

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Message-ID: <99006de3edd3a30d312a837b1c18a136ee3919f2.camel@linux.ibm.com>
Date: Thu, 20 Nov 2025 21:48:22 +0100
From: Niklas Schnelle <schnelle@...ux.ibm.com>
To: Tobias Schumacher <ts@...ux.ibm.com>, Heiko Carstens
 <hca@...ux.ibm.com>,
        Vasily Gorbik <gor@...ux.ibm.com>,
        Alexander Gordeev
 <agordeev@...ux.ibm.com>,
        Christian Borntraeger	
 <borntraeger@...ux.ibm.com>,
        Sven Schnelle <svens@...ux.ibm.com>,
        Gerald
 Schaefer <gerald.schaefer@...ux.ibm.com>,
        Gerd Bayer
 <gbayer@...ux.ibm.com>, Halil Pasic	 <pasic@...ux.ibm.com>,
        Matthew Rosato
 <mjrosato@...ux.ibm.com>,
        Thomas Gleixner	 <tglx@...utronix.de>
Cc: linux-kernel@...r.kernel.org, linux-s390@...r.kernel.org
Subject: Re: [PATCH v4 2/2] s390/pci: Migrate s390 IRQ logic to IRQ domain
 API

On Thu, 2025-11-20 at 11:27 +0100, Tobias Schumacher wrote:
> s390 is one of the last architectures using the legacy API for setup and
> teardown of PCI MSI IRQs. Migrate the s390 IRQ allocation and teardown
> to the MSI parent domain API. For details, see:
> 
> https://lore.kernel.org/lkml/20221111120501.026511281@linutronix.de
> 
> In detail, create an MSI parent domain for each PCI domain. When a PCI
> device sets up MSI or MSI-X IRQs, the library creates a per-device IRQ
> domain for this device, which is used by the device for allocating and
> freeing IRQs.
> 
> The per-device domain delegates this allocation and freeing to the
> parent-domain. In the end, the corresponding callbacks of the parent
> domain are responsible for allocating and freeing the IRQs.
> 
> The allocation is split into two parts:
> - zpci_msi_prepare() is called once for each device and allocates the
>   required resources. On s390, each PCI function has its own airq
>   vector and a summary bit, which must be configured once per function.
>   This is done in prepare().
> - zpci_msi_alloc() can be called multiple times for allocating one or
>   more MSI/MSI-X IRQs. This creates a mapping between the virtual IRQ
>   number in the kernel and the hardware IRQ number.
> 
> Freeing is split into two counterparts:
> - zpci_msi_free() reverts the effects of zpci_msi_alloc() and
> - zpci_msi_teardown() reverts the effects of zpci_msi_prepare(). This is
>   called once when all IRQs are freed before a device is removed.
> 
> Since the parent domain in the end allocates the IRQs, the hwirq
> encoding must be unambiguous for all IRQs of all devices. This is
> achieved by encoding the hwirq using the PCI function id and the MSI
> index.
> 
> Signed-off-by: Tobias Schumacher <ts@...ux.ibm.com>
> ---
>  arch/s390/Kconfig           |   1 +
>  arch/s390/include/asm/pci.h |   4 +
>  arch/s390/pci/pci_bus.c     |  21 ++-
>  arch/s390/pci/pci_irq.c     | 335 +++++++++++++++++++++++++++-----------------
>  4 files changed, 229 insertions(+), 132 deletions(-)
--- snip ---
> +static void zpci_msi_domain_free(struct irq_domain *domain, unsigned int virq,
> +				 unsigned int nr_irqs)
> +{
> +	irq_hw_number_t hwirq;
> +	struct irq_data *d;
> +	u16 msi_index;
> +	int i;
> +
> +	for (i = 0; i < nr_irqs; i++) {
> +		d = irq_domain_get_irq_data(domain, virq + i);
> +		hwirq = d->hwirq;
> +		msi_index = zpci_decode_hwirq_msi_index(hwirq);

The msi_index isn't used or needed so the above two lines can be
removed together with their respective variables.

> +		irq_domain_reset_irq_data(d);
> +	}
> +}
> +
--- snip ---

Apart from the above trivial superfluous code this looks even better
than v3 now. Makes the hwirq encoding smaller and more dense which I
think is also nice and as previously discussed the IRQ domain per
(virtual) PCI domain seems like a good fit. Thanks again for the great
work.

Reviewed-by: Niklas Schnelle <schnelle@...ux.ibm.com>