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

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Message-ID: <ce658d38-0bf7-446c-b225-d434ba6b1e1c@linux.ibm.com>
Date: Thu, 20 Nov 2025 10:34:31 -0800
From: Farhan Ali <alifm@...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>,
        Niklas Schnelle
 <schnelle@...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 11/20/2025 2:27 AM, 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(-)

Thanks for making the changes!

Reviewed-by: Farhan Ali <alifm@...ux.ibm.com>