linux-kernel - Re: [PATCH v7 2/2] s390/pci: Migrate s390 IRQ logic to IRQ domain API

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Message-ID: <47a6dde6a2cd353be7c33a25fc2b6032e9256c9d.camel@linux.ibm.com>
Date: Fri, 28 Nov 2025 10:47:43 +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 v7 2/2] s390/pci: Migrate s390 IRQ logic to IRQ domain
 API

On Thu, 2025-11-27 at 16:07 +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 devfn and the MSI index.
> 
> Signed-off-by: Tobias Schumacher <ts@...ux.ibm.com>
> ---
>  arch/s390/Kconfig           |   1 +
>  arch/s390/include/asm/pci.h |   5 +
>  arch/s390/pci/pci.c         |   6 +
>  arch/s390/pci/pci_bus.c     |  18 ++-
>  arch/s390/pci/pci_irq.c     | 310 ++++++++++++++++++++++++++++----------------
>  5 files changed, 224 insertions(+), 116 deletions(-)
> 
--- snip ---

> +
> +static inline u16 zpci_decode_hwirq_msi_index(u32 hwirq)

I think the parameter's type should by irq_hw_number_t here. It doesn't
matter for correctness since we're only using 32 bits now and the cast
just cuts off the upper 32 bits but I'd like to preserve the type until
you explicitly mask off the bits we don't use.

I also considered making zpci_encode_hwirq()'s return type
irq_hw_number_t but I think it's not needed. This is because there
we're still in the process of encoding the hwirq and want to emphasize
that our encoding output uses 32 bits.

> +{
> +	return hwirq & 0xffff;
> +}
>  

The changes versus v6 look good to me and I agree that putting the
zpci_set_irq() in zpci_reenable_device() looks like the cleanest fix
for the recovery issue. Also good catch on the msg->data assignment. So
feel free to (re-)add my R-b as per below.

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