Re: [PATCH v3 2/2] s390/pci: Migrate s390 IRQ logic to IRQ domain API

[Farhan Ali <alifm@linux.ibm.com>]

On 11/18/2025 8:13 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 zpci which is used by
> all PCI devices.

I think this statement would need to be updated with the new approach of 
MSI parent domain per zpci 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
>    callend 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@linux.ibm.com>
> ---
>   arch/s390/Kconfig           |   1 +
>   arch/s390/include/asm/pci.h |   4 +
>   arch/s390/pci/pci_bus.c     |  11 ++
>   arch/s390/pci/pci_irq.c     | 334 +++++++++++++++++++++++++++-----------------
>   4 files changed, 225 insertions(+), 125 deletions(-)
>
> diff --git a/arch/s390/Kconfig b/arch/s390/Kconfig
> index df22b10d91415e1ed183cc8add9ad0ac4293c50e..48cd6a12bd04dfe4dd61ecc79d3401ba685c51bb 100644
> --- a/arch/s390/Kconfig
> +++ b/arch/s390/Kconfig
> @@ -251,6 +251,7 @@ config S390
>   	select HOTPLUG_SMT
>   	select IOMMU_HELPER		if PCI
>   	select IOMMU_SUPPORT		if PCI
> +	select IRQ_MSI_LIB if PCI
>   	select KASAN_VMALLOC if KASAN
>   	select LOCK_MM_AND_FIND_VMA
>   	select MMU_GATHER_MERGE_VMAS
> diff --git a/arch/s390/include/asm/pci.h b/arch/s390/include/asm/pci.h
> index a32f465ecf73a5cc3408a312d94ec888d62848cc..60abc84cf14fe6fb1ee149df688eea94f0983ed0 100644
> --- a/arch/s390/include/asm/pci.h
> +++ b/arch/s390/include/asm/pci.h
> @@ -5,6 +5,7 @@
>   #include <linux/pci.h>
>   #include <linux/mutex.h>
>   #include <linux/iommu.h>
> +#include <linux/irqdomain.h>
>   #include <linux/pci_hotplug.h>
>   #include <asm/pci_clp.h>
>   #include <asm/pci_debug.h>
> @@ -109,6 +110,7 @@ struct zpci_bus {
>   	struct list_head	resources;
>   	struct list_head	bus_next;
>   	struct resource		bus_resource;
> +	struct irq_domain       *msi_parent_domain;
>   	int			topo;		/* TID if topo_is_tid, PCHID otherwise */
>   	int			domain_nr;
>   	u8			multifunction	: 1;
> @@ -310,6 +312,8 @@ int zpci_dma_exit_device(struct zpci_dev *zdev);
>   /* IRQ */
>   int __init zpci_irq_init(void);
>   void __init zpci_irq_exit(void);
> +int zpci_create_parent_msi_domain(struct zpci_bus *zbus);
> +void zpci_remove_parent_msi_domain(struct zpci_bus *zbus);
>   
>   /* FMB */
>   int zpci_fmb_enable_device(struct zpci_dev *);
> diff --git a/arch/s390/pci/pci_bus.c b/arch/s390/pci/pci_bus.c
> index be8c697fea0cc755cfdb4fb0a9e3b95183bec0dc..4849420d4f72c886edbe9c5e32cf0b1513d0b555 100644
> --- a/arch/s390/pci/pci_bus.c
> +++ b/arch/s390/pci/pci_bus.c
> @@ -15,6 +15,7 @@
>   #include <linux/err.h>
>   #include <linux/delay.h>
>   #include <linux/seq_file.h>
> +#include <linux/irqdomain.h>
>   #include <linux/jump_label.h>
>   #include <linux/pci.h>
>   #include <linux/printk.h>
> @@ -190,6 +191,7 @@ static int zpci_bus_create_pci_bus(struct zpci_bus *zbus, struct zpci_dev *fr, s
>   {
>   	struct pci_bus *bus;
>   	int domain;
> +	int rc;
>   
>   	domain = zpci_alloc_domain((u16)fr->uid);
>   	if (domain < 0)
> @@ -199,6 +201,12 @@ static int zpci_bus_create_pci_bus(struct zpci_bus *zbus, struct zpci_dev *fr, s
>   	zbus->multifunction = zpci_bus_is_multifunction_root(fr);
>   	zbus->max_bus_speed = fr->max_bus_speed;
>   
> +	rc = zpci_create_parent_msi_domain(zbus);
> +	if (rc) {
> +		zpci_free_domain(zbus->domain_nr);
> +		return -EFAULT;
> +	}
> +
>   	/*
>   	 * Note that the zbus->resources are taken over and zbus->resources
>   	 * is empty after a successful call
> @@ -206,10 +214,12 @@ static int zpci_bus_create_pci_bus(struct zpci_bus *zbus, struct zpci_dev *fr, s
>   	bus = pci_create_root_bus(NULL, ZPCI_BUS_NR, ops, zbus, &zbus->resources);
>   	if (!bus) {
>   		zpci_free_domain(zbus->domain_nr);
> +		zpci_remove_parent_msi_domain(zbus);
>   		return -EFAULT;
>   	}
>   

Nit: I wonder if it would be more readable if we used goto statements 
here and above.

<..snip..>

> +
> +/*
> + * Encode the hwirq number for the parent domain. The encoding must be unique
> + * for each IRQ of each device in the parent domain, so it uses the devfn to
> + * identify the device and the msi_index to identify the IRQ within that device.
> + */
> +static inline u32 zpci_encode_hwirq(u8 devfn, u16 msi_index)
> +{
> +	return (devfn << 16) | msi_index;

I see we have changed the encoding from using the fid to using the 
devfn, any reason for changing it? AFAIU the fid is unique for every 
function within the system.

Also thinking it out loud, is it this going to be unique if we have 
multiple IRQ (if nr_irqs in zpci_msi_domain_alloc() is > 1) per MSI 
descriptor, unless I missed something?