Re: [PATCH v2 5/5] irqchip/gic-v3-its: Use allocation size from the prepare call

[Thomas Gleixner <tglx@linutronix.de>]

On Mon, May 19 2025 at 15:28, Marc Zyngier wrote:
> On Mon, 19 May 2025 13:16:58 +0100,
> Thomas Gleixner <tglx@linutronix.de> wrote:
>> > Maybe. It is rather unclear to me what this "dynamic allocation"
>> > actually provides in terms of guarantees to the endpoint driver.
>> 
>> It allows the driver to avoid allocating a gazillion of interrupts
>> upfront during initialization. Instead it can allocate them on demand,
>> when e.g. a queue is initialized. Of course that means that such an
>> allocation can fail, but so can request_irq() and other things. I'm not
>> sure what you mean with guarantees here.
>
> What is the endpoint driver allowed to expect in terms of continuity
> of allocation in the IRQ space? If this is solely limited to MSI-X,
> then the answer probably is "none whatsoever", and the driver should
> only manage the MSI descriptor index.
>
> Can any other MSI-like mechanism end-up with multiple allocations and
> require extra alignment/contiguity guarantees in the hwirq space, more
> or less similar to what MultiMSI requires? Because that'd be much
> harder to provide.

It's only relevant to MSI-X today. That's the only facility, which
actually provides an interface _if_ the underlying parent supports it.

static const struct msi_domain_template pci_msix_template = {
       ....
	.info = {
		.flags			= MSI_COMMON_FLAGS | MSI_FLAG_PCI_MSIX |
					  MSI_FLAG_PCI_MSIX_ALLOC_DYN,
		.bus_token		= DOMAIN_BUS_PCI_DEVICE_MSIX,
	},
};

That's the device domain template, which requests the functionality and
the core then checks whether the parent domain supports it. If so the
functionality is enabled.

Thanks,

        tglx