Re: [PATCH 1/2] genirq/msi, platform-msi: Adjust return value of msi_domain_prepare_irqs()

[Huacai Chen]

Hi, Thomas,

On Sat, May 27, 2023 at 10:03 PM Thomas Gleixner <tglx@xxxxxxxxxxxxx> wrote:
>
> On Sat, May 27 2023 at 13:46, Huacai Chen wrote:
> > Adjust the return value semanteme of msi_domain_prepare_irqs(), which
> > allows us to modify the input nvec by overriding the msi_domain_ops::
> > msi_prepare(). This is necessary for the later patch.
> >
> > Before:
> > 0 on success, others on error.
> >
> > After:
> > = 0: Success;
> >> 0: The modified nvec;
> > < 0: Error code.
>
> This explains what the patch does, but provides zero justification for
> this nor any analysis why this is correct for the existing use cases.
I checked all msi_prepare() callbacks and none of them return positive
values now, so I think it is correct.

>
> That longsoon MSI domain is a PCI MSI domain. PCI/MSI has already a
> mechanism to return the actual possible number of vectors if the
> underlying space is exhausted.
>
> Why is that not sufficient for your problem at hand?
Hmm, maybe I should make things clearer. We want to do some proactive
throttling here. For example, if we have two NICs, we want both of
them to get 32 msi vectors, not one get 64 vectors, and  the other
fallback to use legacy irq.

Huacai
>
> Thanks,
>
>         tglx