Re: [PATCH RFC 0/3] genirq: Allow drivers to respect userspace IRQ affinities

[Florian Bezdeka]

On Thu, 2026-08-20 at 02:10 +0200, Andrew Lunn wrote:
> The problem with using stmmac as an example is that it is very old. It
> was added to Linux in 2009. I guess most of the SoCs at that time were
> single or dual core. CPU affinity was not something developers thought
> about for that class of SoC.

Agree, stmmac might not be the best example, but this is the "driver
under test" here and it shows a pattern that is implemented in ~35
network drivers as well.

Users of irq_set_affinity_hint() in 7.2, see [1], but there is lot more
that needs a revisit, like irq_set_affinity_and_hint().

> 
> Over time the number of cores has gone up and stmmac has got faster
> link speeds. But is the architecture correct? Is the driver following
> best practices?

This are exactly the questions that I would like to discuss. Seems we're
getting closer.

> 
> I would suggest you look at more modern MAC drivers and see how they
> do CPU affinity, etc. Is there anything which can be learned from them
> and implemented in stmmac?

I'm still not sure if there is "the pattern" how modern drivers (btw:
which drivers do you consider "modern"?) implement that.

What I can tell: Most calls to irq_set_affinity_hint() (and friends) are
a real problem on RT enabled systems as they simply overwrite any
userspace settings.

Florian

[1] https://elixir.bootlin.com/linux/v7.2/A/ident/irq_set_affinity_hint