Re: [PATCH] PCI: dwc: Chain the set IRQ affinity request back to the parent

[Thomas Gleixner <tglx@linutronix.de>]

On Fri, Mar 07 2025 at 19:10, Tsai Sung-Fu wrote:
> Thanks for your detailed explanation and feedback, I am a bit confused about the
> #4 you mentioned here ->
>
>>     4) Affinity of the demultiplex interrupt
>
> Are you saying there is a chance to queue this demultiplexing IRQ event
> to the current running CPU ?

The demultiplexing interrupt (currently a chained handler, which is
hidden from /proc/irq/) stays at the affinity which the kernel decided
to assign to it at startup. That means it can't be steered to a
particual CPU and nobody knows to which CPU it is affine. You can only
guess it from /proc/interrupts by observing where the associated
demultiplex interrupts are affine to.

So ideally you want to be able to affine the demultiplexing interrupt
too. That requires to switch it to a regular interrupt for
simplicity. We could expose those hidden chained handlers affinity too,
but that needs some surgery vs. locking etc.

> And that's really an approach worth to try, I will work on it.

I've played with this on top of variant of Marc's changes to use MSI
parent interrupts for such controllers too:

  https://lore.kernel.org/all/20241204124549.607054-1-maz@kernel.org/

A completely untested and largely uncompiled preview is here:

     https://tglx.de/~tglx/patches.tar

The MSI parent parts are in flux. Marc will look at them in the next
weeks, but I picked them up because it simplifies the whole business a
lot. If you find bugs in that series, you can keep them :)

Thanks,

        tglx