Re: [PATCH v6 3/6] irqchip: RISC-V per-HART local interrupt controller driver

[Anup Patel]

On Sun, May 31, 2020 at 4:23 PM Marc Zyngier <maz@xxxxxxxxxx> wrote:
>
> On 2020-05-31 11:06, Anup Patel wrote:
> > On Sun, May 31, 2020 at 3:03 PM Marc Zyngier <maz@xxxxxxxxxx> wrote:
> >>
> >> On 2020-05-31 06:36, Anup Patel wrote:
> >> > On Sat, May 30, 2020 at 5:31 PM Marc Zyngier <maz@xxxxxxxxxx> wrote:
> >>
> >> [...]
> >>
> >> >> >       plic_set_threshold(handler, PLIC_DISABLE_THRESHOLD);
> >> >>
> >> >> Why do you need to both disable the interrupt *and* change the
> >> >> priority
> >> >> threshold? It seems to be that one of them should be enough, but my
> >> >> kno9wledge of the PLIC is limited. In any case, this would deserve a
> >> >> comment.
> >> >
> >> > Okay, I will test and remove "disable the interrupt" part from
> >> > plic_dying_cpu().
> >>
> >> Be careful, as interrupt enabling/disabling is refcounted in order
> >> to allow nesting. If you only enable on CPU_ON and not disable
> >> on CPU_OFF, you will end-up with a depth that only increases,
> >> up to the point where you hit the roof (it will take a while though).
> >>
> >> I would keep the enable/disable as is, and drop the priority
> >> setting from the CPU_OFF path.
> >
> > The PLIC threshold is like GICv2 CPU interface enable/disable.
>
> Looking at the documentation[1], that's not really a correct analogy:
>
> - The PLIC is far removed from the CPU, and is more akin a GICv3
>    distributor. The INTC itself is more like a GICv3 redistributor,
>    as it deals with local interrupts only. I don't see anything
>    in the RISC-V architecture that actually behaves like a GIC
>    CPU interface (not necessarily a bad thing...).
>
> - The threshold register is not an ON/OFF, but a priority mask,
>    similar to the GIC PMR (except that the PMR lives in the CPU
>    interface and affects all interrupts targetting this CPU while
>    this only seem to affect PLIC interrupts and not the INTC interrupts).
>    You may be using it as an ON/OFF for now as you don't support
>    multiple priorities yet, but that's just a SW thing.

Yes, your analogy is correct.

BTW, PLIC does not handle MSI and does not have virtualization support
pass-through interrupts. We will most likely see a new RISC-V interrupt
controller spec for these capabilities.

Also, the PLIC spec is now owned by RISC-V foundation (not SiFive) so
we will have to rename the driver to "irq-riscv-plic" and will have a new
generic compatible string "riscv,plic-1.0.0". One of us (me or Palmer) will
send separate patches for this renaming. I hope you will be fine with this??
(Refer, https://github.com/riscv/riscv-plic-spec)

>
> > Based on your comment, we should only program the PLIC threshold
> > in CPU_ON and don't touch the PLIC threshold in CPU_OFF. Right??
>
> This seems like the correct thing to do.

Sure, I will update.

>
>          M.
>
> [1]
> https://sifive.cdn.prismic.io/sifive%2Fdc4980ff-17db-448b-b521-4c7ab26b7488_sifive+u54-mc+manual+v19.08.pdf
> --
> Jazz is not dead. It just smells funny...

Regards,
Anup