Re: [PATCH v2] irqchip/sifive-plic: enable interrupt if needed before EOI

[Thomas Gleixner]

Nam!

On Wed, Jan 31 2024 at 09:19, Nam Cao wrote:
> RISC-V PLIC cannot "end-of-interrupt" (EOI) disabled interrupts, as
> explained in the description of Interrupt Completion in the PLIC spec:
>
> "The PLIC signals it has completed executing an interrupt handler by
> writing the interrupt ID it received from the claim to the claim/complete
> register. The PLIC does not check whether the completion ID is the same
> as the last claim ID for that target. If the completion ID does not match
> an interrupt source that *is currently enabled* for the target, the
> completion is silently ignored."
>
> Commit 69ea463021be ("irqchip/sifive-plic: Fixup EOI failed when masked")
> ensured that EOI is successful by enabling interrupt first, before EOI.
>
> Commit a1706a1c5062 ("irqchip/sifive-plic: Separate the enable and mask
> operations") removed the interrupt enabling code from the previous
> commit, because it assumes that interrupt should already be enabled at the
> point of EOI. However, this is incorrect: there is a window after a hart
> claiming an interrupt and before irq_desc->lock getting acquired,
> interrupt can be disabled during this window. Thus, EOI can be invoked
> while the interrupt is disabled, effectively nullify this EOI. This
> results in the interrupt never gets asserted again, and the device who
> uses this interrupt appears frozen.

Nice detective work!

> Make sure that interrupt is really enabled before EOI.
>
> Fixes: a1706a1c5062 ("irqchip/sifive-plic: Separate the enable and mask operations")
> Cc: <stable@xxxxxxxxxxxxxxx>
> Signed-off-by: Nam Cao <namcao@xxxxxxxxxxxxx>
> ---
> v2:
>   - add unlikely() for optimization
>   - re-word commit message to make it clearer
>
>  drivers/irqchip/irq-sifive-plic.c | 8 +++++++-
>  1 file changed, 7 insertions(+), 1 deletion(-)
>
> diff --git a/drivers/irqchip/irq-sifive-plic.c b/drivers/irqchip/irq-sifive-plic.c
> index e1484905b7bd..0a233e9d9607 100644
> --- a/drivers/irqchip/irq-sifive-plic.c
> +++ b/drivers/irqchip/irq-sifive-plic.c
> @@ -148,7 +148,13 @@ static void plic_irq_eoi(struct irq_data *d)
>  {
>  	struct plic_handler *handler = this_cpu_ptr(&plic_handlers);
>  
> -	writel(d->hwirq, handler->hart_base + CONTEXT_CLAIM);
> +	if (unlikely(irqd_irq_disabled(d))) {
> +		plic_toggle(handler, d->hwirq, 1);
> +		writel(d->hwirq, handler->hart_base + CONTEXT_CLAIM);
> +		plic_toggle(handler, d->hwirq, 0);

It's unfortunate to have this condition in the hotpath, though it should
be cache hot, easy to predict and compared to the writel() completely in
the noise.

> +	} else {
> +		writel(d->hwirq, handler->hart_base + CONTEXT_CLAIM);
> +	}
>  }

Can the RISCV folks please have a look at this?

Thanks,

        tglx