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

From: Marc Zyngier <maz@kernel.org>
To: Anup Patel <anup@brainfault.org>
Cc: Anup Patel <anup.patel@wdc.com>,
	Palmer Dabbelt <palmer@dabbelt.com>,
	Paul Walmsley <paul.walmsley@sifive.com>,
	Albert Ou <aou@eecs.berkeley.edu>,
	Daniel Lezcano <daniel.lezcano@linaro.org>,
	Thomas Gleixner <tglx@linutronix.de>,
	Jason Cooper <jason@lakedaemon.net>,
	Atish Patra <atish.patra@wdc.com>,
	Alistair Francis <Alistair.Francis@wdc.com>,
	linux-riscv <linux-riscv@lists.infradead.org>,
	"linux-kernel@vger.kernel.org List"
	<linux-kernel@vger.kernel.org>,
	Palmer Dabbelt <palmerdabbelt@google.com>
Subject: Re: [PATCH v6 3/6] irqchip: RISC-V per-HART local interrupt controller driver
Date: Sun, 31 May 2020 10:33:39 +0100	[thread overview]
Message-ID: <a5f1346544aec6e6da69836b7a6e0a6e@kernel.org> (raw)
In-Reply-To: <CAAhSdy3cnZwnjpqWkixmZ5-fi=GK1cSUsjah=P3Yp5hjv382hg@mail.gmail.com>

On 2020-05-31 06:36, Anup Patel wrote:
> On Sat, May 30, 2020 at 5:31 PM Marc Zyngier <maz@kernel.org> 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.

>> >       return 0;
>> > @@ -260,7 +266,11 @@ static int plic_starting_cpu(unsigned int cpu)
>> >  {
>> >       struct plic_handler *handler = this_cpu_ptr(&plic_handlers);
>> >
>> > -     csr_set(CSR_IE, IE_EIE);
>> > +     if (plic_parent_irq)
>> > +             enable_percpu_irq(plic_parent_irq,
>> > +                               irq_get_trigger_type(plic_parent_irq));
>> > +     else
>> > +             pr_warn("cpu%d: parent irq not available\n");
>> 
>> What does it mean to carry on if the interrupt cannot be signaled?
>> Shouldn't you error out instead, and leave the CPU dead?
> 
> The CPU is not dead if we cannot enable RISC-V INTC external
> interrupt because the Timer and IPIs interrupts are always through
> RISC-V INTC. The PLIC external interrupt not present for a CPU
> only means that that CPU cannot receive peripherial interrupts.
> 
> On a sane RISC-V system, if PLIC is present then all CPUs should
> be able to get RISC-V INTC external interrupt. Base on this rationale,
> I have put a warning for plic_parent_irq == 0.

Fair enough.

         M.
-- 
Jazz is not dead. It just smells funny...