Re: [PATCH 5/8] arm64: irq: add a default handle_irq panic function

From: Marc Zyngier <maz@kernel.org>
To: Mark Rutland <mark.rutland@arm.com>
Cc: linux-arm-kernel@lists.infradead.org,
	linux-kernel@vger.kernel.org, catalin.marinas@arm.com,
	james.morse@arm.com, marcan@marcan.st, tglx@linutronix.de,
	will@kernel.org
Subject: Re: [PATCH 5/8] arm64: irq: add a default handle_irq panic function
Date: Mon, 22 Feb 2021 11:43:13 +0000	[thread overview]
Message-ID: <2e6a9659eabcccb355318ff7214c8d1f@kernel.org> (raw)
In-Reply-To: <20210222112544.GB70951@C02TD0UTHF1T.local>

On 2021-02-22 11:25, Mark Rutland wrote:
> On Mon, Feb 22, 2021 at 10:48:11AM +0000, Marc Zyngier wrote:
>> On 2021-02-22 09:59, Mark Rutland wrote:
>> > On Fri, Feb 19, 2021 at 11:39:01AM +0000, Mark Rutland wrote:
>> > > +void (*handle_arch_irq)(struct pt_regs *) __ro_after_init =
>> > > default_handle_irq;
>> > >
>> > >  int __init set_handle_irq(void (*handle_irq)(struct pt_regs *))
>> > >  {
>> > > -	if (handle_arch_irq)
>> > > +	if (handle_arch_irq != default_handle_irq)
>> > >  		return -EBUSY;
>> > >
>> > >  	handle_arch_irq = handle_irq;
>> > > @@ -87,7 +92,7 @@ void __init init_IRQ(void)
>> > >  	init_irq_stacks();
>> > >  	init_irq_scs();
>> > >  	irqchip_init();
>> > > -	if (!handle_arch_irq)
>> > > +	if (handle_arch_irq == default_handle_irq)
>> > >  		panic("No interrupt controller found.");
>> 
>> It also seems odd to have both default_handle_irq() that panics,
>> and init_IRQ that panics as well. Not a big deal, but maybe
>> we should just drop this altogether and get the firework on the
>> first interrupt.
> 
> My gut feeling was that both were useful, and served slightly different
> cases:
> 
> * The panic in default_handle_irq() helps if we unexpectedly unmask IRQ
>   too early. This is mostly a nicety over the current behaviour of
>   branching to NULL in this case.
> 
> * The panic in init_IRQ() gives us a consistent point at which we can
>   note the absence of a root IRQ controller even if all IRQs are
>   quiescent. This is a bit nicer to debug than seeing a load of driver
>   probes fail their request_irq() or whatever.
> 
> ... so I'd err on the side of keeping both, but if you think otherwise
> I'm happy to change this.

As I said, it's not a big deal. I doubt that we'll see 
default_handle_irq()
exploding in practice. But the real nit here is the difference of 
treatment
between IRQ and FIQ. *IF* we ever get a system that only signals its
interrupt as FIQ (and I don't see why we'd forbid that), then we would

To be clear, I don't think we should care too much either way, and I'm
fine with the code as is.

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