linux-kernel - Re: [PATCH 5/8] arm64: irq: add a default handle

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Date:   Mon, 22 Feb 2021 11:25:44 +0000
From:   Mark Rutland <mark.rutland@....com>
To:     Marc Zyngier <maz@...nel.org>
Cc:     linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        catalin.marinas@....com, james.morse@....com, marcan@...can.st,
        tglx@...utronix.de, will@...nel.org
Subject: Re: [PATCH 5/8] arm64: irq: add a default handle_irq panic function

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.

Thanks,
Mark.