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

[Marc Zyngier]

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...