Re: [PATCH] irqchip: mips-cpu: Remove eoi operation

[Marc Zyngier]

On 2020-01-14 23:30, Paul Burton wrote:
> Hi Jiaxun,
>
> On Mon, Jan 13, 2020 at 06:12:51PM +0800, Jiaxun Yang wrote:
> > The eoi opreation in mips_cpu_irq_controller caused chained_irq_enter
> > falsely consider CPU IP interrupt as a FastEOI type IRQ. So the 
> > interrupt
> > won't be masked during in handler. Which might lead to spurious 
> > interrupt.
> >
> > Thus we simply remove eoi operation for mips_cpu_irq_controller,
> >
> > Signed-off-by: Jiaxun Yang <jiaxun.yang@xxxxxxxxxxx>
> > ---
> >  drivers/irqchip/irq-mips-cpu.c | 1 -
> >  1 file changed, 1 deletion(-)
> >
> > diff --git a/drivers/irqchip/irq-mips-cpu.c 
> > b/drivers/irqchip/irq-mips-cpu.c
> > index 95d4fd8f7a96..0ad7f1f9a58b 100644
> > --- a/drivers/irqchip/irq-mips-cpu.c
> > +++ b/drivers/irqchip/irq-mips-cpu.c
> > @@ -55,7 +55,6 @@ static struct irq_chip mips_cpu_irq_controller = {
> >  	.irq_mask	= mask_mips_irq,
> >  	.irq_mask_ack	= mask_mips_irq,
> >  	.irq_unmask	= unmask_mips_irq,
> > -	.irq_eoi	= unmask_mips_irq,
> >  	.irq_disable	= mask_mips_irq,
> >  	.irq_enable	= unmask_mips_irq,
> >  };
>
> This one scares me; something doesn't seem right. The irq_eoi (nÃe eoi)
> callback was first added way back in commit 1417836e81c0 ("[MIPS] use
> generic_handle_irq, handle_level_irq, handle_percpu_irq"). The commit
> message there states that the motivation was to allow use of
> handle_percpu_irq(), and indeed handle_percpu_irq() does:
>
>     irq_ack() (ie. mask)
>     invoke the handler(s)
>     irq_eoi() (ie. unmask)
>
> By removing the irq_eoi callback I don't see how we'd ever unmask the
> interrupt again..?

To be completely blunt, the fact that unmask and eoi are implemented the
same way is a clear sign that this is a bit broken.

irq_eoi is used if the irqchip tracks the IRQ life-cycle in HW, and it's
not obvious that this is the case. The fact that ack is also mapped to 
mask
just adds to my feeling...

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