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

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Date:   Wed, 15 Jan 2020 15:22:23 +0000
From:   Marc Zyngier <maz@...nel.org>
To:     Jiaxun Yang <jiaxun.yang@...goat.com>
Cc:     Paul Burton <paulburton@...nel.org>, linux-mips@...r.kernel.org,
        chenhc@...ote.com, paul.burton@...s.com, tglx@...utronix.de,
        jason@...edaemon.net, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] irqchip: mips-cpu: Remove eoi operation

On 2020-01-15 14:23, Jiaxun Yang wrote:
> 于 2020年1月15日 GMT+08:00 下午9:40:31, Marc Zyngier <maz@...nel.org> 写到:
>> 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@...goat.com>
>>>> ---
>>>>  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 (née
>> 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
> 
> It's just a kind of hack to workaround the fact that our current
> percpu irq handler assumed
> all percpu irqs are edge triggered or fasteoi type.
> 
> However MIPS processor implemented it in level triggered way.
> 
> My solution would be add a check. If neither ack nor eoi exist for the 
> chip,
> than we assume it's level triggered and process precpu irq in 
> mask/unmask way.
> 
> Could it be a possible option?

Post the patch, and we'll discuss it.

Thanks,

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