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Date:   Wed, 16 Dec 2020 17:42:18 +0100
From:   "Enrico Weigelt, metux IT consult" <info@...ux.net>
To:     Thomas Gleixner <tglx@...utronix.de>,
        Michael Ellerman <mpe@...erman.id.au>,
        "Enrico Weigelt, metux IT consult" <info@...ux.net>,
        linux-kernel@...r.kernel.org
Cc:     James.Bottomley@...senPartnership.com, deller@....de,
        benh@...nel.crashing.org, paulus@...ba.org, jdike@...toit.com,
        richard@....at, anton.ivanov@...bridgegreys.com, mingo@...hat.com,
        bp@...en8.de, x86@...nel.org, hpa@...or.com,
        linux-parisc@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org,
        linux-s390@...r.kernel.org, linux-um@...ts.infradead.org
Subject: Re: [PATCH] arch: fix 'unexpected IRQ trap at vector' warnings

On 15.12.20 23:12, Thomas Gleixner wrote:
> On Tue, Dec 15 2020 at 21:12, Enrico Weigelt wrote:
>> On 09.12.20 00:01, Thomas Gleixner wrote:
>>>   3) It's invoked from __handle_domain_irq() when the 'hwirq' which is
>>>      handed in by the caller does not resolve to a mapped Linux
>>>      interrupt which is pretty much the same as the x86 situation above
>>>      in #1, but it prints useless data.
>>>
>>>      It prints 'irq' which is invalid but it does not print the really
>>>      interesting 'hwirq' which was handed in by the caller and did
>>>      not resolve.
>>
>> I wouldn't say the irq-nr isn't interesting. In my particular case it
>> was quite what I've been looking for. But you're right, hwirq should
>> also be printed.
> 
> The number is _not_ interesting in this case. It's useless because the
> function does:

Oh, I've mixed up the cases - I only had the other one, down below.

>     irq = hwirq;
> 
>     if (lookup)
>         irq = find_mapping(hwirq);
> 
>     if (!irq || irq >= nr_irqs)
>        -> BAD

When exactly can that happen ? Only when some hardware sending an IRQ,
but no driver listening to it, or are there other cases ?

By the way: who's supposed to call that function ? Only irqchip's
(and the few soc specific 1st-level irq handlers) ? I'm asking, because
we have lots of gpio drivers, which have their own irq domain, but go
the generic_handle_irq() route. Same for some SOC-specific irqchips.

Should they also call handle_domain_irq() instead ?

> In both cases the only interesting information is that hwirq does not
> resolve to a valid Linux interrupt number and which hwirq number caused
> that.

Don't we also need know which irqchip the hwirq number belongs to ?

> If you look really then you find out that there is exactly _ONE_
> architecture which does anything else than incrementing a counter and/or
> printing stuff: X86, which has a big fat comment explaining why. The
> only way to ack an interrupt on X86 is to issue EOI on the local APIC,
> i.e. it does _not_ need any further information.

Yeah, found it :)

At this point I wonder whether the ack_APIC_irq() call could be done
somewhere further up in the call chain, eg. handle_irq() or
common_interrupt() ?

If that works, we IMHO could drop ack_bad_irq() completely (except for
the counter and printk, which we could consolidate elsewhere anyways)

>> ... rethinking this further ... shouldn't we also pass in even more data
>> (eg. irq_desc, irqchip, ...), so this function can check which hw to
>> actually talk to ?
> 
> There are 3 ways to get there:
> 
>       1) via dummy chip which obviously has no hardware associated

... which also calls print_irq_desc() ..

>       2) via handle_bad_irq() which prints the info already

print_irq_desc() doesn't seem to print the hwirq ... shall we fix this ?

>       3) __handle_domain_irq() which cannot print anything and obviously
>          cannot figure out the hw to talk to because there is no irq
>          descriptor associated.

Okay, what's the conclusion ? Drop printouts in the ack_bad_irq()'s ?

>>>   4) It's invoked from the dummy irq chip which is installed for a
>>>      couple of truly virtual interrupts where the invocation of
>>>      dummy_irq_chip::irq_ack() is indicating wreckage.
>>>
>>>      In that case the Linux irq number is the thing which is printed.
>>>
>>> So no. It's not just inconsistent it's in some places outright
>>> wrong. What we really want is:
>>>
>>> ack_bad_irq(int hwirq, int virq)
>>
>> is 'int' correct here ?
> 
> This was just for illustration.

Okay, thanks. Just discovered already have an irq_hw_number_t, which
doesn't seem to be used everywhere ... shall we fix that ?

>> OTOH: since both callers (dummychip.c, handle.c) already dump out before
>> ack_bad_irq(), do we need to print out anything at all ?
> 
> Not all callers print something, but yes this could do with some general
> cleanup.

I've found three callers, only one (__handle_domain_irq() in irqdesc.c)
doesn't print out anything. I belive, adding a pr_warn() here and drop
all the printouts in ack_bad_irq()'s makes sense.

> The error counter is independent of that, but yes there is room for
> consolidation.

Ok, I've already started hacking a bit here: adding an atomic_t counter
in kernel/irq/handle.c and inline'd accessor functions in
include/asm-generic/irq.h (just feeling that accessors are a bit cleaner
than direct access). Would that be okay ?

By the way: I still wonder whether my case should have ever reached
ack_bad_irq().

The irqdescs had been allocated via devm_irq_alloc_descs(), and the
driver just called generic_handle_irq() with base irq + gpio nr.
So, IMHO it was a valid linux irq number, but no (explicit) handler.

I wonder whether ack'ing those virtual irqs onto hw could be harmful.


--mtx

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Enrico Weigelt, metux IT consult
Free software and Linux embedded engineering
info@...ux.net -- +49-151-27565287

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