Message-ID: <78a17bae-435b-e35e-b2dc-1166777725a0@omp.ru>
Date:   Wed, 12 Jan 2022 18:05:24 +0300
From:   Sergey Shtylyov <s.shtylyov@....ru>
To:     "Rafael J. Wysocki" <rafael@...nel.org>,
        Geert Uytterhoeven <geert@...ux-m68k.org>
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Subject: Re: [PATCH 1/2] platform: make platform_get_irq_optional() optional

On 1/12/22 5:41 PM, Rafael J. Wysocki wrote:

[...]
>>>> If an optional IRQ is not present, drivers either just ignore it (e.g.
>>>> for devices that can have multiple interrupts or a single muxed IRQ),
>>>> or they have to resort to polling. For the latter, fall-back handling
>>>> is needed elsewhere in the driver.
>>>> To me it sounds much more logical for the driver to check if an
>>>> optional irq is non-zero (available) or zero (not available), than to
>>>> sprinkle around checks for -ENXIO. In addition, you have to remember
>>>> that this one returns -ENXIO, while other APIs use -ENOENT or -ENOSYS
>>>> (or some other error code) to indicate absence. I thought not having
>>>> to care about the actual error code was the main reason behind the
>>>> introduction of the *_optional() APIs.
>>>
>>> The *_optional() functions return an error code if there has been a
>>> real error which should be reported up the call stack. This excludes
>>> whatever error code indicates the requested resource does not exist,
>>> which can be -ENODEV etc. If the device does not exist, a magic cookie
>>> is returned which appears to be a valid resources but in fact is
>>> not. So the users of these functions just need to check for an error
>>> code, and fail the probe if present.
>>
>> Agreed.
>>
>> Note that in most (all?) other cases, the return type is a pointer
>> (e.g. to struct clk), and NULL is the magic cookie.
>>
>>> You seems to be suggesting in binary return value: non-zero
>>> (available) or zero (not available)
>>
>> Only in case of success. In case of a real failure, an error code
>> must be returned.
>>
>>> This discards the error code when something goes wrong. That is useful
>>> information to have, so we should not be discarding it.
>>
>> No, the error code must be retained in case of failure.
>>
>>> IRQ don't currently have a magic cookie value. One option would be to
>>> add such a magic cookie to the subsystem. Otherwise, since 0 is
>>> invalid, return 0 to indicate the IRQ does not exist.
>>
>> Exactly. And using 0 means the similar code can be used as for other
>> subsystems, where NULL would be returned.
>>
>> The only remaining difference is the "dummy cookie can be passed
>> to other functions" behavior.  Which is IMHO a valid difference,
>> as unlike with e.g. clk_prepare_enable(), you do pass extra data to
>> request_irq(), and sometimes you do need to handle the absence of
>> the interrupt using e.g. polling.
>>
>>> The request for a script checking this then makes sense. However, i
>>> don't know how well coccinelle/sparse can track values across function
>>> calls. They probably can check for:
>>>
>>>    ret = irq_get_optional()
>>>    if (ret < 0)
>>>       return ret;
>>>
>>> A missing if < 0 statement somewhere later is very likely to be an
>>> error. A comparison of <= 0 is also likely to be an error. A check for
>>>> 0 before calling any other IRQ functions would be good. I'm
>>> surprised such a check does not already existing in the IRQ API, but
>>> there are probably historical reasons for that.
>>
>> There are still a few platforms where IRQ 0 does exist.
> 
> Not just a few even.  This happens on a reasonably recent x86 PC:
> 
> rafael@...tch:~/work/linux-pm> head -2 /proc/interrupts
>            CPU0       CPU1       CPU2       CPU3       CPU4       CPU5
>   0:         10          0          0          0          0          0
>  IR-IO-APIC    2-edge
> timer

   IIRC Linus has proclaimed that IRQ0 was valid for the i8253 driver (living in
arch/x86/); IRQ0 only was frowned upon when returned by platform_get_irq() and its
ilk.

MBR, Sergey

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