Date:   Wed, 12 Jan 2022 16:13:54 +0100
From:   Hans de Goede <hdegoede@...hat.com>
To:     Sergey Shtylyov <s.shtylyov@....ru>,
        "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

Hi,

On 1/12/22 16:05, Sergey Shtylyov wrote:
> 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.
>>>>Hi,
>>>> 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

Right, platform_get_irq() has this:

        WARN(ret == 0, "0 is an invalid IRQ number\n");

So given that platform_get_irq() returning 0 is not expected, it seems
reasonable for platform_get_irq_optional() to use 0 as a special
"no irq available" return value, matching the NULL returned by
gpiod_get_optional().

Regards,

Hans

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