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Subject: Re: [PATCH 1/2] platform: make platform_get_irq_optional() optional
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From:   Sergey Shtylyov <s.shtylyov@omp.ru>
Organization: Open Mobile Platform
Message-ID: <6370798a-7a7e-243d-99f9-09bf772ddbac@omp.ru>
Date:   Wed, 19 Jan 2022 19:12:23 +0300
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On 1/18/22 5:29 PM, Uwe Kleine-K?nig wrote:

>> nst the magic not-found value (so no implementation detail magic
>>>>> leaks into the caller code) and just pass it to the next API function=
>> .
>>>>> (And my expectation would be that if you chose to represent not-found=
>>  by
>>>>> (void *)66 instead of NULL, you won't have to adapt any user, just th=
>> e
>>>>> framework internal checks. This is a good thing!)
>>>>
>>>> Ah, there is the wrong assumption: drivers sometimes do need to know
>>>> if the resource was found, and thus do need to know about (void *)66,
>>>> -ENODEV, or -ENXIO.  I already gave examples for IRQ and clk before.
>>>> I can imagine these exist for gpiod and regulator, too, as soon as
>>>> you go beyond the trivial "enable" and "disable" use-cases.
>>>
>>> My premise is that every user who has to check for "not found"
>>> explicitly should not use (clk|gpiod)_get_optional() but
>>> (clk|gpiod)_get() and do proper (and explicit) error handling for
>>> -ENODEV. (clk|gpiod)_get_optional() is only for these trivial use-cases.
>>>
>>>> And 0/NULL vs. > 0 is the natural check here: missing, but not
>>>> an error.
>>>
>>> For me it it 100% irrelevant if "not found" is an error for the query
>>> function or not. I just have to be able to check for "not found" and
>>> react accordingly.
>>>
>>> And adding a function
>>>
>>>         def platform_get_irq_opional():
>>>                 ret =3D platform_get_irq()
>>>                 if ret =3D=3D -ENXIO:
>>>                         return 0
>>>                 return ret
>>>
>>> it's not a useful addition to the API if I cannot use 0 as a dummy
>>> because it doesn't simplify the caller enough to justify the additional
>>> function.
>>>
>>> The only thing I need to be able is to distinguish the cases "there is
>>> an irq", "there is no irq" and anything else is "there is a problem I
>>> cannot handle and so forward it to my caller". The semantic of
>>> platform_get_irq() is able to satisfy this requirement[1], so why introdu=
>> ce
>>> platform_get_irq_opional() for the small advantage that I can check for
>>> not-found using
>>>
>>>         if (!irq)
>>>
>>> instead of
>>>
>>>         if (irq !=3D -ENXIO)
>>>
>>> ? The semantic of platform_get_irq() is easier ("Either a usable
>>> non-negative irq number or a negative error number") compared to
>>> platform_get_irq_optional() ("Either a usable positive irq number or a
>>> negative error number or 0 meaning not found"). Usage of
>>> platform_get_irq() isn't harder or more expensive (neither for a human
>>> reader nor for a maching running the resulting compiled code).
>>> For a human reader
>>>
>>>         if (irq !=3D -ENXIO)
>>>
>>> is even easier to understand because for
>>>
>>>         if (!irq)
>>>
>>> they have to check where the value comes from, see it's
>>> platform_get_irq_optional() and understand that 0 means not-found.
>>
>> "vIRQ zero does not exist."
> 
> With that statement in mind I would expect that a function that gives me
> an (v)irq number never returns 0.
> 
>>> This function just adds overhead because as a irq framework user I have
>>> to understand another function. For me the added benefit is too small to
>>> justify the additional function. And you break out-of-tree drivers.
>>> These are all no major counter arguments, but as the advantage isn't
>>> major either, they still matter.
>>>
>>> Best regards
>>> Uwe
>>>
>>> [1] the only annoying thing is the error message.
>>
>> So there's still a need for two functions.
> 
> Or a single function not emitting an error message together with the
> callers being responsible for calling dev_err().
> 
> So the options in my preference order (first is best) are:
> 
>  - Remove the printk from platform_get_irq() and remove
>    platform_get_irq_optional();

   Strong NAK here:
- dev_err() in our function saves a lot of (repeatable!) comments;
- we've already discussed that it's more optimal to check againt 0 than
  against -ENXIO in the callers.

>  - Rename platform_get_irq_optional() to platform_get_irq_silently()

   NAK as well. We'd better off complaining about irq < 0 in this function.

>  - Keep platform_get_irq_optional() as is

   NAK, it's suboptimal in the call sites.

>  - Collect underpants
> 
>  - ?

   You're on your own here. :-)

>  - Change semantic of platform_get_irq_optional()

   Yes, we should change the semantics if it serves our goals better. 

> Best regards
> Uwe

MBR, Sergey