linux-kernel - Re: [PATCH] mtd: spi-nor: cqspi: remove duplicate const

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Message-ID: <7e54bc24-45a0-fb70-8bee-a7de6b6aae16@wedev4u.fr>
Date:   Wed, 28 Jun 2017 00:08:16 +0200
From:   Cyrille Pitchen <cyrille.pitchen@...ev4u.fr>
To:     Marek Vasut <marek.vasut@...il.com>, Arnd Bergmann <arnd@...db.de>,
        David Woodhouse <dwmw2@...radead.org>,
        Brian Norris <computersforpeace@...il.com>,
        Boris Brezillon <boris.brezillon@...e-electrons.com>,
        Richard Weinberger <richard@....at>
Cc:     linux-mtd@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mtd: spi-nor: cqspi: remove duplicate const

Le 27/06/2017 à 22:53, Marek Vasut a écrit :
> On 06/27/2017 10:21 PM, Cyrille Pitchen wrote:
>> Le 27/06/2017 à 21:51, Marek Vasut a écrit :
>>> On 06/27/2017 09:48 PM, Cyrille Pitchen wrote:
>>>> Le 27/06/2017 à 18:11, Marek Vasut a écrit :
>>>>> On 06/27/2017 05:34 PM, Arnd Bergmann wrote:
>>>>>> The variable was already marked 'const' before the previous
>>>>>> patch, but the qualifier was in an unusual place, and now the
>>>>>> extra 'const' causes a harmless warning:
>>>>>>
>>>>>> drivers/mtd/spi-nor/cadence-quadspi.c:1286:34: error: duplicate 'const' declaration specifier [-Werror=duplicate-decl-specifier]
>>>>>>
>>>>>> This removes the other 'const' instead.
>>>>>
>>>>> Isn't that const array const elements , thus two consts ?
>>>>> IMO the original code is correct.
>>>>
>>>> Indeed the 2 'const' don't have the same meaning so the original code
>>>> may be correct but with an array like cqspi_dt_ids[] we can't write
>>>> "cqspi_dt_ids = (const struct of_device_id *)<right value>;" anyway.
>>>
>>> Not sure I understand what you're trying to say here.
>>>
>>
>> Just that once an array like cqspi_dt_ids[] has been defined, you can
>> never assign it a new value later:
>>
>> cqspi_dt_ids = <new value>; /* that doesn't work */
>>
>> For this specific point, an array behaves like a constant pointer (but
>> otherwise I agree that arrays and constant pointers are not the same
>> things). So I guess the 2nd 'const' could be considered as implicit :)
> 
> I had to think about this a little and I think what you're getting at is
> something slightly different.

Indeed, you're right, I've misunderstood the 2nd 'const' ;)

So this time if I understand correctly, patch f993c123b461 was actually
useless since the elements of the array were already constant before
that patch, the syntax "Type const array[] = {...};" being equivalent to
"const Type array[] = {...};"

Anyway, the issue is fixed :)

> 
> IIUC if this was array of pointers (which it is not), you'd need this
> const Type *const name[] , but since this is array of Type , you can do
> with const Type name[] and the whole thing is implicitly const.
> 
> But in that case:
> Acked-by: Marek Vasut <marek.vasut@...il.com>
> 
>>>> So I think the 2nd 'const' is useless here then if this patch removes a
>>>> warning, let's apply it, right?
>>>>
>>>>>
>>>>>> Fixes: f993c123b461 ("mtd: spi-nor: cqspi: make of_device_ids const")
>>>>>> Signed-off-by: Arnd Bergmann <arnd@...db.de>
>>>>>> ---
>>>>>>  drivers/mtd/spi-nor/cadence-quadspi.c | 2 +-
>>>>>>  1 file changed, 1 insertion(+), 1 deletion(-)
>>>>>>
>>>>>> diff --git a/drivers/mtd/spi-nor/cadence-quadspi.c b/drivers/mtd/spi-nor/cadence-quadspi.c
>>>>>> index d315c326e72f..53c7d8e0327a 100644
>>>>>> --- a/drivers/mtd/spi-nor/cadence-quadspi.c
>>>>>> +++ b/drivers/mtd/spi-nor/cadence-quadspi.c
>>>>>> @@ -1283,7 +1283,7 @@ static const struct dev_pm_ops cqspi__dev_pm_ops = {
>>>>>>  #define CQSPI_DEV_PM_OPS	NULL
>>>>>>  #endif
>>>>>>  
>>>>>> -static const struct of_device_id const cqspi_dt_ids[] = {
>>>>>> +static const struct of_device_id cqspi_dt_ids[] = {
>>>>>>  	{.compatible = "cdns,qspi-nor",},
>>>>>>  	{ /* end of table */ }
>>>>>>  };
>>>>>>
>>>>>
>>>>>
>>>>
>>>
>>>
>>
> 
>