linux-kernel - Re: [PATCH v5 06/12] mm, s390: constify mapping related test functions for improved const-correctness

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Message-ID: <4597944e-a8f5-44df-adf3-558940e88598@redhat.com>
Date: Mon, 1 Sep 2025 17:31:01 +0200
From: David Hildenbrand <david@...hat.com>
To: Max Kellermann <max.kellermann@...os.com>
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Subject: Re: [PATCH v5 06/12] mm, s390: constify mapping related test
 functions for improved const-correctness

On 01.09.25 17:22, Max Kellermann wrote:
> On Mon, Sep 1, 2025 at 5:11 PM David Hildenbrand <david@...hat.com> wrote:
>>>> Should this also be *const ?
>>>
>>> No. These are function protoypes. A "const" on a parameter value
>>> (pointer address, not pointed-to memory) makes no sense on a
>>> prototype.
>>
>> But couldn't you argue the same about variable names? In most (not all
>> :) ) we keep declaration + definition in sync. So thus my confusion.
> 
> Variable names in the prototypes have no effect either, but they serve
> as useful documentation.
> 
> Whereas the "const" on a parameter value documents nothing - it's an
> implementation detail whether the function would like to modify
> parameter values. That implementation detail has no effect for the
> caller.
> 
> Of course, we could have "const" in the prototype as well. This boils
> down to personal taste. It's not my taste (has no use, has no effect,
> documents nothing, only adds noise for no gain), so I didn't add it.
> If you prefer to have that, I'll leave my taste and home and add it,
> but only after you guys make up your minds about whether you want to
> have const parameters at all.

Valid points. The problem is that it could very soon become inconsistent.

For example, when I write a new function I usually just copy what I have
from the definition into the declaration.

For example, checkpatch complains about missing variable names and I 
think it complains when "extern" is used for functions.

If we were to decide to go that route (not keep them in perfect sync), I 
guess it would be reasonable to extend checkpatch to warn if "*const" is 
used in a declaration. (perl magic, no idea how hard that would be)

I'm sure there are false positives in the following:

$ git grep "\*const" *.h | grep -v inline | wc -l
403

-- 
Cheers

David / dhildenb