linux-kernel - Re: [PATCH 21/21] Docs: add Functions parameters order section

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Message-ID: <2ab04392-f133-4ebe-943a-c58050b36f13@infradead.org>
Date: Mon, 27 Oct 2025 11:43:27 -0700
From: Randy Dunlap <rdunlap@...radead.org>
To: Jani Nikula <jani.nikula@...el.com>,
 "Yury Norov (NVIDIA)" <yury.norov@...il.com>,
 Linus Walleij <linus.walleij@...aro.org>, Lee Jones <lee@...nel.org>,
 linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
 Jonathan Corbet <corbet@....net>, workflows@...r.kernel.org,
 linux-doc@...r.kernel.org
Subject: Re: [PATCH 21/21] Docs: add Functions parameters order section



On 10/27/25 2:02 AM, Jani Nikula wrote:
> On Sat, 25 Oct 2025, "Yury Norov (NVIDIA)" <yury.norov@...il.com> wrote:
>> Standardize parameters ordering in some typical cases to minimize
>> confusion.
>>
>> Signed-off-by: Yury Norov (NVIDIA) <yury.norov@...il.com>
>> ---
>>  Documentation/process/coding-style.rst | 48 ++++++++++++++++++++++++++
>>  1 file changed, 48 insertions(+)
>>
>> diff --git a/Documentation/process/coding-style.rst b/Documentation/process/coding-style.rst
>> index d1a8e5465ed9..dde24148305c 100644
>> --- a/Documentation/process/coding-style.rst
>> +++ b/Documentation/process/coding-style.rst
>> @@ -523,6 +523,54 @@ below, compared to the **declaration** example above)::
>>  	...
>>   }
>>  
>> +6.2) Function parameters order
>> +------------------------------
>> +
>> +The order of parameters is important both for code generation and readability.
>> +Passing parameters in an unusual order is a common source of bugs. Listing
>> +them in standard widely adopted order helps to avoid confusion.
>> +
>> +Many ABIs put first function parameter and return value in R0. If your
>> +function returns one of its parameters, passing it at the very beginning
>> +would lead to a better code generation. For example::
>> +
>> +        void *memset64(uint64_t *s, uint64_t v, size_t count);
>> +        void *memcpy(void *dest, const void *src, size_t count);
>> +
>> +If your function doesn't propagate a parameter, but has a meaning of copying
>> +and/or processing data, the best practice is following the traditional order:
>> +destination, source, options, flags.
>> +
>> +for_each()-like iterators should take an enumerator the first. For example::
>> +
>> +        for_each_set_bit(bit, mask, nbits);
>> +                do_something(bit);
>> +
>> +        list_for_each_entry(pos, head, member);
>> +                do_something(pos);
>> +
>> +If function operates on a range or ranges of data, corresponding parameters
>> +may be described as ``start - end`` or ``start - size`` pairs. In both cases,
>> +the parameters should follow each other. For example::
>> +
>> +        int
>> +        check_range(unsigned long vstart, unsigned long vend,
>> +                    unsigned long kstart, unsigned long kend);
>> +
>> +        static inline void flush_icache_range(unsigned long start, unsigned long end);
>> +
>> +        static inline void flush_icache_user_page(struct vm_area_struct *vma,
>> +                                            struct page *page,
>> +                                            unsigned long addr, int len);
>> +
>> +Both ``start`` and ``end`` of the interval are inclusive.
>> +
>> +Describing intervals in order ``end - start`` is unfavorable. One notable
>> +example is the ``GENMASK(high, low)`` macro. While such a notation is popular
>> +in hardware context, particularly to describe registers structure, in context
>> +of software development it looks counter intuitive and confusing. Please switch
>> +to an equivalent ``BITS(low, high)`` version.
>> +
> 
> GENMASK when used for defining hardware registers is completely fine,
> and *much* easier to deal with when you cross check against the specs
> that almost invariably define high:low.
> 
> Which other parts of coding style take on specific interfaces and tell
> you to switch? Weird. I for one don't want to encourage an influx of
> trivial patches doing GENMASK to BITS conversions, and then keep
> rejecting them. It's just a huge collective waste of time.
> 
> Anyway, that's a lot of text on "function parameter order" to justify
> BITS(), but completely skips more important principles such as "context
> parameter first", or "destination first".

and usually flags or gfp_t last (if they are used).

There are several exceptions to these, but consistency helps and
lack of it has caused some argument problems in the past.

-- 
~Randy