On 10/27/25 2:02 AM, Jani Nikula wrote:

On Sat, 25 Oct 2025, "Yury Norov (NVIDIA)" [off-list ref] wrote:

quoted

Standardize parameters ordering in some typical cases to minimize
confusion.

Signed-off-by: Yury Norov (NVIDIA) <yury.norov@gmail.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