linux-kernel - Re: [PATCH v5 04/23] rust: add new `num` module with `PowerOfTwo` type

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Message-ID: <529736c9-15a1-4777-be30-cc9aa9d11a87@nvidia.com>
Date: Thu, 12 Jun 2025 13:08:25 -0700
From: John Hubbard <jhubbard@...dia.com>
To: Boqun Feng <boqun.feng@...il.com>
Cc: Alexandre Courbot <acourbot@...dia.com>, Miguel Ojeda <ojeda@...nel.org>,
 Alex Gaynor <alex.gaynor@...il.com>, Gary Guo <gary@...yguo.net>,
 Björn Roy Baron <bjorn3_gh@...tonmail.com>,
 Andreas Hindborg <a.hindborg@...nel.org>, Alice Ryhl <aliceryhl@...gle.com>,
 Trevor Gross <tmgross@...ch.edu>, Danilo Krummrich <dakr@...nel.org>,
 David Airlie <airlied@...il.com>, Simona Vetter <simona@...ll.ch>,
 Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
 Maxime Ripard <mripard@...nel.org>, Thomas Zimmermann <tzimmermann@...e.de>,
 Benno Lossin <lossin@...nel.org>, Ben Skeggs <bskeggs@...dia.com>,
 Joel Fernandes <joelagnelf@...dia.com>, Timur Tabi <ttabi@...dia.com>,
 Alistair Popple <apopple@...dia.com>, linux-kernel@...r.kernel.org,
 rust-for-linux@...r.kernel.org, nouveau@...ts.freedesktop.org,
 dri-devel@...ts.freedesktop.org
Subject: Re: [PATCH v5 04/23] rust: add new `num` module with `PowerOfTwo`
 type

On 6/12/25 1:05 PM, Boqun Feng wrote:
> On Thu, Jun 12, 2025 at 01:00:12PM -0700, John Hubbard wrote:
>> On 6/12/25 8:07 AM, Boqun Feng wrote:
>>> On Thu, Jun 12, 2025 at 11:01:32PM +0900, Alexandre Courbot wrote:
>> ...
>>>> +                #[inline(always)]
>>>> +                pub const fn align_down(self, value: $t) -> $t {
>>>
>>> I'm late to party, but could we instead implement:
>>>
>>>     pub const fn round_down<i32>(value: i32, shift: i32) -> i32 {
>>>         value & !((1 << shift) - 1)
>>>     }
>>>
>>>     pub const fn round_up<i32>(value: i32, shift: i32) -> i32 {
>>>         let mask = (1 << shift) - 1;
>>>         value.wrapping_add(mask) & !mask
>>>     }
>>
>> Just a naming concern here.
>>
>> The function name, and the "shift" argument is extremely odd there.
>> And that's because it is re-inventing the concept of align_down()
>> and align_up(), but with a misleading name and a hard to understand
>> "shift" argument.
>>
>> If you are "rounding" to a power of two, that's normally called
>> alignment, at least in kernel code. And if you are rounding to the
>> nearest...integer, for example, that's rounding.
>>
>> But "rounding" with a "shift" argument? That's a little too 
>> creative! :) 
>>
> 
> Oh, sorry, I should have mentioned where I got these names, see
> round_up() and round_down() in include/linux/math.h. But no objection to
> find a better name for "shift".

lol, perfect response! So my complaint is really about the kernel's existing
math.h, rather than your proposal. OK then. :)

thanks,
-- 
John Hubbard