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

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Message-ID: <f8d9af76-fc1d-4f7a-8dfb-a0606e44c56b@nvidia.com>
Date: Thu, 12 Jun 2025 13:00:12 -0700
From: John Hubbard <jhubbard@...dia.com>
To: Boqun Feng <boqun.feng@...il.com>, Alexandre Courbot <acourbot@...dia.com>
Cc: 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 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! :) 

> 
> ? It's much harder to pass an invalid alignment with this.

Hopefully we can address argument validation without blowing up
the usual naming conventions.


thanks,
-- 
John Hubbard