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

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Message-ID: <aEsy8XDy6JW8zb6v@tardis.local>
Date: Thu, 12 Jun 2025 13:05:05 -0700
From: Boqun Feng <boqun.feng@...il.com>
To: John Hubbard <jhubbard@...dia.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 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".

Regards,
Boqun

> > 
> > ? 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
>