linux-kernel - Re: [PATCH 2/2] rust: time: Implement basic arithmetic operations for Delta

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Message-ID: <CAH5fLgi63Avw2VYvaOEdZhw93Qb+1isuW-CsyaD-_ask62_tcA@mail.gmail.com>
Date: Fri, 1 Aug 2025 00:10:20 +0200
From: Alice Ryhl <aliceryhl@...gle.com>
To: Lyude Paul <lyude@...hat.com>
Cc: rust-for-linux@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>, 
	Boqun Feng <boqun.feng@...il.com>, linux-kernel@...r.kernel.org, 
	Andreas Hindborg <a.hindborg@...nel.org>, FUJITA Tomonori <fujita.tomonori@...il.com>, 
	Frederic Weisbecker <frederic@...nel.org>, Anna-Maria Behnsen <anna-maria@...utronix.de>, 
	John Stultz <jstultz@...gle.com>, Stephen Boyd <sboyd@...nel.org>, Miguel Ojeda <ojeda@...nel.org>, 
	Alex Gaynor <alex.gaynor@...il.com>, Gary Guo <gary@...yguo.net>, 
	Björn Roy Baron <bjorn3_gh@...tonmail.com>, 
	Benno Lossin <lossin@...nel.org>, Trevor Gross <tmgross@...ch.edu>, 
	Danilo Krummrich <dakr@...nel.org>
Subject: Re: [PATCH 2/2] rust: time: Implement basic arithmetic operations for Delta

On Thu, Jul 31, 2025 at 10:47 PM Lyude Paul <lyude@...hat.com> wrote:
>
> On Tue, 2025-07-29 at 12:15 +0000, Alice Ryhl wrote:
> >
> >
> > The reason I bring up the example is that once you add code using these
> > impls, you're going to get kernel build bot errors from your code not
> > compiling on 32-bit. And as seen in the linked one, code may be compiled
> > for 32-bit when setting CONFIG_COMPILE_TEST even if you don't support it
> > for real.
> >
> > > This being said, the kernel does have a math library that we can call into
> > > that emulates operations like this on 32 bit - which I'd be willing to convert
> > > these implementations over to using. I just put the CONFIG_64BIT there because
> > > if we do use the kernel math library, I just want to make sure I don't end up
> > > being the oen who has to figure out how to hook up the kernel math library for
> > > 64 bit division outside of simple time value manipulation. I've got enough
> > > dependencies on my plate to get upstream as it is :P
> >
> > If you just want to call the relevant bindings:: method directly without
> > any further logic that seems fine to me.
>
> Gotcha, I will do that. Ideally I would at least like to have us only call the
> bindings:: method so long as we're on a config where we really need it. Which
> brings me to ask - do we actually have a way of checking BITS_PER_LONG in
> #[cfg()]? I would have assumed it'd be simple but I don't actually seem to be
> able to reference BITS_PER_LONG.

There is:
#[cfg(target_pointer_width = "32")]
or
#[cfg(target_pointer_width = "64")]

Alice