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

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Message-ID: <87ms8b713z.fsf@kernel.org>
Date: Thu, 07 Aug 2025 14:44:16 +0200
From: Andreas Hindborg <a.hindborg@...nel.org>
To: Lyude Paul <lyude@...hat.com>, Alice Ryhl <aliceryhl@...gle.com>
Cc: rust-for-linux@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>,
 Boqun Feng <boqun.feng@...il.com>, linux-kernel@...r.kernel.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

"Lyude Paul" <lyude@...hat.com> writes:

> 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.

We took a similar approach in `Delta::as_micros_ceil`:

    /// Return the smallest number of microseconds greater than or equal
    /// to the value in the [`Delta`].
    #[inline]
    pub fn as_micros_ceil(self) -> i64 {
        #[cfg(CONFIG_64BIT)]
        {
            self.as_nanos().saturating_add(NSEC_PER_USEC - 1) / NSEC_PER_USEC
        }

        #[cfg(not(CONFIG_64BIT))]
        // SAFETY: It is always safe to call `ktime_to_us()` with any value.
        unsafe {
            bindings::ktime_to_us(self.as_nanos().saturating_add(NSEC_PER_USEC - 1))
        }
    }


Best regards,
Andreas Hindborg