linux-kernel - Re: [PATCH 1/2] rust: time: Implement Add<Delta>/Sub<Delta> for Instant

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Message-Id: <DBKQMYTGDFDY.2N70A5XF1TIIM@nvidia.com>
Date: Fri, 25 Jul 2025 10:17:20 +0900
From: "Alexandre Courbot" <acourbot@...dia.com>
To: "Lyude Paul" <lyude@...hat.com>, <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>
Cc: "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>, "Alice Ryhl" <aliceryhl@...gle.com>, "Trevor Gross"
 <tmgross@...ch.edu>, "Danilo Krummrich" <dakr@...nel.org>
Subject: Re: [PATCH 1/2] rust: time: Implement Add<Delta>/Sub<Delta> for
 Instant

On Fri Jul 25, 2025 at 3:54 AM JST, Lyude Paul wrote:
> In order to maintain the invariants of Instant, we use saturating
> addition/subtraction that is clamped to the valid value range for a
> non-negative Ktime.
>
> Signed-off-by: Lyude Paul <lyude@...hat.com>
> ---
>  rust/kernel/time.rs | 29 ++++++++++++++++++++++++++++-
>  1 file changed, 28 insertions(+), 1 deletion(-)
>
> diff --git a/rust/kernel/time.rs b/rust/kernel/time.rs
> index 64c8dcf548d63..ac5cab62070c6 100644
> --- a/rust/kernel/time.rs
> +++ b/rust/kernel/time.rs
> @@ -25,6 +25,7 @@
>  //! C header: [`include/linux/ktime.h`](srctree/include/linux/ktime.h).
>  
>  use core::marker::PhantomData;
> +use core::ops;
>  
>  pub mod delay;
>  pub mod hrtimer;
> @@ -202,7 +203,7 @@ pub(crate) fn as_nanos(&self) -> i64 {
>      }
>  }
>  
> -impl<C: ClockSource> core::ops::Sub for Instant<C> {
> +impl<C: ClockSource> ops::Sub for Instant<C> {
>      type Output = Delta;
>  
>      // By the type invariant, it never overflows.
> @@ -214,6 +215,32 @@ fn sub(self, other: Instant<C>) -> Delta {
>      }
>  }
>  
> +impl<T: ClockSource> ops::Add<Delta> for Instant<T> {
> +    type Output = Self;
> +
> +    #[inline]
> +    fn add(self, rhs: Delta) -> Self::Output {
> +        // INVARIANT: We clamp the resulting value to be between `0` and `KTIME_MAX`.

Not directly related, but I see `KTIME_MAX` being mentioned several
times in this file, but it doesn't seem to be declared anywhere in Rust?
Shall we have an alias/binding for it?

Otherwise,

Reviewed-by: Alexandre Courbot <acourbot@...dia.com>