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

[Lyude Paul <lyude@redhat.com>]

On Fri, 2025-07-25 at 10:17 +0900, Alexandre Courbot wrote:
> 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@redhat.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?

Yeah - I considered adding one but I haven't bothered because KTIME_MAX is
just i64::MAX. We could add one though.

> 
> Otherwise,
> 
> Reviewed-by: Alexandre Courbot <acourbot@nvidia.com>
> 

-- 
Cheers,
 Lyude Paul (she/her)
 Software Engineer at Red Hat

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