linux-kernel - Re: [PATCH net-next v3 4/8] rust: time: Implement addition of Ktime and Delta

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Message-ID: <CAH5fLgjHf3Z5HHOLnzZkk-Q5MOwz_57LQc6scr9yDy1j89HSCw@mail.gmail.com>
Date: Thu, 17 Oct 2024 11:33:09 +0200
From: Alice Ryhl <aliceryhl@...gle.com>
To: FUJITA Tomonori <fujita.tomonori@...il.com>
Cc: boqun.feng@...il.com, netdev@...r.kernel.org, 
	rust-for-linux@...r.kernel.org, andrew@...n.ch, hkallweit1@...il.com, 
	tmgross@...ch.edu, ojeda@...nel.org, alex.gaynor@...il.com, gary@...yguo.net, 
	bjorn3_gh@...tonmail.com, benno.lossin@...ton.me, a.hindborg@...sung.com, 
	anna-maria@...utronix.de, frederic@...nel.org, tglx@...utronix.de, 
	arnd@...db.de, jstultz@...gle.com, sboyd@...nel.org, 
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH net-next v3 4/8] rust: time: Implement addition of Ktime
 and Delta

On Thu, Oct 17, 2024 at 11:31 AM FUJITA Tomonori
<fujita.tomonori@...il.com> wrote:
>
> On Wed, 16 Oct 2024 12:54:07 -0700
> Boqun Feng <boqun.feng@...il.com> wrote:
>
> >> diff --git a/rust/kernel/time.rs b/rust/kernel/time.rs
> >> index 8c00854db58c..9b0537b63cf7 100644
> >> --- a/rust/kernel/time.rs
> >> +++ b/rust/kernel/time.rs
> >> @@ -155,3 +155,14 @@ pub fn as_secs(self) -> i64 {
> >>          self.nanos / NSEC_PER_SEC
> >>      }
> >>  }
> >> +
> >> +impl core::ops::Add<Delta> for Ktime {
> >> +    type Output = Ktime;
> >> +
> >> +    #[inline]
> >> +    fn add(self, delta: Delta) -> Ktime {
> >> +        Ktime {
> >> +            inner: self.inner + delta.as_nanos(),
> >
> > What if overflow happens in this addition? Is the expectation that user
> > should avoid overflows?
>
> Yes, I'll add a comment.
>
> > I asked because we have ktime_add_safe() which saturate at
> > KTIME_SEC_MAX.
>
> We could add the Rust version of add_safe method. But looks like
> ktime_add_safe() is used by only some core systems so we don't need to
> add it now?

I think it makes sense to follow the standard Rust addition
conventions here. Rust normally treats + as addition that BUGs on
overflow (with the appropriate configs set), and then there's a
saturating_add function for when you want it to saturate.

Alice