linux-kernel - Re: [PATCH v2] rust: time: New module for timekeeping functions

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Message-ID: <20230714100525.2192448-1-aliceryhl@google.com>
Date:   Fri, 14 Jul 2023 10:05:25 +0000
From:   Alice Ryhl <aliceryhl@...gle.com>
To:     lina@...hilina.net
Cc:     alex.gaynor@...il.com, asahi@...ts.linux.dev,
        bjorn3_gh@...tonmail.com, boqun.feng@...il.com, gary@...yguo.net,
        gbs@...ishe.com, heghedus.razvan@...tonmail.com,
        jistone@...hat.com, jstultz@...gle.com,
        linux-kernel@...r.kernel.org, ojeda@...nel.org,
        rust-for-linux@...r.kernel.org, sboyd@...nel.org,
        tglx@...utronix.de, wedsonaf@...il.com
Subject: Re: [PATCH v2] rust: time: New module for timekeeping functions

Asahi Lina <lina@...hilina.net> writes:
> +/// Marker trait for clock sources that represent a calendar (wall clock)
> +/// relative to the UNIX epoch.
> +pub trait WallTime {}

What's the purpose of this trait? Perhaps it should have a method to get
the UNIX epoch as an Instant?

> +    /// Returns the time elapsed since an earlier Instant<t>, or
> +    /// None if the argument is a later Instant.
> +    pub fn since(&self, earlier: Instant<T>) -> Option<Duration> {
> +        if earlier.nanoseconds > self.nanoseconds {
> +            None
> +        } else {
> +            // Casting to u64 and subtracting is guaranteed to give the right
> +            // result for all inputs, as long as the condition we checked above
> +            // holds.
> +            Some(Duration::from_nanos(
> +                self.nanoseconds as u64 - earlier.nanoseconds as u64,
> +            ))

It looks like you intend to use wrapping semantics for this subtraction
so that self=1,earlier=-1 results in a difference of two.

In that case, you should explicitly use `.wrapping_sub` instead to
convey your intent.

I guess you could also use `abs_diff`, which takes two i64s and returns
an u64.

> +/// Contains the various clock source types available to the kernel.
> +pub mod clock {
> +    use super::*;
> +
> +    /// A clock representing the default kernel time source.
> +    ///
> +    /// This is `CLOCK_MONOTONIC` (though it is not the only
> +    /// monotonic clock) and also the default clock used by
> +    /// `ktime_get()` in the C API.
> +    ///
> +    /// This is like `BootTime`, but does not include time
> +    /// spent sleeping.
> +
> +    pub struct KernelTime;
> +
> +    impl Clock for KernelTime {}
> +    impl Monotonic for KernelTime {}
> +    impl Now for KernelTime {
> +        fn now() -> Instant<Self> {
> +            Instant::<Self>::new(unsafe { bindings::ktime_get() })
> +        }
> +    }

We usually add a SAFETY comment even if it is trivial.

// SAFETY: Just an FFI call without any safety requirements.

Alice