linux-kernel - Re: [PATCH 3/5] rust: time: Introduce clock reading framework

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Date: Tue, 26 Mar 2024 17:00:39 +0000
From: Benno Lossin <benno.lossin@...ton.me>
To: Boqun Feng <boqun.feng@...il.com>
Cc: rust-for-linux@...r.kernel.org, linux-kernel@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>, Alice Ryhl <aliceryhl@...gle.com>, Miguel Ojeda <ojeda@...nel.org>, Alex Gaynor <alex.gaynor@...il.com>, Wedson Almeida Filho <wedsonaf@...il.com>, Gary Guo <gary@...yguo.net>, Björn Roy Baron <bjorn3_gh@...tonmail.com>, Andreas Hindborg <a.hindborg@...sung.com>, John Stultz <jstultz@...gle.com>, Stephen Boyd <sboyd@...nel.org>, Valentin Obst <kernel@...entinobst.de>, Heghedus Razvan <heghedus.razvan@...tonmail.com>, Asahi Lina <lina@...hilina.net>
Subject: Re: [PATCH 3/5] rust: time: Introduce clock reading framework

On 24.03.24 23:33, Boqun Feng wrote:
> To make sure Rust code doesn't mix timestamps from different clocks, a
> type safe clock reading framework is introduced. It includes:
> 
> * A `Clock` trait that represents different clocks, to read a particular
>   clock, one needs implement the `Clock::now()` function.
> 
> * A `Instant<Clock>` type that represents timestamps of different
>   clocks, whose implementation is just a `ktime_t`, so all the
>   calculation on `ktime_t` should apply to it as well.
> 
> Co-developed-by: Heghedus Razvan <heghedus.razvan@...tonmail.com>
> Signed-off-by: Heghedus Razvan <heghedus.razvan@...tonmail.com>
> Co-developed-by: Asahi Lina <lina@...hilina.net>
> Signed-off-by: Asahi Lina <lina@...hilina.net>
> Signed-off-by: Boqun Feng <boqun.feng@...il.com>
> ---
>  rust/kernel/time.rs | 49 +++++++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 49 insertions(+)
> 
> diff --git a/rust/kernel/time.rs b/rust/kernel/time.rs
> index b238b3a4e899..0f9f5605ed48 100644
> --- a/rust/kernel/time.rs
> +++ b/rust/kernel/time.rs
> @@ -6,6 +6,9 @@
>  //! have been ported or wrapped for usage by Rust code in the kernel.
>  //!
>  //! C header: [`include/linux/jiffies.h`](srctree/include/linux/jiffies.h).
> +//! C header: [`include/linux/ktime.h`](srctree/include/linux/ktime.h).
> +
> +use core::marker::PhantomData;
> 
>  /// The number of nanoseconds per millisecond.
>  pub const NSEC_PER_MSEC: i64 = bindings::NSEC_PER_MSEC as i64;
> @@ -64,3 +67,49 @@ pub fn to_ns(self) -> i64 {
>          self.inner
>      }
>  }
> +
> +/// Represents a clock, that is, a unique time source and it can be queried for the current time.
> +pub trait Clock: Sized {
> +    /// Returns the current time for this clock.
> +    fn now() -> Instant<Self>;
> +}
> +
> +/// Marker trait for clock sources that are guaranteed to be monotonic.
> +pub trait Monotonic {}

Why is this trait not an extension of `Clock`?

> +
> +/// A timestamp of a given [`Clock`]

Missing '.'.

> +#[repr(transparent)]
> +#[derive(Debug)]
> +pub struct Instant<T: Clock> {
> +    inner: bindings::ktime_t,
> +    clock: PhantomData<T>,
> +}
> +
> +impl<T: Clock> Clone for Instant<T> {
> +    fn clone(&self) -> Self {
> +        *self
> +    }
> +}
> +
> +impl<T: Clock> Copy for Instant<T> {}
> +
> +impl<T: Clock> Instant<T> {
> +    fn new(ktime: bindings::ktime_t) -> Self {

When compiling, this function is marked as dead-code in this patch.

-- 
Cheers,
Benno

> +        Self {
> +            inner: ktime,
> +            clock: PhantomData,
> +        }
> +    }
> +}
> +
> +impl<T: Clock> core::ops::Sub for Instant<T> {
> +    type Output = Duration;
> +
> +    /// Returns the difference of two timestamps.
> +    #[inline]
> +    fn sub(self, other: Self) -> Self::Output {
> +        // `ktime_t` is an `i64` value of nanoseconds, and kernel defines signed overflow to behave
> +        // like 2s-complement, hence `wrapping_sub()` is used here to mirror `ktime_sub()`.
> +        Duration::new(self.inner.wrapping_sub(other.inner))
> +    }
> +}
> --
> 2.44.0
>