linux-kernel - Re: [PATCH 2/5] rust: time: Introduce Duration type

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Date: Tue, 26 Mar 2024 16:50:02 +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>
Subject: Re: [PATCH 2/5] rust: time: Introduce Duration type

On 24.03.24 23:33, Boqun Feng wrote:> From: Alice Ryhl <aliceryhl@...gle.com>
> 
> Introduce a type representing time duration. Define our own type instead
> of using `core::time::Duration` because in kernel C code, an i64
> (ktime_t) is used for representing time durations, an i64 backed
> duration type is more efficient when interacting with time APIs in C.
> 
> Signed-off-by: Alice Ryhl <aliceryhl@...gle.com>
> [boqun: Rename `Ktime` to `Duration`, and make it a type of durations]
> Signed-off-by: Boqun Feng <boqun.feng@...il.com>
> ---
>  rust/kernel/time.rs | 44 ++++++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 44 insertions(+)
> 
> diff --git a/rust/kernel/time.rs b/rust/kernel/time.rs
> index bbb666e64dd7..b238b3a4e899 100644
> --- a/rust/kernel/time.rs
> +++ b/rust/kernel/time.rs
> @@ -7,6 +7,9 @@
>  //!
>  //! C header: [`include/linux/jiffies.h`](srctree/include/linux/jiffies.h).
> 
> +/// The number of nanoseconds per millisecond.
> +pub const NSEC_PER_MSEC: i64 = bindings::NSEC_PER_MSEC as i64;
> +
>  /// The time unit of Linux kernel. One jiffy equals (1/HZ) second.
>  pub type Jiffies = core::ffi::c_ulong;
> 
> @@ -20,3 +23,44 @@ pub fn msecs_to_jiffies(msecs: Msecs) -> Jiffies {
>      // matter what the argument is.
>      unsafe { bindings::__msecs_to_jiffies(msecs) }
>  }
> +
> +/// A time duration.
> +///
> +/// # Examples
> +///
> +/// ```
> +/// let one_second = kernel::time::Duration::new(1000_000_000);
> +///
> +/// // 1 second is 1000 milliseconds.
> +/// assert_eq!(one_second.to_ms(), 1000);
> +/// ```
> +#[repr(transparent)]
> +#[derive(Copy, Clone, Debug)]
> +pub struct Duration {
> +    inner: i64,

Why not use the name `ns` or `nanos`?

> +}
> +
> +impl Duration {
> +    /// Creates a new duration of `ns` nanoseconds.
> +    pub const fn new(ns: i64) -> Self {
> +        Self { inner: ns }
> +    }
> +
> +    /// Divides the number of nanoseconds by a compile-time constant.
> +    #[inline]
> +    fn divns_constant<const DIV: i64>(self) -> i64 {
> +        self.to_ns() / DIV
> +    }

I am a bit confused, why is this better than writing
`self.to_ns() / DIV` at the callsite?

-- 
Cheers,
Benno

> +
> +    /// Returns the number of milliseconds.
> +    #[inline]
> +    pub fn to_ms(self) -> i64 {
> +        self.divns_constant::<NSEC_PER_MSEC>()
> +    }
> +
> +    /// Returns the number of nanoseconds.
> +    #[inline]
> +    pub fn to_ns(self) -> i64 {
> +        self.inner
> +    }
> +}
> --
> 2.44.0
>