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

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Date: Tue, 26 Mar 2024 10:11:07 -0700
From: Boqun Feng <boqun.feng@...il.com>
To: Benno Lossin <benno.lossin@...ton.me>
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 Tue, Mar 26, 2024 at 04:50:02PM +0000, Benno Lossin wrote:
> 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`?
> 

Good point, I will rename it in next version.

> > +}
> > +
> > +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?
> 

Hmm.. you're right, there should be no difference I think. If there is
nothing I'm missing from Alice, I will drop this function in the next
version.

Regards,
Boqun

> -- 
> 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
> >