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

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Date: Wed, 27 Mar 2024 10:49:49 -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>,
	Heghedus Razvan <heghedus.razvan@...tonmail.com>,
	Asahi Lina <lina@...hilina.net>
Subject: Re: [PATCH 3/5] rust: time: Introduce clock reading framework

On Wed, Mar 27, 2024 at 12:50:50PM +0000, Benno Lossin wrote:
> On 26.03.24 20:19, Boqun Feng wrote:
> > On Tue, Mar 26, 2024 at 05:00:39PM +0000, Benno Lossin wrote:
> >> 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`?
> >>
> > 
> > This was carried over from the old version, for myself, it doesn't make
> > much difference between:
> > 
> > 	trait A { .. }
> > 	trait B { .. }
> > 
> > 	impl<T: A + B> ...
> > 
> > vs
> > 
> > 	trait A { .. }
> > 	trait B: A { .. }
> > 
> > 	impl<T: B> ...
> > 
> > hence I kept it as it is, but yes, it a `Monotonic` *`Clock`*, so I will
> > change it in the next version.
> 
> I think it would also make sense to rename it to `MonotonicClock`.
> 
> > 
> >>> +
> >>> +/// A timestamp of a given [`Clock`]
> >>
> >> Missing '.'.
> >>
> > 
> > Fixed locally.
> > 
> >>> +#[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.
> >>
> > 
> > Hmm... I cannot trigger any compile errors with this patch. If you see
> > an error, could you share the build command? It's not a `pub` function
> > though.
> 
> I created a fresh config using `menuconfig` and enabled CONFIG_RUST.
> Then I get this error:
> 
>     error: associated function `new` is never used
>       --> rust/kernel/time.rs:97:8
>        |
>     96 | impl<T: Clock> Instant<T> {
>        | ------------------------- associated function in this implementation
>     97 |     fn new(ktime: bindings::ktime_t) -> Self {
>        |        ^^^
>        |
>        = note: `-D dead-code` implied by `-D warnings`
>        = help: to override `-D warnings` add `#[allow(dead_code)]`
>     
>     error: aborting due to 1 previous error
> 
> Note that the error vanishes in the next patch, since there you do have
> a user for `new`.
> 

Seems I didn't select CONFIG_WERROR=y, so I missed it as a warning.
and I can hit it now with that config. Will fix it in the next version.
Thanks!

Regards

> -- 
> Cheers,
> Benno