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

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Date:   Tue, 21 Feb 2023 17:02:40 +0100
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Boqun Feng <boqun.feng@...il.com>
Cc:     Asahi Lina <lina@...hilina.net>, 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>,
        John Stultz <jstultz@...gle.com>,
        Stephen Boyd <sboyd@...nel.org>, linux-kernel@...r.kernel.org,
        rust-for-linux@...r.kernel.org, asahi@...ts.linux.dev
Subject: Re: [PATCH] rust: time: New module for timekeeping functions

On Tue, Feb 21 2023 at 06:06, Boqun Feng wrote:
> On Tue, Feb 21, 2023 at 01:32:51PM +0100, Thomas Gleixner wrote:
>> Similar to 'Instant' 'SystemTime' is strictly bound to CLOCK_REALTIME
>> by specification and there is no way to read CLOCK_TAI.
>> 
> ..'Instant' and 'SystemTime' are in Rust std, we cannot use them
> directly, similar as we cannot use userspace libc.

Sure. Was Rust std defined based on SysV from 30 years ago? :)

> To me, there seems two options to provide Rust types for kernel time
> management:
>
> *	Use KTime which maps to ktime_t, then we have the similar
> 	semantics around it: sometimes it's a duration, sometimes it's
> 	a point of time.. but I know "this is a safe language, you
> 	should do more" ;-)
>
> *	Introduce kernel's own types, e.g. BootTime, RawTime, TAI,
> 	RealTime, and make them play with Duration (actually I'd prefer
> 	we have own Duration, because Rust core::time::Duration takes
> 	more than u64), something like below:
>
>
> 	pub struct BootTime {
> 	    d: Duration
> 	}
>
> 	impl BootTime {
> 	    fn now() -> Self {
> 	        unsafe { BootTime { d: ktime_to_duration(ktime_get_boottime())} }
> 	    }
> 	    fn add(self, d: Duration) -> Self {
> 	        <Add a duration, similar to ktime_add>
> 	    }
> 	    fn sub(self, other: Self) -> Duration {
> 	        ...
> 	    }

I'm not rusty enough, but you really want two types:

    timestamp and timedelta

timestamp is an absolute time on a specific clock which is read via
now() and you can add time deltas to it. The latter is required for
arming an absolute timer on the clock.

timedelta is a relative time and completely independent of any
clock. That's what you get when you subtract two timestamps, but you can
also initialize it from a constant or some other source. timedelta can
be used to arm a relative timer on any clock.

Playing games with a single type is neither safe nor intuitive, right?

Thanks,

        tglx