Message-ID: <7b93bf74-abdc-f8c1-9a12-7c7f080f9e19@asahilina.net>
Date:   Wed, 22 Feb 2023 01:31:28 +0900
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Subject: Re: [PATCH] rust: time: New module for timekeeping functions
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To:     Thomas Gleixner <tglx@linutronix.de>,
        Boqun Feng <boqun.feng@gmail.com>
Cc:     Miguel Ojeda <ojeda@kernel.org>,
        Alex Gaynor <alex.gaynor@gmail.com>,
        Wedson Almeida Filho <wedsonaf@gmail.com>,
        Gary Guo <gary@garyguo.net>,
        =?UTF-8?Q?Bj=c3=b6rn_Roy_Baron?= <bjorn3_gh@protonmail.com>,
        John Stultz <jstultz@google.com>,
        Stephen Boyd <sboyd@kernel.org>, linux-kernel@vger.kernel.org,
        rust-for-linux@vger.kernel.org, asahi@lists.linux.dev
References: <20230221-gpu-up-time-v1-1-bf8fe74b7f55@asahilina.net>
 <87v8jvnqq4.ffs@tglx> <Y/TP6as7qqwfcI42@boqun-archlinux>
 <87h6vfnh0f.ffs@tglx>
From:   Asahi Lina <lina@asahilina.net>
In-Reply-To: <87h6vfnh0f.ffs@tglx>
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On 22/02/2023 01.02, Thomas Gleixner wrote:
> On Tue, Feb 21 2023 at 06:06, Boqun Feng wrote:
>> 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.

If all clocks end up as the same `timestamp` though, then this isn't
fully safe, because you could subtract `timestamp`s that came from
different clocks and the result would be meaningless. That's why the
Rust std Instant is specifically tied to one and only one system clock
on each platform.

~~ Lina