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

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Message-ID: <87edqioo1e.ffs@tglx>
Date:   Tue, 21 Feb 2023 19:45:33 +0100
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Asahi Lina <lina@...hilina.net>, Boqun Feng <boqun.feng@...il.com>
Cc:     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 Wed, Feb 22 2023 at 01:31, Asahi Lina wrote:
> On 22/02/2023 01.02, Thomas Gleixner wrote:
>> 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.

Fine, but do you agree that:

      ts1 = tboot.now()
      ...
      ts2 = tboot.now()

      xb = ts2 - ts1

then the result x1 cannot be the same data type as ts1, ts2.

>From a typesafety perspective

      ts1 = treal.now()
      ...
      ts2 = tboot.now()

      x = ts2 - ts1

would be an invalid operation, but

      ts1 = treal.now()
      ...
      ts2 = treal.now()

      xr = ts2 - ts1

is obviously valid.

But xb abd xr are the same datatype because they represent a time delta.

That's the same the Rust std time semantics:

       Duration = Instance - Instance           valid
       Duration = Systemtime - SystemTime       valid
       Duration = Systemtime - Instance         invalid

No?

Thanks,

        tglx