From:   Thomas Gleixner <tglx@linutronix.de>
To:     Asahi Lina <lina@asahilina.net>, 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
Subject: Re: [PATCH] rust: time: New module for timekeeping functions
In-Reply-To: <7b93bf74-abdc-f8c1-9a12-7c7f080f9e19@asahilina.net>
References: <20230221-gpu-up-time-v1-1-bf8fe74b7f55@asahilina.net>
 <87v8jvnqq4.ffs@tglx> <Y/TP6as7qqwfcI42@boqun-archlinux>
 <87h6vfnh0f.ffs@tglx> <7b93bf74-abdc-f8c1-9a12-7c7f080f9e19@asahilina.net>
Date:   Tue, 21 Feb 2023 19:45:33 +0100
Message-ID: <87edqioo1e.ffs@tglx>
MIME-Version: 1.0
Content-Type: text/plain
Precedence: bulk

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