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

[Thomas Gleixner <tglx@linutronix.de>]

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