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


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