linux-kernel - Re: [PATCH net-next v2 2/6] rust: time: Introduce Delta type

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Message-Id: <20241015.211207.1963272330702106304.fujita.tomonori@gmail.com>
Date: Tue, 15 Oct 2024 21:12:07 +0900 (JST)
From: FUJITA Tomonori <fujita.tomonori@...il.com>
To: andrew@...n.ch
Cc: fujita.tomonori@...il.com, netdev@...r.kernel.org,
 rust-for-linux@...r.kernel.org, hkallweit1@...il.com, tmgross@...ch.edu,
 ojeda@...nel.org, alex.gaynor@...il.com, gary@...yguo.net,
 bjorn3_gh@...tonmail.com, benno.lossin@...ton.me, a.hindborg@...sung.com,
 aliceryhl@...gle.com, anna-maria@...utronix.de, frederic@...nel.org,
 tglx@...utronix.de, arnd@...db.de, linux-kernel@...r.kernel.org
Subject: Re: [PATCH net-next v2 2/6] rust: time: Introduce Delta type

On Sat, 5 Oct 2024 20:02:55 +0200
Andrew Lunn <andrew@...n.ch> wrote:

>> +/// A span of time.
>> +#[derive(Copy, Clone)]
>> +pub struct Delta {
>> +    nanos: i64,
> 
> Is there are use case for negative Deltas ? Should this be u64?

After investigating ktime_us_delta() and ktime_ms_delta() users, I
found that ten or so places which use nagative Deltas like:

timedout_ktime = ktime_add_us(ktime_get(), some_usecs);
// Do something that takes time
remaining = ktime_us_delta(timedout_ktime, ktime_get());
if (remaining > 0)
     fsleep(remaining)


Looks straightforward. On second thought, I feel it would be better to
support nagative Deltas. We could use i64 everywhere.

And i64 is more compatible with Ktime Delta APIs; ktime_us_delta and
ktime_ms_delta returns s64; we create Delta without type conversion.