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

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Message-ID: <3848736d-7cc8-44f4-9386-c30f0658ed9b@lunn.ch>
Date: Sat, 5 Oct 2024 20:02:55 +0200
From: Andrew Lunn <andrew@...n.ch>
To: FUJITA Tomonori <fujita.tomonori@...il.com>
Cc: 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

> +/// A span of time.
> +#[derive(Copy, Clone)]
> +pub struct Delta {
> +    nanos: i64,

Is there are use case for negative Deltas ? Should this be u64?

A u64 would allow upto 500 years, if i did my back of an envelope
maths correct. So i suppose 250 years allowing negative delta would
also work.

> +}
> +
> +impl Delta {
> +    /// Create a new `Delta` from a number of nanoseconds.
> +    #[inline]
> +    pub fn from_nanos(nanos: u16) -> Self {

So here you don't allow negative values.

But why limit it to u16, when the base value is a 63 bits? 65535 nS is
not very long.

> +        Self {
> +            nanos: nanos.into(),
> +        }
> +    }
> +
> +    /// Create a new `Delta` from a number of microseconds.
> +    #[inline]
> +    pub fn from_micros(micros: u16) -> Self {

A u32 should not overflow when converted to nS in an i64.

Dumb question. What does Rust in the kernel do if there is an
overflow?

> +    /// Return the number of nanoseconds in the `Delta`.
> +    #[inline]
> +    pub fn as_nanos(self) -> i64 {
> +        self.nanos
> +    }
> +
> +    /// Return the number of microseconds in the `Delta`.
> +    #[inline]
> +    pub fn as_micros(self) -> i64 {
> +        self.nanos / NSEC_PER_USEC
> +    }
> +}

So here we are back to signed values. And also you cannot create a
Delta from a Delta because the types are not transitive.

	Andrew