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

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Message-ID: <7e6e0f84-1eef-4c7c-872a-3852a9a80034@lunn.ch>
Date: Sat, 5 Oct 2024 23:09:48 +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

On Sat, Oct 05, 2024 at 09:25:27PM +0900, FUJITA Tomonori wrote:
> Introduce a type representing a span of time. Define our own type
> because `core::time::Duration` is large and could panic during
> creation.
> 
> We could use time::Ktime for time duration but timestamp and timedelta
> are different so better to use a new type.
> 
> Signed-off-by: FUJITA Tomonori <fujita.tomonori@...il.com>
> ---
>  rust/kernel/time.rs | 64 +++++++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 64 insertions(+)
> 
> diff --git a/rust/kernel/time.rs b/rust/kernel/time.rs
> index c40105941a2c..6c5a1c50c5f1 100644
> --- a/rust/kernel/time.rs
> +++ b/rust/kernel/time.rs
> @@ -8,9 +8,15 @@
>  //! C header: [`include/linux/jiffies.h`](srctree/include/linux/jiffies.h).
>  //! C header: [`include/linux/ktime.h`](srctree/include/linux/ktime.h).
>  
> +/// The number of nanoseconds per microsecond.
> +pub const NSEC_PER_USEC: i64 = bindings::NSEC_PER_USEC as i64;
> +
>  /// The number of nanoseconds per millisecond.
>  pub const NSEC_PER_MSEC: i64 = bindings::NSEC_PER_MSEC as i64;
>  
> +/// The number of nanoseconds per second.
> +pub const NSEC_PER_SEC: i64 = bindings::NSEC_PER_SEC as i64;
> +
>  /// The time unit of Linux kernel. One jiffy equals (1/HZ) second.
>  pub type Jiffies = core::ffi::c_ulong;
>  
> @@ -103,3 +109,61 @@ fn partial_cmp(&self, other: &Self) -> Option<core::cmp::Ordering> {
>          }
>      }
>  }
> +
> +/// A span of time.
> +#[derive(Copy, Clone)]
> +pub struct Delta {
> +    nanos: i64,
> +}
> +
> +impl Delta {
> +    /// Create a new `Delta` from a number of nanoseconds.
> +    #[inline]
> +    pub fn from_nanos(nanos: u16) -> Self {
> +        Self {
> +            nanos: nanos.into(),
> +        }
> +    }

Just throwing out an idea:

How about we clamp delay to ~1 year, with a pr_warn() if it needs to
actually clamp. All the APIs take or return a u64.

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

Another dumb rust question. How does the Rust compiler implement 64
bit division on 32 bit systems? GCC with C calls out to a library to
do it, and the kernel does not have that library. So you need to use
the kernel div_u64() function.

Did you compiler this code for a 32 bit system?

	Andrew