Re: [PATCH v2 1/4] rust: time: add jiffies conversion helpers to Delta

[Andreas Hindborg <a.hindborg@kernel.org>]

"FUJITA Tomonori" <tomo@flapping.org> writes:

> From: FUJITA Tomonori <fujita.tomonori@gmail.com>
>
> Callers that hand a timeout to some C APIs need conversion between
> Delta and jiffies.
>
> Signed-off-by: FUJITA Tomonori <fujita.tomonori@gmail.com>
> ---
>  rust/kernel/time.rs | 30 ++++++++++++++++++++++++++++++
>  1 file changed, 30 insertions(+)
>
> diff --git a/rust/kernel/time.rs b/rust/kernel/time.rs
> index 363e93cbb139..cd054ea5df02 100644
> --- a/rust/kernel/time.rs
> +++ b/rust/kernel/time.rs
> @@ -377,6 +377,20 @@ impl Delta {
>      /// A span of time equal to zero.
>      pub const ZERO: Self = Self { nanos: 0 };
>
> +    /// Create a new [`Delta`] from a number of jiffies.
> +    ///
> +    /// If `jiffies` is large enough that the corresponding number of nanoseconds
> +    /// would overflow an `i64`, the result saturates to `i64::MAX` nanoseconds.
> +    /// The exact threshold depends on `CONFIG_HZ`.
> +    #[inline]
> +    pub fn from_jiffies(jiffies: u64) -> Self {
> +        let nanos = (u128::from(jiffies) * NSEC_PER_SEC as u128 / u128::from(bindings::HZ))
> +            .min(i64::MAX as u128);

In general for this patch, why are some widening operations using `from`
and others use `as u128`? Is it not possible to use into/form for all
the widening operations?

For the narrowing operations, I think we need `// CAST:` annotations.

Best regards,
Andreas Hindborg