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:

> On Wed, 15 Jul 2026 08:57:03 +0200
> Andreas Hindborg <a.hindborg@kernel.org> wrote:
>
>> "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?
>
> When NSEC_PER_SEC % HZ != 0, we need to multiply jiffies *
> NSEC_PER_SEC before dividing by HZ.
>
> What the C side does is:
>
> u64 jiffies64_to_nsecs(u64 j)
> {
> #if !(NSEC_PER_SEC % HZ)
> 	return (NSEC_PER_SEC / HZ) * j;
> # else
> 	return div_u64(j * HZ_TO_NSEC_NUM, HZ_TO_NSEC_DEN);
> #endif
> }
>
> We could replicate this with a cfg/the C side's const-eval
> computing. But I think using u128/i128 is simpler. What do you think?

I think widening to 128 bits is fine. My question is why `NSEC_PER_SEC
as u128` instead of `u128::from(NSEC_PER_SEC)`?

Best regards,
Andreas Hindborg