Re: [PATCH v5 4/7] rust: time: add Delta::to_jiffies() for timeout conversion

[FUJITA Tomonori <tomo@flapping.org>]

On Fri, 07 Aug 2026 13:59:20 +0100
"Gary Guo" <gary@garyguo.net> wrote:

> On Fri Aug 7, 2026 at 1:38 PM BST, FUJITA Tomonori wrote:
>> On Thu, 06 Aug 2026 12:50:55 +0100
>> "Gary Guo" <gary@garyguo.net> wrote:
>>
>>> On Thu Aug 6, 2026 at 8:32 AM BST, FUJITA Tomonori wrote:
>>>> From: FUJITA Tomonori <fujita.tomonori@gmail.com>
>>>>
>>>> Add Delta<Nsec>::to_jiffies() conversion that rounds up so the
>>>> resulting timeout is never shorter than the requested span, clamps a
>>>> negative span to an immediate timeout, and saturates an overlong span
>>>> to the kernel's MAX_JIFFY_OFFSET "wait forever" value.
>>>>
>>>> Reviewed-by: Gary Guo <gary@garyguo.net>
>>>> Signed-off-by: FUJITA Tomonori <fujita.tomonori@gmail.com>
>>>> ---
>>>>  rust/kernel/time.rs | 21 +++++++++++++++++++++
>>>>  1 file changed, 21 insertions(+)
>>>>
>>>> diff --git a/rust/kernel/time.rs b/rust/kernel/time.rs
>>>> index 9d65a6c1992a..5844ee985a7b 100644
>>>> --- a/rust/kernel/time.rs
>>>> +++ b/rust/kernel/time.rs
>>>> @@ -551,6 +551,27 @@ pub fn as_millis_ceil(self) -> i64 {
>>>>          }
>>>>      }
>>>>  
>>>> +    /// Convert this span to a [`Delta<Jiffy>`] suitable for use as a timeout.
>>>> +    ///
>>>> +    /// The value is rounded up to the next whole jiffy, so the resulting
>>>> +    /// timeout is never shorter than `self` (as `msecs_to_jiffies()` does).
>>>> +    /// A negative span clamps to zero jiffies (an immediate timeout).
>>>> +    #[inline]
>>>> +    pub fn to_jiffies(self) -> Delta<Jiffy> {
>>>> +        let msecs = self.as_millis_ceil();
>>>> +
>>>> +        // CAST: `msecs` is clamped to `0..=c_uint::MAX`, so it is non-negative and
>>>> +        // fits in `c_uint`.
>>>> +        let msecs = msecs.clamp(0, i64::from(crate::ffi::c_uint::MAX)) as crate::ffi::c_uint;
>>>> +
>>>> +        // SAFETY: `__msecs_to_jiffies()` is always safe to call.
>>>> +        let jiffies = unsafe { bindings::__msecs_to_jiffies(msecs) };
>>>> +
>>>> +        // CAST: `__msecs_to_jiffies()` returns a value in `0..=MAX_JIFFY_OFFSET`, i.e.
>>>> +        // `((LONG_MAX >> 1) - 1)`, which is non-negative and well within `isize`.
>>>> +        Delta::<Jiffy>::from_jiffies(jiffies as isize)
>>>> +    }
>>> 
>>> This should be using nsec_to_jiffies?
>>
>> nsecs_to_jiffies() is documented as unsuitable here, time.c says:
>>
>>     Unlike {m,u}secs_to_jiffies, type of input is not unsigned int but
>>     u64.  And this doesn't return MAX_JIFFY_OFFSET since this function
>>     is designed for scheduler, not for use in device drivers to
>>     calculate timeout value.
>>
>> It also truncates, so it's not suitable for a timeout.
>>
>> __msecs_to_jiffies() is the timeout-oriented helper: it rounds up and
>> saturates to MAX_JIFFY_OFFSET, which is what a timeout wants.
> 
> Which sounds like is what exactly is suitable here. What you want should be a
> different named function that does the ceiling and clamping?

Agreed, the ceiling and clamping should not hide behind a plan
conversion name.

I will rename it to to_jiffies_timeout() as Andreas suggested.