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

["Gary Guo" <gary@garyguo.net>]

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?

Best,
Gary