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

[FUJITA Tomonori <tomo@flapping.org>]

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.