Re: [PATCH v2 2/2] rust: time: Implement basic arithmetic operations for Delta

["Benno Lossin" <lossin@kernel.org>]

On Fri Aug 8, 2025 at 11:26 AM CEST, Andreas Hindborg wrote:
> "Benno Lossin" <lossin@kernel.org> writes:
>
>> On Thu Aug 7, 2025 at 9:06 PM CEST, Lyude Paul wrote:
>>> While rvkms is only going to be using a few of these, since Deltas are
>>> basically the same as i64 it's easy enough to just implement all of the
>>> basic arithmetic operations for Delta types.
>>>
>>> Keep in mind there's one quirk here - the kernel has no support for
>>> i64 % i64 on 32 bit platforms, the closest we have is i64 % i32 through
>>> div_s64_rem(). So, instead of implementing ops::Rem or ops::RemAssign we
>>> simply provide Delta::rem_nanos().
>>
>> We could still provide the trait implementations on CONFIG_64BIT? WDYT?
>>
>>> +impl ops::Div for Delta {
>>> +    type Output = Self;
>>> +
>>> +    #[inline]
>>> +    fn div(self, rhs: Self) -> Self::Output {
>>> +        #[cfg(CONFIG_64BIT)]
>>> +        {
>>
>> This pattern seems to be rather common in this patchset & in general I
>> think I've also seen it elsewhere. We should think about adding a
>> `if_cfg!` macro:
>>
>>     Self {
>>         nanos: if_cfg! {
>>             if CONFIG_64BIT {
>>                 self.nanos / rhs.nanos
>>             } else {
>>                 unsafe { ... }
>>             }
>>         },
>>     }
>>
>
> Why the need for a macro. `cfg!` is built-in [1]:
>
>   if cfg!(CONFIG_64BIT) {
>      ...
>   } else {
>      ...
>   }
>
> The conditional expression should be optimized statically and the dead
> part should be removed.

AIUI the `self.nanos / rhs.nanos` won't compile on 32 bit platforms.

---
Cheers,
Benno