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

[Andreas Hindborg <a.hindborg@kernel.org>]

"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.

Best regards,
Andreas Hindborg


[1] https://doc.rust-lang.org/reference/conditional-compilation.html#the-cfg-macro