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

[Alice Ryhl <aliceryhl@google.com>]

On Fri, Aug 08, 2025 at 08:42:30AM +0200, Benno Lossin wrote:
> 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 { ... }
>             }
>         },
>     }
> 
> But we can do that later. I'll file a good-first-issue.

This kind of macro breaks rustfmt, so I wouldn't recommend it. I would
suggest just using the native Rust pattern rather than introduce even
more macros.

Alice