linux-kernel - Re: [PATCH net-next v2 1/6] rust: time: Implement PartialEq and PartialOrd for Ktime

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Message-ID: <CAH5fLghaLTMFS18cdjY6ntQ8BE85APjG71F1MvEEhxG667HU4A@mail.gmail.com>
Date: Mon, 7 Oct 2024 15:59:07 +0200
From: Alice Ryhl <aliceryhl@...gle.com>
To: Andrew Lunn <andrew@...n.ch>
Cc: FUJITA Tomonori <fujita.tomonori@...il.com>, finn@...enk.dev, netdev@...r.kernel.org, 
	rust-for-linux@...r.kernel.org, hkallweit1@...il.com, tmgross@...ch.edu, 
	ojeda@...nel.org, alex.gaynor@...il.com, gary@...yguo.net, 
	bjorn3_gh@...tonmail.com, benno.lossin@...ton.me, a.hindborg@...sung.com, 
	anna-maria@...utronix.de, frederic@...nel.org, tglx@...utronix.de, 
	arnd@...db.de, linux-kernel@...r.kernel.org
Subject: Re: [PATCH net-next v2 1/6] rust: time: Implement PartialEq and
 PartialOrd for Ktime

On Mon, Oct 7, 2024 at 3:16 PM Andrew Lunn <andrew@...n.ch> wrote:
>
> On Mon, Oct 07, 2024 at 10:41:23AM +0200, Alice Ryhl wrote:
> > On Mon, Oct 7, 2024 at 7:37 AM FUJITA Tomonori
> > <fujita.tomonori@...il.com> wrote:
> > >
> > > On Sun, 06 Oct 2024 12:28:59 +0200
> > > Fiona Behrens <finn@...enk.dev> wrote:
> > >
> > > >> Implement PartialEq and PartialOrd trait for Ktime by using C's
> > > >> ktime_compare function so two Ktime instances can be compared to
> > > >> determine whether a timeout is met or not.
> > > >
> > > > Why is this only PartialEq/PartialOrd? Could we either document why or implement Eq/Ord as well?
> > >
> > > Because what we need to do is comparing two Ktime instances so we
> > > don't need them?
> >
> > When you implement PartialEq without Eq, you are telling the reader
> > that this is a weird type such as floats where there exists values
> > that are not equal to themselves. That's not the case here, so don't
> > confuse the reader by leaving out `Eq`.
>
> This might be one of those areas where there needs to be a difference
> between C and Rust in terms of kernel rules. For C, there would need
> to be a user. Here you seem to be saying the type system needs it, for
> the type to be meaningful, even if there is no user?
>
> Without Eq, would the compiler complain on an == operation, saying it
> is not a valid operation? Is there a clear difference between nobody
> has implemented this yet, vs such an operation is impossible, such as
> your float example?

Think of it this way: I wrote an implementation of something that
works in situations A and B, but I only use it in situation A. Must I
write my program in a way to make it impossible to use it in situation
B? That's how I see this case. Implementing Eq does not involve adding
any new functions.

Alice