linux-kernel - Re: [PATCH v9 04/13] rust: hrtimer: allow timer restart from timer handler

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Message-ID: <38d342776eee31c697095c9c9c7c68463910a584.camel@redhat.com>
Date: Wed, 26 Feb 2025 14:26:35 -0500
From: Lyude Paul <lyude@...hat.com>
To: Andreas Hindborg <a.hindborg@...nel.org>
Cc: Miguel Ojeda <ojeda@...nel.org>, Anna-Maria Behnsen	
 <anna-maria@...utronix.de>, Frederic Weisbecker <frederic@...nel.org>, 
 Thomas Gleixner <tglx@...utronix.de>, Danilo Krummrich <dakr@...nel.org>,
 Alex Gaynor <alex.gaynor@...il.com>,  Boqun Feng <boqun.feng@...il.com>,
 Gary Guo <gary@...yguo.net>, Björn Roy Baron	
 <bjorn3_gh@...tonmail.com>, Benno Lossin <benno.lossin@...ton.me>, Alice
 Ryhl	 <aliceryhl@...gle.com>, Trevor Gross <tmgross@...ch.edu>, Guangbo Cui
	 <2407018371@...com>, Dirk Behme <dirk.behme@...il.com>, Daniel Almeida	
 <daniel.almeida@...labora.com>, Tamir Duberstein <tamird@...il.com>, 
	rust-for-linux@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v9 04/13] rust: hrtimer: allow timer restart from timer
 handler

On Wed, 2025-02-26 at 14:43 +0100, Andreas Hindborg wrote:
> "Lyude Paul" <lyude@...hat.com> writes:
> 
> > On Tue, 2025-02-25 at 09:58 +0100, Andreas Hindborg wrote:
> > > "Lyude Paul" <lyude@...hat.com> writes:
> > > 
> > > > On Mon, 2025-02-24 at 13:03 +0100, Andreas Hindborg wrote:
> > > > 
> > > > Also, I feel like I might have asked this a few versions ago so hopefully i'm
> > > > not asking again: but what's the reason for us not just using the
> > > > discriminants of `HrTimerRestart` here:
> > > > 
> > > > /// Restart policy for timers.
> > > > #[repr(u32)]
> > > > pub enum HrTimerRestart {
> > > >     /// Timer should not be restarted.
> > > >     NoRestart = bindings::hrtimer_restart_HRTIMER_NORESTART,
> > > >     /// Timer should be restarted.
> > > >     Restart = bindings::hrtimer_restart_HRTIMER_RESTART,
> > > > }
> > > 
> > > I forget if we discussed this, but it does not make much of a
> > > difference, does it?
> > > 
> > > With a Rust enum, we get a smaller storage type maybe with better
> > > support for niche optimizations? And then pay a bit more for conversion.
> > > All in all, I don't think it makes much difference.
> > 
> > No idea about performance wise, but I -think- it would actually cut down on
> > the code that you need - particularly for the larger enums here. Mainly
> > because you only would need to manually specify each variant for converting
> > from bindings::hrtimer_restart to HrTimerRestart, but not the other way
> > around:
> > 
> >    /// Restart policy for timers.
> >    #[repr(u32)]
> >    pub enum HrTimerRestart {
> >        /// Timer should not be restarted.
> >        NoRestart = bindings::hrtimer_restart_HRTIMER_NORESTART,
> >        /// Timer should be restarted.
> >        Restart = bindings::hrtimer_restart_HRTIMER_RESTART,
> >    }
> > 
> >    impl From<bindings::hrtimer_restart> for HrTimerRestart {
> >        fn from(value: u32) -> Self {
> >            match value {
> >                bindings::hrtimer_restart_HRTIMER_NORESTART => Self::NoRestart,
> >                _ => Self::Restart,
> >            }
> >        }
> >    }
> > 
> >    impl From<HrTimerRestart> for bindings::hrtimer_restart {
> >        fn from(value: HrTimerRestart) -> Self {
> >            value as Self
> >        }
> >    }
> 
> I was implementing this, and it is fine for `HrTimerRestart`, but for
> `HrTimerMode` it does not work out. We have multiple flags with the same
> value:
> 
>   error[E0081]: discriminant value `2` assigned more than once
>     --> /home/aeh/src/linux-rust/hrtimer/rust/kernel/time/hrtimer.rs:689:1
>       |
>   689 | pub enum HrTimerMode {
>       | ^^^^^^^^^^^^^^^^^^^^
>   ...
>   695 |     Pinned = bindings::hrtimer_mode_HRTIMER_MODE_PINNED,
>       |              ------------------------------------------ `2` assigned here
>   ...
>   702 |     AbsolutePinned = bindings::hrtimer_mode_HRTIMER_MODE_ABS_PINNED,
>       |                      ---------------------------------------------- `2` assigned here
> 
> 
> Which is unfortunate. I'll keep the old style for this one and convert
> the others where applicable.

Interesting - I'm curious if maybe this is something that needs cleaning up on
the C side, just a side thought though.

> 
> 
> Best regards,
> Andreas Hindborg
> 
> 

-- 
Cheers,
 Lyude Paul (she/her)
 Software Engineer at Red Hat

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