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

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Message-ID: <877c5exuwu.fsf@kernel.org>
Date: Tue, 25 Feb 2025 09:58:41 +0100
From: Andreas Hindborg <a.hindborg@...nel.org>
To: "Lyude Paul" <lyude@...hat.com>
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

"Lyude Paul" <lyude@...hat.com> writes:

> On Mon, 2025-02-24 at 13:03 +0100, Andreas Hindborg wrote:
>> This patch allows timer handlers to report that they want a timer to be
>> restarted after the timer handler has finished executing.
>>
>> Also update the `hrtimer` documentation to showcase the new feature.
>>
>> Acked-by: Frederic Weisbecker <frederic@...nel.org>
>> Signed-off-by: Andreas Hindborg <a.hindborg@...nel.org>
>> ---
>>  rust/kernel/time/hrtimer.rs     | 19 ++++++++++++++++++-
>>  rust/kernel/time/hrtimer/arc.rs |  4 +---
>>  2 files changed, 19 insertions(+), 4 deletions(-)
>>
>> diff --git a/rust/kernel/time/hrtimer.rs b/rust/kernel/time/hrtimer.rs
>> index d08fd7de158d..a431c8b728ae 100644
>> --- a/rust/kernel/time/hrtimer.rs
>> +++ b/rust/kernel/time/hrtimer.rs
>> @@ -207,7 +207,7 @@ pub trait HrTimerCallback {
>>      type Pointer<'a>: RawHrTimerCallback;
>>
>>      /// Called by the timer logic when the timer fires.
>> -    fn run(this: <Self::Pointer<'_> as RawHrTimerCallback>::CallbackTarget<'_>)
>> +    fn run(this: <Self::Pointer<'_> as RawHrTimerCallback>::CallbackTarget<'_>) -> HrTimerRestart
>>      where
>>          Self: Sized;
>>  }
>> @@ -313,6 +313,23 @@ unsafe fn start(self_ptr: *const Self, expires: Ktime) {
>>      }
>>  }
>>
>> +/// Restart policy for timers.
>> +pub enum HrTimerRestart {
>> +    /// Timer should not be restarted.
>> +    NoRestart,
>> +    /// Timer should be restarted.
>> +    Restart,
>> +}
>
> Should we have #[derive(Copy, Clone, PartialEq, Eq)] here?

Yea, lets do that. `Debug` as well?

>
> 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.


Best regards,
Andreas Hindborg