linux-kernel - Re: [PATCH v8 02/14] rust: hrtimer: introduce hrtimer support

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Message-ID: <875xl3fvtm.fsf@kernel.org>
Date: Fri, 21 Feb 2025 11:15:01 +0100
From: Andreas Hindborg <a.hindborg@...nel.org>
To: "Benno Lossin" <benno.lossin@...ton.me>
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>,  "Alice Ryhl" <aliceryhl@...gle.com>,  "Trevor
 Gross" <tmgross@...ch.edu>,  "Lyude Paul" <lyude@...hat.com>,  "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 v8 02/14] rust: hrtimer: introduce hrtimer support

Andreas Hindborg <a.hindborg@...nel.org> writes:

> "Benno Lossin" <benno.lossin@...ton.me> writes:
>
>> On 18.02.25 14:27, Andreas Hindborg wrote:
>>> +pub trait HrTimerCallback {
>>> +    /// The type whose [`RawHrTimerCallback::run`] method will be invoked when
>>> +    /// the timer expires.
>>> +    type CallbackTarget<'a>: RawHrTimerCallback;
>>> +
>>> +    /// This type is passed to the timer callback function. It may be a borrow
>>> +    /// of [`Self::CallbackTarget`], or it may be `Self::CallbackTarget` if the
>>> +    /// implementation can guarantee exclusive access to the target during timer
>>
>> Technically "exclusive" access is correct if the `CallbackTarget` is
>> `Pin<&Self>`, since you will get exclusive access to a `Pin<&Self>`, but
>> it might confuse people, because there can be multiple `Pin<&Self>`. So
>> I would just drop the word "exclusive" here.
>
> Yes, maybe it should be "shared or exclusive access, depending on the type"?
>
>>
>>> +    /// handler execution.
>>> +    type CallbackTargetParameter<'a>;
>>
>> Also why can't this type be an associated type of `HrTimerPointer`?
>> Since this seems to always be constrained in the impls of
>> `RawHrTimerCallback`.
>
> That might be a nice improvement, I'll try that out.

Looking closer at this, I don't see how to achieve this. We need access
to the type here, because it is used in the signature of `run`.
`HrTimerCallback` has no bounds on it, and that is nice. If we want to
move these associated types, we have to introduce a bound here.

We need to be generic over the type of the parameter to `run`, and by
the time the user implements this trait, the type must be known and so
the user has to specify somehow.


Best regards,
Andreas Hindborg