linux-kernel - Re: [PATCH v4 5/5] rust: devres: implement register

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Message-Id: <DAWULS8SIOXS.1O4PLL2WCLX74@kernel.org>
Date: Fri, 27 Jun 2025 01:19:53 +0200
From: "Benno Lossin" <lossin@...nel.org>
To: "Danilo Krummrich" <dakr@...nel.org>, "Boqun Feng"
 <boqun.feng@...il.com>
Cc: <gregkh@...uxfoundation.org>, <rafael@...nel.org>, <ojeda@...nel.org>,
 <alex.gaynor@...il.com>, <gary@...yguo.net>, <bjorn3_gh@...tonmail.com>,
 <a.hindborg@...nel.org>, <aliceryhl@...gle.com>, <tmgross@...ch.edu>,
 <david.m.ertman@...el.com>, <ira.weiny@...el.com>, <leon@...nel.org>,
 <kwilczynski@...nel.org>, <bhelgaas@...gle.com>,
 <rust-for-linux@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
 <linux-pci@...r.kernel.org>
Subject: Re: [PATCH v4 5/5] rust: devres: implement register_release()

On Thu Jun 26, 2025 at 10:48 PM CEST, Danilo Krummrich wrote:
> On Thu, Jun 26, 2025 at 01:37:22PM -0700, Boqun Feng wrote:
>> On Thu, Jun 26, 2025 at 10:00:43PM +0200, Danilo Krummrich wrote:
>> > +/// [`Devres`]-releaseable resource.
>> > +///
>> > +/// Register an object implementing this trait with [`register_release`]. Its `release`
>> > +/// function will be called once the device is being unbound.
>> > +pub trait Release {
>> > +    /// The [`ForeignOwnable`] pointer type consumed by [`register_release`].
>> > +    type Ptr: ForeignOwnable;
>> > +
>> > +    /// Called once the [`Device`] given to [`register_release`] is unbound.
>> > +    fn release(this: Self::Ptr);
>> > +}
>> > +
>> 
>> I would like to point out the limitation of this design, say you have a
>> `Foo` that can ipml `Release`, with this, I think you could only support
>> either `Arc<Foo>` or `KBox<Foo>`. You cannot support both as the input
>> for `register_release()`. Maybe we want:
>> 
>>     pub trait Release<Ptr: ForeignOwnable> {
>>         fn release(this: Ptr);
>>     }
>
> Good catch! I think this wasn't possible without ForeignOwnable::Target.

Hmm do we really need that? Normally you either store a type in a shared
or a non-shared manner and not both...

---
Cheers,
Benno