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

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Message-ID: <aF8V8hqUzjdZMZNe@tardis.local>
Date: Fri, 27 Jun 2025 15:06:42 -0700
From: Boqun Feng <boqun.feng@...il.com>
To: Benno Lossin <lossin@...nel.org>
Cc: Danilo Krummrich <dakr@...nel.org>, 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 Fri, Jun 27, 2025 at 01:19:53AM +0200, Benno Lossin wrote:
> 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

I think it might be quite common, for example, `Foo` may be a general
watchdog for a subsystem, for one driver, there might be multiple
devices that could feed the dog, for another driver, there might be only
one. For the first case we need Arc<Watchdog> or the second we can do
Box<Watchdog>.

What's the downside?

Regards,
Boqun

> or a non-shared manner and not both...
> 
> ---
> Cheers,
> Benno
>