linux-kernel - Re: [PATCH v3 4/4] rust: devres: implement register

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Message-ID: <aF1sEnxdJaVeBZ3x@cassiopeiae>
Date: Thu, 26 Jun 2025 17:49:38 +0200
From: Danilo Krummrich <dakr@...nel.org>
To: Benno Lossin <lossin@...nel.org>
Cc: gregkh@...uxfoundation.org, rafael@...nel.org, ojeda@...nel.org,
	alex.gaynor@...il.com, boqun.feng@...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 v3 4/4] rust: devres: implement register_release()

On Thu, Jun 26, 2025 at 05:32:25PM +0200, Danilo Krummrich wrote:
> On Thu, Jun 26, 2025 at 04:41:55PM +0200, Benno Lossin wrote:
> > On Thu Jun 26, 2025 at 4:02 PM CEST, Danilo Krummrich wrote:
> > > On Thu, Jun 26, 2025 at 01:15:34PM +0200, Danilo Krummrich wrote:
> > >> On Thu, Jun 26, 2025 at 12:36:23PM +0200, Benno Lossin wrote:
> > >> > Or, we could change `Release` to be:
> > >> > 
> > >> >     pub trait Release {
> > >> >         type Ptr: ForeignOwnable;
> > >> > 
> > >> >         fn release(this: Self::Ptr);
> > >> >     }
> > >> > 
> > >> > and then `register_release` is:
> > >> > 
> > >> >     pub fn register_release<T: Release>(dev: &Device<Bound>, data: T::Ptr) -> Result
> > >> > 
> > >> > This way, one can store a `Box<T>` and get access to the `T` at the end.
> > >> 
> > >> I think this was also the case before? Well, it was P::Borrowed instead.
> > >> 
> > >> > Or if they store the value in an `Arc<T>`, they have the option to clone
> > >> > it and give it to somewhere else.
> > >> 
> > >> Anyways, I really like this proposal of implementing the Release trait.
> > >
> > > One downside seems to be that the compiler cannot infer T anymore with this
> > > function signature.
> > 
> > Yeah... That's a bit annoying.
> > 
> > We might be able to add an associated type to `ForeignOwnable` like
> > `Target` or `Inner` or whatever.
> 
> I think we already have `PointedTo` [1]? But I remember that I've seen a patch
> to remove it again [2].

Well, not exactly, I think. Arc, for instance, defines PointedTo as ArcInner<T>.
So, I think we indeed want something different.

> [1] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/rust/kernel/types.rs#n32
> [2] https://lore.kernel.org/all/20250612-pointed-to-v3-1-b009006d86a1@kernel.org/
> 
> > Then we could do:
> > 
> >     pub fn register_release<P>(dev: &Device<Bound>, data: P) -> Result
> >     where
> >         P: ForeignOwnable,
> >         P::Inner: Release<Ptr = P>,
> > 
> > ---
> > Cheers,
> > Benno