Re: [PATCH v4 5/5] rust: devres: implement register_release()

[Danilo Krummrich <dakr@kernel.org>]

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:
> > register_release() is useful when a device resource has associated data,
> > but does not require the capability of accessing it or manually releasing
> > it.
> > 
> > If we would want to be able to access the device resource and release the
> > device resource manually before the device is unbound, but still keep
> > access to the associated data, we could implement it as follows.
> > 
> > 	struct Registration<T> {
> > 	   inner: Devres<RegistrationInner>,
> > 	   data: T,
> > 	}
> > 
> > However, if we never need to access the resource or release it manually,
> > register_release() is great optimization for the above, since it does not
> > require the synchronization of the Devres type.
> > 
> > Suggested-by: Alice Ryhl <aliceryhl@google.com>
> > Signed-off-by: Danilo Krummrich <dakr@kernel.org>
> > ---
> >  rust/kernel/devres.rs | 73 +++++++++++++++++++++++++++++++++++++++++++
> >  1 file changed, 73 insertions(+)
> > 
> > diff --git a/rust/kernel/devres.rs b/rust/kernel/devres.rs
> > index 3ce8d6161778..92aca78874ff 100644
> > --- a/rust/kernel/devres.rs
> > +++ b/rust/kernel/devres.rs
> > @@ -353,3 +353,76 @@ pub fn register<T, E>(dev: &Device<Bound>, data: impl PinInit<T, E>, flags: Flag
> >  
> >      register_foreign(dev, data)
> >  }
> > +
> > +/// [`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.

Here's the diff for the change:

diff --git a/rust/kernel/devres.rs b/rust/kernel/devres.rs
index 92aca78874ff..42a9cd2812d8 100644
--- a/rust/kernel/devres.rs
+++ b/rust/kernel/devres.rs
@@ -358,12 +358,9 @@ pub fn register<T, E>(dev: &Device<Bound>, data: impl PinInit<T, E>, flags: Flag
 ///
 /// 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;
-
+pub trait Release<Ptr: ForeignOwnable> {
     /// Called once the [`Device`] given to [`register_release`] is unbound.
-    fn release(this: Self::Ptr);
+    fn release(this: Ptr);
 }

 /// Consume the `data`, [`Release::release`] and [`Drop::drop`] `data` once `dev` is unbound.
@@ -384,9 +381,7 @@ pub trait Release {
 ///     }
 /// }
 ///
-/// impl Release for Registration {
-///     type Ptr = Arc<Self>;
-///
+/// impl Release<Arc<Self>> for Registration {
 ///     fn release(this: Arc<Self>) {
 ///        // unregister
 ///     }
@@ -401,7 +396,7 @@ pub trait Release {
 pub fn register_release<P>(dev: &Device<Bound>, data: P) -> Result
 where
     P: ForeignOwnable,
-    P::Target: Release<Ptr = P> + Send,
+    P::Target: Release<P> + Send,
 {
     let ptr = data.into_foreign();

@@ -409,7 +404,7 @@ pub fn register_release<P>(dev: &Device<Bound>, data: P) -> Result
     unsafe extern "C" fn callback<P>(ptr: *mut kernel::ffi::c_void)
     where
         P: ForeignOwnable,
-        P::Target: Release<Ptr = P>,
+        P::Target: Release<P>,
     {
         // SAFETY: `ptr` is the pointer to the `ForeignOwnable` leaked above and hence valid.
         let data = unsafe { P::from_foreign(ptr.cast()) };