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

[Boqun Feng <boqun.feng@gmail.com>]

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);
    }

?

Regards,
Boqun

> +/// Consume the `data`, [`Release::release`] and [`Drop::drop`] `data` once `dev` is unbound.
> +///
> +/// # Examples
> +///
> +/// ```no_run
> +/// use kernel::{device::{Bound, Device}, devres, devres::Release, sync::Arc};
> +///
> +/// /// Registration of e.g. a class device, IRQ, etc.
> +/// struct Registration;
> +///
> +/// impl Registration {
> +///     fn new() -> Result<Arc<Self>> {
> +///         // register
> +///
> +///         Ok(Arc::new(Self, GFP_KERNEL)?)
> +///     }
> +/// }
> +///
> +/// impl Release for Registration {
> +///     type Ptr = Arc<Self>;
> +///
> +///     fn release(this: Arc<Self>) {
> +///        // unregister
> +///     }
> +/// }
> +///
> +/// fn from_bound_context(dev: &Device<Bound>) -> Result {
> +///     let reg = Registration::new()?;
> +///
> +///     devres::register_release(dev, reg.clone())
> +/// }
> +/// ```
> +pub fn register_release<P>(dev: &Device<Bound>, data: P) -> Result
> +where
> +    P: ForeignOwnable,
> +    P::Target: Release<Ptr = P> + Send,
> +{
> +    let ptr = data.into_foreign();
> +
> +    #[allow(clippy::missing_safety_doc)]
> +    unsafe extern "C" fn callback<P>(ptr: *mut kernel::ffi::c_void)
> +    where
> +        P: ForeignOwnable,
> +        P::Target: Release<Ptr = P>,
> +    {
> +        // SAFETY: `ptr` is the pointer to the `ForeignOwnable` leaked above and hence valid.
> +        let data = unsafe { P::from_foreign(ptr.cast()) };
> +
> +        P::Target::release(data);
> +    }
> +
> +    // SAFETY:
> +    // - `dev.as_raw()` is a pointer to a valid and bound device.
> +    // - `ptr` is a valid pointer the `ForeignOwnable` devres takes ownership of.
> +    to_result(unsafe {
> +        // `devm_add_action_or_reset()` also calls `callback` on failure, such that the
> +        // `ForeignOwnable` is released eventually.
> +        bindings::devm_add_action_or_reset(dev.as_raw(), Some(callback::<P>), ptr.cast())
> +    })
> +}
> -- 
> 2.49.0
>