Re: [PATCH 2/4] rust: devres: replace Devres::new_foreign_owned()

["Benno Lossin" <lossin@kernel.org>]

On Sat Jun 21, 2025 at 11:45 PM CEST, Danilo Krummrich wrote:
> On Sat, Jun 21, 2025 at 11:10:14PM +0200, Benno Lossin wrote:
>> On Thu Jun 12, 2025 at 4:51 PM CEST, Danilo Krummrich wrote:
>> > diff --git a/rust/kernel/cpufreq.rs b/rust/kernel/cpufreq.rs
>> > index b0a9c6182aec..f20636079f7a 100644
>> > --- a/rust/kernel/cpufreq.rs
>> > +++ b/rust/kernel/cpufreq.rs
>> > @@ -12,7 +12,7 @@
>> >      clk::Hertz,
>> >      cpumask,
>> >      device::{Bound, Device},
>> > -    devres::Devres,
>> > +    devres,
>> >      error::{code::*, from_err_ptr, from_result, to_result, Result, VTABLE_DEFAULT_ERROR},
>> >      ffi::{c_char, c_ulong},
>> >      prelude::*,
>> > @@ -910,7 +910,7 @@ unsafe impl<T: Driver> Sync for Registration<T> {}
>> >  /// thread.
>> >  unsafe impl<T: Driver> Send for Registration<T> {}
>> >  
>> > -impl<T: Driver> Registration<T> {
>> > +impl<T: Driver + 'static> Registration<T> {
>> 
>> This change should probably be its own patch? If not, then it should be
>> mentioned in the commit message.
>
> It's a consequence of register_foreign_boxed() requiring T: 'static.

Then let's put the bound on that function, since putting it on the impl
block also affects `Registration::new()`.

>> >      const VTABLE: bindings::cpufreq_driver = bindings::cpufreq_driver {
>> >          name: Self::copy_name(T::NAME),
>> >          boost_enabled: T::BOOST_ENABLED,
>> > @@ -1044,10 +1044,10 @@ pub fn new() -> Result<Self> {
>> >  
>> >      /// Same as [`Registration::new`], but does not return a [`Registration`] instance.
>> >      ///
>> > -    /// Instead the [`Registration`] is owned by [`Devres`] and will be revoked / dropped, once the
>> > +    /// Instead the [`Registration`] is owned by [`kernel::devres`] and will be dropped, once the
>> >      /// device is detached.
>> >      pub fn new_foreign_owned(dev: &Device<Bound>) -> Result {
>> 
>> I think we can improve the names here. How about `new_attached`? See
>> more below.
>
> I feel like the name pretty much nails it: it's a new instance that is not
> owned, by the Rust side, but by the C devres implementation (i.e. foreign
> owned), which automatically drops it when the device is unbound.

Yeah, but `foreign` is so unspecific... With `ForeignOwnable`, it makes
sense, since it could be anything.

> Maybe Registration::new_devres_owned() instead?

I like that one better, let's go with that.

>> > -        Devres::new_foreign_owned(dev, Self::new()?, GFP_KERNEL)
>> > +        devres::register_foreign_boxed(dev, Self::new()?, GFP_KERNEL)
>> >      }
>> >  }
>> 
>> > +/// Encapsulate `data` in a [`KBox`] and [`Drop::drop`] `data` once `dev` is unbound.
>> > +///
>> > +/// # Examples
>> > +///
>> > +/// ```no_run
>> > +/// use kernel::{device::{Bound, Device}, devres};
>> > +///
>> > +/// struct Registration;
>> > +///
>> > +/// impl Registration {
>> > +///     fn new() -> Self {
>> > +///         // register (e.g. class device, IRQ, etc.)
>> > +///
>> > +///         Self
>> > +///     }
>> > +/// }
>> > +///
>> > +/// impl Drop for Registration {
>> > +///     fn drop(&mut self) {
>> > +///        // unregister
>> > +///     }
>> > +/// }
>> > +///
>> > +/// fn from_bound_context(dev: &Device<Bound>) -> Result {
>> > +///     devres::register_foreign_boxed(dev, Registration::new(), GFP_KERNEL)
>> > +/// }
>> > +/// ```
>> > +pub fn register_foreign_boxed<T, E>(
>> 
>> I don't really get the name of this function. The data isn't really
>> foreign and that the user also shouldn't really care about the fact that
>> you use `KBox` under the hood.
>> 
>> How about we call this something like `attach_data`?
>
> Hm, I think attach_data() doesn't quite hit the point. Maybe just
> devres::register_owned() instead. I agree that 'boxed' is an unnecessary
> implementation detail.

I like `register_owned` better, but I'm not 100% convinced by the
`owned` part... The regular devres creation is called `Devres::new`,
right? How about we just call this `register`?

---
Cheers,
Benno