Re: [PATCH v3 15/16] rust: platform: add basic platform device / driver abstractions

[Danilo Krummrich <dakr@kernel.org>]

On Wed, Oct 30, 2024 at 04:50:43PM +0100, Alice Ryhl wrote:
> On Tue, Oct 22, 2024 at 11:33 PM Danilo Krummrich <dakr@kernel.org> wrote:
> > +/// Drivers must implement this trait in order to get a platform driver registered. Please refer to
> > +/// the `Adapter` documentation for an example.
> > +pub trait Driver {
> > +    /// The type holding information about each device id supported by the driver.
> > +    ///
> > +    /// TODO: Use associated_type_defaults once stabilized:
> > +    ///
> > +    /// type IdInfo: 'static = ();
> > +    type IdInfo: 'static;
> > +
> > +    /// The table of device ids supported by the driver.
> > +    const ID_TABLE: IdTable<Self::IdInfo>;
> > +
> > +    /// Platform driver probe.
> > +    ///
> > +    /// Called when a new platform device is added or discovered.
> > +    /// Implementers should attempt to initialize the device here.
> > +    fn probe(dev: &mut Device, id_info: Option<&Self::IdInfo>) -> Result<Pin<KBox<Self>>>;
> 
> This forces the user to put their driver data in a KBox, but they
> might want to use an Arc instead. You don't actually *need* a KBox -
> any ForeignOwnable seems to fit your purposes.

This is intentional, I do need a `KBox` here.

The reason is that I want to enforce that the returned `Pin<KBox<Self>>` has
exactly the lifetime of the binding of the device and driver, i.e. from probe()
until remove(). This is the lifetime the structure should actually represent.

This way we can attach things like `Registration` objects to this structure, or
anything else that should only exist from probe() until remove().

If a driver needs some private driver data that needs to be reference counted,
it is usually attached to the class representation of the driver.

For instance, in Nova the reference counted stuff is attached to the DRM device
and then I just have the DRM device (which itself is reference counted) embedded
in the `Driver` structure.

In any case, drivers can always embed a separate `Arc` in their `Driver`
structure if they really have a need for that.

- Danilo

> 
> Please see my miscdevice and shrinker patchsets for examples of how
> you can extend this to allow any ForeignOwnable.
> 
> Alice
>