linux-kernel - Re: [PATCH v3 1/2] rust: miscdevice: add additional data to MiscDeviceRegistration

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Message-ID: <2025052104-mockup-dupe-2573@gregkh>
Date: Wed, 21 May 2025 14:04:22 +0200
From: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To: Danilo Krummrich <dakr@...nel.org>
Cc: Christian Schrefl <chrisi.schrefl@...il.com>,
	Miguel Ojeda <ojeda@...nel.org>,
	Alex Gaynor <alex.gaynor@...il.com>,
	Boqun Feng <boqun.feng@...il.com>, Gary Guo <gary@...yguo.net>,
	Björn Roy Baron <bjorn3_gh@...tonmail.com>,
	Benno Lossin <benno.lossin@...ton.me>,
	Andreas Hindborg <a.hindborg@...nel.org>,
	Alice Ryhl <aliceryhl@...gle.com>, Trevor Gross <tmgross@...ch.edu>,
	Arnd Bergmann <arnd@...db.de>, Lee Jones <lee@...nel.org>,
	Daniel Almeida <daniel.almeida@...labora.com>,
	Gerald Wisböck <gerald.wisboeck@...ther.ink>,
	rust-for-linux@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 1/2] rust: miscdevice: add additional data to
 MiscDeviceRegistration

On Wed, May 21, 2025 at 01:55:36PM +0200, Greg Kroah-Hartman wrote:
> On Sat, May 17, 2025 at 03:42:29PM +0200, Danilo Krummrich wrote:
> > On Sat, May 17, 2025 at 01:33:49PM +0200, Christian Schrefl wrote:
> > > +pub struct MiscDeviceRegistration<T: MiscDevice> {
> > >      #[pin]
> > >      inner: Opaque<bindings::miscdevice>,
> > > +    #[pin]
> > > +    data: UnsafePinned<T::RegistrationData>,
> > >      _t: PhantomData<T>,
> > >  }
> > 
> > I recommend not to store data within a Registration type itself.
> > 
> > I know that this is designed with the focus on using misc device directly from
> > the module scope; and in this context it works great.
> > 
> > However, it becomes quite suboptimal when used from a driver scope. For
> > instance, if the misc device is registered within a platform driver's probe()
> > function.
> > 
> > I know this probably isn't supported yet. At least, I assume it isn't supported
> > "officially", given that the abstraction does not provide an option to set a
> > parent device. Yet I think we should consider it.
> 
> It's going to be a requirement to properly set the parent device, and
> as you point out, this really should be in some sort of scope, not just
> a module.
> 
> But, we have two types of users of a misc device, one like this is
> written, for a module-scope, and one for the "normal" device scope.  The
> device scope is going to be tricker as it can, and will, get
> disconnected from the device separately from the misc device lifespan,
> so when that logic is added, it's going to be tricky as you point out.
> 
> So I'll take this now, but in the future this is going to have to be
> cleaned up and modified.

Nope, can't take it, it breaks the build from my tree:

error[E0432]: unresolved import `crate::types::UnsafePinned`
  --> rust/kernel/miscdevice.rs:20:37
   |
20 |     types::{ForeignOwnable, Opaque, UnsafePinned},
   |                                     ^^^^^^^^^^^^ no `UnsafePinned` in `types`

error[E0432]: unresolved import `pin_init::Wrapper`
  --> rust/kernel/miscdevice.rs:23:5
   |
23 | use pin_init::Wrapper;
   |     ^^^^^^^^^^^^^^^^^ no `Wrapper` in the root

error: aborting due to 2 previous errors

:(