Re: [PATCH] rust: miscdevice: add registration data to MiscDevice

["Gary Guo" <gary@garyguo.net>]

On Fri Aug 7, 2026 at 3:22 PM BST, Alice Ryhl wrote:
> Currently when fds for a miscdevice are opened, the only "global" data
> they are given access to is the MiscDeviceRegistration value. However,
> this value doesn't let you store any user-provided data, so there is no
> way for different fds from the same miscdevice to interact with each
> other. Thus, let the user specify a type to be stored in the
> MiscDeviceRegistration in which the user can store whichever data they
> would like.
>
> The intended user of this patch is Rust course material. Miscdevice is a
> nice and relatively easy to use API for someone's first driver, and
> being able to persist data from fd to fd allows the student to interact
> with their driver using 'cat' and 'echo', even though each call opens a
> new fd.
>
> Signed-off-by: Alice Ryhl <aliceryhl@google.com>
> ---
>  rust/kernel/miscdevice.rs        | 42 ++++++++++++++++++++++++++++++----------
>  samples/rust/rust_misc_device.rs |  3 ++-
>  2 files changed, 34 insertions(+), 11 deletions(-)
>
> diff --git a/rust/kernel/miscdevice.rs b/rust/kernel/miscdevice.rs
> index 3abef1b8543d..c4910918cd15 100644
> --- a/rust/kernel/miscdevice.rs
> +++ b/rust/kernel/miscdevice.rs
> @@ -31,7 +31,10 @@
>          Opaque, //
>      },
>  };
> -use core::marker::PhantomData;
> +use core::{
> +    marker::PhantomData,
> +    ops::Deref, //
> +};
>  
>  /// Options for creating a misc device.
>  #[derive(Copy, Clone)]
> @@ -62,25 +65,30 @@ pub const fn into_raw<T: MiscDevice>(self) -> bindings::miscdevice {
>  /// - Deregistration occurs exactly once in [`Drop`] via `misc_deregister()`.
>  /// - `inner` wraps a valid, pinned `miscdevice` created using
>  ///   [`MiscDeviceOptions::into_raw`].
> -#[repr(transparent)]
> +#[repr(C)]
>  #[pin_data(PinnedDrop)]
> -pub struct MiscDeviceRegistration<T> {
> +pub struct MiscDeviceRegistration<T: MiscDevice> {
>      #[pin]
>      inner: Opaque<bindings::miscdevice>,
> -    _t: PhantomData<T>,
> +    #[pin]
> +    data: T::RegistrationData,
>  }
>  
>  // SAFETY: It is allowed to call `misc_deregister` on a different thread from where you called
>  // `misc_register`.
> -unsafe impl<T> Send for MiscDeviceRegistration<T> {}
> +unsafe impl<T: MiscDevice> Send for MiscDeviceRegistration<T> where T::RegistrationData: Send {}
>  // SAFETY: All `&self` methods on this type are written to ensure that it is safe to call them in
> -// parallel.
> -unsafe impl<T> Sync for MiscDeviceRegistration<T> {}
> +// parallel. The `RegistrationData` type is always `Sync`.
> +unsafe impl<T: MiscDevice> Sync for MiscDeviceRegistration<T> {}
>  
>  impl<T: MiscDevice> MiscDeviceRegistration<T> {
>      /// Register a misc device.
> -    pub fn register(opts: MiscDeviceOptions) -> impl PinInit<Self, Error> {
> +    pub fn register(
> +        opts: MiscDeviceOptions,
> +        data: impl PinInit<T::RegistrationData, Error>,

We can just skip the `RegistrationData` and use `T`?

Best,
Gary


> +    ) -> impl PinInit<Self, Error> {
>          try_pin_init!(Self {
> +            data <- data,
>              inner <- Opaque::try_ffi_init(move |slot: *mut bindings::miscdevice| {
>                  // SAFETY: The initializer can write to the provided `slot`.
>                  unsafe { slot.write(opts.into_raw::<T>()) };
> @@ -88,11 +96,14 @@ pub fn register(opts: MiscDeviceOptions) -> impl PinInit<Self, Error> {
>                  // SAFETY: We just wrote the misc device options to the slot. The miscdevice will
>                  // get unregistered before `slot` is deallocated because the memory is pinned and
>                  // the destructor of this type deallocates the memory.
> +                //
> +                // The `data` field is `Sync + 'static`, so it's okay for the `open` callback to
> +                // access it until the destructor is invoked.
> +                //
>                  // INVARIANT: If this returns `Ok(())`, then the `slot` will contain a registered
>                  // misc device.
>                  to_result(unsafe { bindings::misc_register(slot) })
>              }),
> -            _t: PhantomData,
>          })
>      }
>  
> @@ -112,8 +123,16 @@ pub fn device(&self) -> &Device {
>      }
>  }
>  
> +impl<T: MiscDevice> Deref for MiscDeviceRegistration<T> {
> +    type Target = T::RegistrationData;
> +    #[inline]
> +    fn deref(&self) -> &T::RegistrationData {
> +        &self.data
> +    }
> +}
> +
>  #[pinned_drop]
> -impl<T> PinnedDrop for MiscDeviceRegistration<T> {
> +impl<T: MiscDevice> PinnedDrop for MiscDeviceRegistration<T> {
>      fn drop(self: Pin<&mut Self>) {
>          // SAFETY: We know that the device is registered by the type invariants.
>          unsafe { bindings::misc_deregister(self.inner.get()) };
> @@ -126,6 +145,9 @@ pub trait MiscDevice: Sized {
>      /// What kind of pointer should `Self` be wrapped in.
>      type Ptr: ForeignOwnable + Send + Sync;
>  
> +    /// The registration data shared between all open files for this character device.
> +    type RegistrationData: Sync + 'static;
> +
>      /// Called when the misc device is opened.
>      ///
>      /// The returned pointer will be stored as the private data for the file.
> diff --git a/samples/rust/rust_misc_device.rs b/samples/rust/rust_misc_device.rs
> index 41e26c825060..0bde071743ef 100644
> --- a/samples/rust/rust_misc_device.rs
> +++ b/samples/rust/rust_misc_device.rs
> @@ -156,7 +156,7 @@ fn init(_module: &'static ThisModule) -> impl PinInit<Self, Error> {
>          };
>  
>          try_pin_init!(Self {
> -            _miscdev <- MiscDeviceRegistration::register(options),
> +            _miscdev <- MiscDeviceRegistration::register(options, Ok(())),
>          })
>      }
>  }
> @@ -176,6 +176,7 @@ struct RustMiscDevice {
>  #[vtable]
>  impl MiscDevice for RustMiscDevice {
>      type Ptr = Pin<KBox<Self>>;
> +    type RegistrationData = ();
>  
>      fn open(_file: &File, misc: &MiscDeviceRegistration<Self>) -> Result<Pin<KBox<Self>>> {
>          let dev = ARef::from(misc.device());
>
> ---
> base-commit: 220190f97da558e67cd01c62f1b84fe77b267a5a
> change-id: 20260807-miscdevice-data-71c727e8b2e5
>
> Best regards,