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

[Alice Ryhl <aliceryhl@google.com>]

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>,
+    ) -> 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,
-- 
Alice Ryhl <aliceryhl@google.com>