Claude review: samples: rust: add Rust serial device bus sample device driver

Claude review: samples: rust: add Rust serial device bus sample device driver

[Claude Code Review Bot]

Patch Review

The sample is clean and demonstrates the abstraction well. A few notes:

**`set_baudrate` error handling is odd due to the API design:**

```rust
if sdev
    .set_baudrate(
        dev.fwnode()
            .and_then(|fwnode| fwnode.property_read(c"baudrate").optional())
            .unwrap_or(115200),
    )
    .is_err()
{
    return Err(EINVAL);
}
```

This discards the actual baudrate returned in the error. If `set_baudrate` returned `u32` instead of `Result<(), u32>`, this would be simpler and more informative.

**Echo server with `Timeout::Max` would hang:** Due to the `Timeout::Max` bug (returning 0), `write_all` in receive would return immediately or with ETIMEDOUT rather than actually waiting. This makes the sample non-functional in practice.

**The sample stores an `ARef<serdev::Device>` but never uses it** (except in Drop for a debug message). This is fine for a sample but worth noting.

---
Generated by Claude Code Patch Reviewer

Claude review: samples: rust: add Rust serial device bus sample device driver

[Claude Code Review Bot]

Patch Review

The sample is clean and demonstrates the API well.

**Missing dependency in Kconfig:**

```
+config SAMPLE_RUST_DRIVER_SERDEV
+	tristate "Serial Device Bus Device Driver"
+	depends on SERIAL_DEV_BUS
```

This should also `depends on RUST_SERIAL_DEV_BUS_ABSTRACTIONS` (or `select` it), since the sample uses `kernel::serdev` which is gated behind that config option.

**The sample stores an `ARef<serdev::Device>` but never uses it functionally:**

```rust
+struct SampleDriver {
+    sdev: ARef<serdev::Device>,
+}
```

It's only used in `Drop` for a debug message. This is fine for a sample, but the reference counting should be correct — `sdev.into()` in `probe` calls `impl_device_context_into_aref` which should increment the refcount.

**Minor: `"GPL v2"` vs `"GPL"`:**

```rust
+    license: "GPL v2",
```

Modern kernel modules typically use just `"GPL"`. `"GPL v2"` is also accepted but `"GPL"` is the recommended form.

---
Generated by Claude Code Patch Reviewer

Claude review: samples: rust: add Rust serial device bus sample device driver

[Claude Code Review Bot]

Patch Review

A clean sample that demonstrates probe, receive (echo/loopback), and cleanup. A few observations:

**Error in receive is swallowed**:

```rust
+    fn receive(sdev: &serdev::Device<Bound>, _this: Pin<&Self>, data: &[u8]) -> usize {
+        let _ = sdev.write_all(data, serdev::Timeout::Max);
+        data.len()
+    }
```

The `write_all` error is discarded with `let _ = ...`. For a sample this is acceptable, but a comment like `// Ignore write errors for simplicity` would be helpful.

**Strict baudrate matching**:

```rust
+        if sdev
+            .set_baudrate(...)
+            .is_err()
+        {
+            return Err(EINVAL);
+        }
```

This fails probe if the exact baudrate isn't matched. As noted above, many serial controllers approximate the baudrate. For a sample this is fine, but it could be confusing if someone tries to run this on real hardware.

**License string**: `"GPL v2"` is used, which is the correct form for `MODULE_LICENSE`.

**Overall the sample is clean and well-structured**, demonstrating the key abstractions (OF/ACPI ID tables, probe, receive, device reference management via `ARef`, Drop-based cleanup).

---
Generated by Claude Code Patch Reviewer

[PATCH v5 0/4] rust: add basic serial device bus abstractions

[Markus Probst]

This patch series adds the serdev device bus rust abstraction into the
kernel.

This abstraction will be used by a driver,
which targets the MCU devices in Synology devices.

Kari Argillander also messaged me, stating that he wants to write a
watchdog driver with this abstraction (needing initial device data).

@Rob: Are you willing to maintain these rust abstractions yourself,
as you are the expert on this subsystem, otherwise I would take care of
it with a "SERIAL DEVICE BUS [RUST]" section in the MAINTAINERS file. In
the second case, I assume you are going to pick those patches as-is into
your tree, after they have been reviewed?

Signed-off-by: Markus Probst <markus.probst@posteo.de>
---
Changes in v5:
- fix typo in documentation
- Link to v4: https://lore.kernel.org/r/20260411-rust_serdev-v4-0-845e960c6627@posteo.de

Changes in v4:
- fixed not selecting rust serdev abstraction in sample
- Link to v3: https://lore.kernel.org/r/20260313-rust_serdev-v3-0-c9a3af214f7f@posteo.de

Changes in v3:
- fix vertical import style
- add Kconfig entry for the rust abstraction
- fix documentation in include/linux/serdev.h
- rename private_data to rust_private_data
- fix `complete_all` <-> `wait_for_completion` typo
- move drvdata_borrow call after the completion
- Link to v2: https://lore.kernel.org/r/20260306-rust_serdev-v2-0-e9b23b42b255@posteo.de

Changes in v2:
- fix documentation in `serdev::Driver::write` and
  `serdev::Driver::write_all`
- remove use of `dev_info` in probe from the sample
- remove `properties_parse` from the sample
- add optional `baudrate` property to the sample
- remove 1. patch
- remove `TryFrom<&device::Device<Ctx>> for &serdev::Device<Ctx>`
  implementation
- fix import style
- add patch to return reference in `devres::register` to fix safety
  issue
- add patch to add private data to serdev_device, to fix
  `Device.drvdata()` from failing
- simplify abstraction by removing ability to receive the initial
  transmission. It may be added later in a separate patch series if
  needed.
- Link to v1: https://lore.kernel.org/r/20251220-rust_serdev-v1-0-e44645767621@posteo.de

---
Markus Probst (4):
      rust: devres: return reference in `devres::register`
      serdev: add rust private data to serdev_device
      rust: add basic serial device bus abstractions
      samples: rust: add Rust serial device bus sample device driver

 drivers/tty/serdev/Kconfig         |   7 +
 include/linux/serdev.h             |  15 +-
 rust/bindings/bindings_helper.h    |   1 +
 rust/helpers/helpers.c             |   1 +
 rust/helpers/serdev.c              |  22 ++
 rust/kernel/cpufreq.rs             |   3 +-
 rust/kernel/devres.rs              |  15 +-
 rust/kernel/drm/driver.rs          |   3 +-
 rust/kernel/lib.rs                 |   2 +
 rust/kernel/serdev.rs              | 536 +++++++++++++++++++++++++++++++++++++
 samples/rust/Kconfig               |  11 +
 samples/rust/Makefile              |   1 +
 samples/rust/rust_driver_serdev.rs |  86 ++++++
 13 files changed, 693 insertions(+), 10 deletions(-)
---
base-commit: 3131ff5a117498bb4b9db3a238bb311cbf8383ce
change-id: 20251217-rust_serdev-ee5481e9085c

[PATCH v5 1/4] rust: devres: return reference in `devres::register`

[Markus Probst]

Return the reference to the initialized data in the `devres::register`
function.

This is needed in a following commit (rust: add basic serial device bus
abstractions).

Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Markus Probst <markus.probst@posteo.de>
---
 rust/kernel/cpufreq.rs    |  3 ++-
 rust/kernel/devres.rs     | 15 +++++++++++++--
 rust/kernel/drm/driver.rs |  3 ++-
 3 files changed, 17 insertions(+), 4 deletions(-)

diff --git a/rust/kernel/cpufreq.rs b/rust/kernel/cpufreq.rs
index f5adee48d40c..31bf7e685097 100644
--- a/rust/kernel/cpufreq.rs
+++ b/rust/kernel/cpufreq.rs
@@ -1052,7 +1052,8 @@ pub fn new_foreign_owned(dev: &Device<Bound>) -> Result
     where
         T: 'static,
     {
-        devres::register(dev, Self::new()?, GFP_KERNEL)
+        devres::register(dev, Self::new()?, GFP_KERNEL)?;
+        Ok(())
     }
 }
 
diff --git a/rust/kernel/devres.rs b/rust/kernel/devres.rs
index 6afe196be42c..f882bace8601 100644
--- a/rust/kernel/devres.rs
+++ b/rust/kernel/devres.rs
@@ -326,15 +326,26 @@ fn register_foreign<P>(dev: &Device<Bound>, data: P) -> Result
 /// }
 ///
 /// fn from_bound_context(dev: &Device<Bound>) -> Result {
-///     devres::register(dev, Registration::new(), GFP_KERNEL)
+///     devres::register(dev, Registration::new(), GFP_KERNEL)?;
+///     Ok(())
 /// }
 /// ```
-pub fn register<T, E>(dev: &Device<Bound>, data: impl PinInit<T, E>, flags: Flags) -> Result
+pub fn register<'a, T, E>(
+    dev: &'a Device<Bound>,
+    data: impl PinInit<T, E>,
+    flags: Flags,
+) -> Result<&'a T>
 where
     T: Send + 'static,
     Error: From<E>,
 {
     let data = KBox::pin_init(data, flags)?;
 
+    let data_ptr = &raw const *data;
+
     register_foreign(dev, data)
+        // SAFETY: `dev` is valid for the lifetime of 'a. As long as there is a reference to
+        // `Device<Bound>`, it is guaranteed that the device is not unbound and data has not been
+        // dropped. Thus `data_ptr` is also valid for the lifetime of 'a.
+        .map(|()| unsafe { &*data_ptr })
 }
diff --git a/rust/kernel/drm/driver.rs b/rust/kernel/drm/driver.rs
index e09f977b5b51..51e0c7e30cc2 100644
--- a/rust/kernel/drm/driver.rs
+++ b/rust/kernel/drm/driver.rs
@@ -145,7 +145,8 @@ pub fn new_foreign_owned(
 
         let reg = Registration::<T>::new(drm, flags)?;
 
-        devres::register(dev, reg, GFP_KERNEL)
+        devres::register(dev, reg, GFP_KERNEL)?;
+        Ok(())
     }
 
     /// Returns a reference to the `Device` instance for this registration.

-- 
2.52.0

[PATCH v5 2/4] serdev: add rust private data to serdev_device

[Markus Probst]

Add rust private data to `struct serdev_device`, as it is required by the
rust abstraction added in the following commit
(rust: add basic serial device bus abstractions).

Signed-off-by: Markus Probst <markus.probst@posteo.de>
---
 include/linux/serdev.h | 15 +++++++++------
 1 file changed, 9 insertions(+), 6 deletions(-)

diff --git a/include/linux/serdev.h b/include/linux/serdev.h
index 5654c58eb73c..c74c345d60ae 100644
--- a/include/linux/serdev.h
+++ b/include/linux/serdev.h
@@ -33,12 +33,14 @@ struct serdev_device_ops {
 
 /**
  * struct serdev_device - Basic representation of an serdev device
- * @dev:	Driver model representation of the device.
- * @nr:		Device number on serdev bus.
- * @ctrl:	serdev controller managing this device.
- * @ops:	Device operations.
- * @write_comp	Completion used by serdev_device_write() internally
- * @write_lock	Lock to serialize access when writing data
+ * @dev:	       Driver model representation of the device.
+ * @nr:		       Device number on serdev bus.
+ * @ctrl:	       serdev controller managing this device.
+ * @ops:	       Device operations.
+ * @write_comp:	       Completion used by serdev_device_write() internally
+ * @write_lock:	       Lock to serialize access when writing data
+ * @rust_private_data: Private data for the rust abstraction. This should
+ *		       not be used by the C drivers.
  */
 struct serdev_device {
 	struct device dev;
@@ -47,6 +49,7 @@ struct serdev_device {
 	const struct serdev_device_ops *ops;
 	struct completion write_comp;
 	struct mutex write_lock;
+	void *rust_private_data;
 };
 
 static inline struct serdev_device *to_serdev_device(struct device *d)

-- 
2.52.0

[PATCH v5 3/4] rust: add basic serial device bus abstractions

[Markus Probst]

Implement the basic serial device bus abstractions required to write a
serial device bus device driver with or without the need for initial device
data. This includes the following data structures:

The `serdev::Driver` trait represents the interface to the driver.

The `serdev::Device` abstraction represents a `struct serdev_device`.

In order to provide the Serdev specific parts to a generic
`driver::Registration` the `driver::RegistrationOps` trait is
implemented by `serdev::Adapter`.

Signed-off-by: Markus Probst <markus.probst@posteo.de>
---
 drivers/tty/serdev/Kconfig      |   7 +
 rust/bindings/bindings_helper.h |   1 +
 rust/helpers/helpers.c          |   1 +
 rust/helpers/serdev.c           |  22 ++
 rust/kernel/lib.rs              |   2 +
 rust/kernel/serdev.rs           | 536 ++++++++++++++++++++++++++++++++++++++++
 6 files changed, 569 insertions(+)

diff --git a/drivers/tty/serdev/Kconfig b/drivers/tty/serdev/Kconfig
index 46ae732bfc68..e6dfe949ad01 100644
--- a/drivers/tty/serdev/Kconfig
+++ b/drivers/tty/serdev/Kconfig
@@ -9,6 +9,13 @@ menuconfig SERIAL_DEV_BUS
 
 	  Note that you typically also want to enable TTY port controller support.
 
+config RUST_SERIAL_DEV_BUS_ABSTRACTIONS
+	bool "Rust Serial device bus abstractions"
+	depends on RUST
+	select SERIAL_DEV_BUS
+	help
+	  This enables the Rust abstraction for the serial device bus API.
+
 if SERIAL_DEV_BUS
 
 config SERIAL_DEV_CTRL_TTYPORT
diff --git a/rust/bindings/bindings_helper.h b/rust/bindings/bindings_helper.h
index 083cc44aa952..ab521ba42673 100644
--- a/rust/bindings/bindings_helper.h
+++ b/rust/bindings/bindings_helper.h
@@ -80,6 +80,7 @@
 #include <linux/regulator/consumer.h>
 #include <linux/sched.h>
 #include <linux/security.h>
+#include <linux/serdev.h>
 #include <linux/slab.h>
 #include <linux/sys_soc.h>
 #include <linux/task_work.h>
diff --git a/rust/helpers/helpers.c b/rust/helpers/helpers.c
index a3c42e51f00a..9b87e9591cfd 100644
--- a/rust/helpers/helpers.c
+++ b/rust/helpers/helpers.c
@@ -53,6 +53,7 @@
 #include "regulator.c"
 #include "scatterlist.c"
 #include "security.c"
+#include "serdev.c"
 #include "signal.c"
 #include "slab.c"
 #include "spinlock.c"
diff --git a/rust/helpers/serdev.c b/rust/helpers/serdev.c
new file mode 100644
index 000000000000..c52b78ca3fc7
--- /dev/null
+++ b/rust/helpers/serdev.c
@@ -0,0 +1,22 @@
+// SPDX-License-Identifier: GPL-2.0
+
+#include <linux/serdev.h>
+
+__rust_helper
+void rust_helper_serdev_device_driver_unregister(struct serdev_device_driver *sdrv)
+{
+	serdev_device_driver_unregister(sdrv);
+}
+
+__rust_helper
+void rust_helper_serdev_device_put(struct serdev_device *serdev)
+{
+	serdev_device_put(serdev);
+}
+
+__rust_helper
+void rust_helper_serdev_device_set_client_ops(struct serdev_device *serdev,
+					      const struct serdev_device_ops *ops)
+{
+	serdev_device_set_client_ops(serdev, ops);
+}
diff --git a/rust/kernel/lib.rs b/rust/kernel/lib.rs
index d93292d47420..5107c9c1be07 100644
--- a/rust/kernel/lib.rs
+++ b/rust/kernel/lib.rs
@@ -144,6 +144,8 @@
 pub mod scatterlist;
 pub mod security;
 pub mod seq_file;
+#[cfg(CONFIG_RUST_SERIAL_DEV_BUS_ABSTRACTIONS)]
+pub mod serdev;
 pub mod sizes;
 pub mod slice;
 #[cfg(CONFIG_SOC_BUS)]
diff --git a/rust/kernel/serdev.rs b/rust/kernel/serdev.rs
new file mode 100644
index 000000000000..f1464e6a3fd6
--- /dev/null
+++ b/rust/kernel/serdev.rs
@@ -0,0 +1,536 @@
+// SPDX-License-Identifier: GPL-2.0
+
+//! Abstractions for the serial device bus.
+//!
+//! C header: [`include/linux/serdev.h`](srctree/include/linux/serdev.h)
+
+use crate::{
+    acpi,
+    device,
+    devres,
+    driver,
+    error::{
+        from_result,
+        to_result,
+        VTABLE_DEFAULT_ERROR, //
+    },
+    of,
+    prelude::*,
+    sync::Completion,
+    time::{
+        msecs_to_jiffies,
+        Jiffies,
+        Msecs, //
+    },
+    types::{
+        AlwaysRefCounted,
+        Opaque, //
+    }, //
+};
+
+use core::{
+    cell::UnsafeCell,
+    marker::PhantomData,
+    mem::offset_of,
+    num::NonZero,
+    ptr::NonNull, //
+};
+
+/// Parity bit to use with a serial device.
+#[repr(u32)]
+pub enum Parity {
+    /// No parity bit.
+    None = bindings::serdev_parity_SERDEV_PARITY_NONE,
+    /// Even partiy.
+    Even = bindings::serdev_parity_SERDEV_PARITY_EVEN,
+    /// Odd parity.
+    Odd = bindings::serdev_parity_SERDEV_PARITY_ODD,
+}
+
+/// Timeout in Jiffies.
+pub enum Timeout {
+    /// Wait for a specific amount of [`Jiffies`].
+    Jiffies(NonZero<Jiffies>),
+    /// Wait for a specific amount of [`Msecs`].
+    Milliseconds(NonZero<Msecs>),
+    /// Wait as long as possible.
+    ///
+    /// This is equivalent to [`kernel::task::MAX_SCHEDULE_TIMEOUT`].
+    Max,
+}
+
+impl Timeout {
+    fn into_jiffies(self) -> isize {
+        match self {
+            Self::Jiffies(value) => value.get().try_into().unwrap_or_default(),
+            Self::Milliseconds(value) => {
+                msecs_to_jiffies(value.get()).try_into().unwrap_or_default()
+            }
+            Self::Max => 0,
+        }
+    }
+}
+
+/// An adapter for the registration of serial device bus device drivers.
+pub struct Adapter<T: Driver>(T);
+
+// SAFETY:
+// - `bindings::serdev_device_driver` is a C type declared as `repr(C)`.
+// - `Drvdata<T>` is the type of the driver's device private data.
+// - `struct serdev_device_driver` embeds a `struct device_driver`.
+// - `DEVICE_DRIVER_OFFSET` is the correct byte offset to the embedded `struct device_driver`.
+unsafe impl<T: Driver + 'static> driver::DriverLayout for Adapter<T> {
+    type DriverType = bindings::serdev_device_driver;
+    type DriverData = T;
+    const DEVICE_DRIVER_OFFSET: usize = core::mem::offset_of!(Self::DriverType, driver);
+}
+
+// SAFETY: A call to `unregister` for a given instance of `DriverType` is guaranteed to be valid if
+// a preceding call to `register` has been successful.
+unsafe impl<T: Driver + 'static> driver::RegistrationOps for Adapter<T> {
+    unsafe fn register(
+        sdrv: &Opaque<Self::DriverType>,
+        name: &'static CStr,
+        module: &'static ThisModule,
+    ) -> Result {
+        let of_table = match T::OF_ID_TABLE {
+            Some(table) => table.as_ptr(),
+            None => core::ptr::null(),
+        };
+
+        let acpi_table = match T::ACPI_ID_TABLE {
+            Some(table) => table.as_ptr(),
+            None => core::ptr::null(),
+        };
+
+        // SAFETY: It's safe to set the fields of `struct serdev_device_driver` on initialization.
+        unsafe {
+            (*sdrv.get()).driver.name = name.as_char_ptr();
+            (*sdrv.get()).probe = Some(Self::probe_callback);
+            (*sdrv.get()).remove = Some(Self::remove_callback);
+            (*sdrv.get()).driver.of_match_table = of_table;
+            (*sdrv.get()).driver.acpi_match_table = acpi_table;
+        }
+
+        // SAFETY: `sdrv` is guaranteed to be a valid `DriverType`.
+        to_result(unsafe { bindings::__serdev_device_driver_register(sdrv.get(), module.0) })
+    }
+
+    unsafe fn unregister(sdrv: &Opaque<Self::DriverType>) {
+        // SAFETY: `sdrv` is guaranteed to be a valid `DriverType`.
+        unsafe { bindings::serdev_device_driver_unregister(sdrv.get()) };
+    }
+}
+
+#[pin_data]
+struct PrivateData {
+    #[pin]
+    probe_complete: Completion,
+    error: UnsafeCell<bool>,
+}
+
+impl<T: Driver + 'static> Adapter<T> {
+    const OPS: &'static bindings::serdev_device_ops = &bindings::serdev_device_ops {
+        receive_buf: if T::HAS_RECEIVE {
+            Some(Self::receive_buf_callback)
+        } else {
+            None
+        },
+        write_wakeup: Some(bindings::serdev_device_write_wakeup),
+    };
+
+    extern "C" fn probe_callback(sdev: *mut bindings::serdev_device) -> kernel::ffi::c_int {
+        // SAFETY: The serial device bus only ever calls the probe callback with a valid pointer to
+        // a `struct serdev_device`.
+        //
+        // INVARIANT: `sdev` is valid for the duration of `probe_callback()`.
+        let sdev = unsafe { &*sdev.cast::<Device<device::CoreInternal>>() };
+        let id_info = <Self as driver::Adapter>::id_info(sdev.as_ref());
+
+        from_result(|| {
+            let private_data = devres::register(
+                sdev.as_ref(),
+                try_pin_init!(PrivateData {
+                    probe_complete <- Completion::new(),
+                    error: false.into(),
+                }),
+                GFP_KERNEL,
+            )?;
+
+            // SAFETY: `sdev.as_raw()` is guaranteed to be a valid pointer to `serdev_device`.
+            unsafe {
+                (*sdev.as_raw()).rust_private_data =
+                    (&raw const *private_data).cast::<c_void>().cast_mut()
+            };
+
+            // SAFETY: `sdev.as_raw()` is guaranteed to be a valid pointer to `serdev_device`.
+            unsafe { bindings::serdev_device_set_client_ops(sdev.as_raw(), Self::OPS) };
+
+            // SAFETY: The serial device bus only ever calls the probe callback with a valid pointer
+            // to a `serdev_device`.
+            to_result(unsafe {
+                bindings::devm_serdev_device_open(sdev.as_ref().as_raw(), sdev.as_raw())
+            })?;
+
+            let data = T::probe(sdev, id_info);
+            let result = sdev.as_ref().set_drvdata(data);
+
+            // SAFETY: We have exclusive access to `private_data.error`.
+            unsafe { *private_data.error.get() = result.is_err() };
+
+            private_data.probe_complete.complete_all();
+
+            result.map(|()| 0)
+        })
+    }
+
+    extern "C" fn remove_callback(sdev: *mut bindings::serdev_device) {
+        // SAFETY: The serial device bus only ever calls the remove callback with a valid pointer
+        // to a `struct serdev_device`.
+        //
+        // INVARIANT: `sdev` is valid for the duration of `remove_callback()`.
+        let sdev = unsafe { &*sdev.cast::<Device<device::CoreInternal>>() };
+
+        // SAFETY: `remove_callback` is only ever called after a successful call to
+        // `probe_callback`, hence it's guaranteed that `Device::set_drvdata()` has been called
+        // and stored a `Pin<KBox<T>>`.
+        let data = unsafe { sdev.as_ref().drvdata_borrow::<T>() };
+
+        T::unbind(sdev, data);
+    }
+
+    extern "C" fn receive_buf_callback(
+        sdev: *mut bindings::serdev_device,
+        buf: *const u8,
+        length: usize,
+    ) -> usize {
+        // SAFETY: The serial device bus only ever calls the receive buf callback with a valid
+        // pointer to a `struct serdev_device`.
+        //
+        // INVARIANT: `sdev` is valid for the duration of `receive_buf_callback()`.
+        let sdev = unsafe { &*sdev.cast::<Device<device::CoreInternal>>() };
+
+        // SAFETY:
+        // - The serial device bus only ever calls the receive buf callback with a valid pointer to
+        //   a `struct serdev_device`.
+        // - `receive_buf_callback` is only ever called after a successful call to
+        //   `probe_callback`, hence it's guaranteed that `sdev.private_data` is a pointer
+        //   to a valid `PrivateData`.
+        let private_data = unsafe { &*(*sdev.as_raw()).rust_private_data.cast::<PrivateData>() };
+
+        private_data.probe_complete.wait_for_completion();
+
+        // SAFETY: No one has exclusive access to `private_data.error`.
+        if unsafe { *private_data.error.get() } {
+            return length;
+        }
+
+        // SAFETY: `receive_buf_callback` is only ever called after a successful call to
+        // `probe_callback`, hence it's guaranteed that `Device::set_drvdata()` has been called
+        // and stored a `Pin<KBox<T>>`.
+        let data = unsafe { sdev.as_ref().drvdata_borrow::<T>() };
+
+        // SAFETY: `buf` is guaranteed to be non-null and has the size of `length`.
+        let buf = unsafe { core::slice::from_raw_parts(buf, length) };
+
+        T::receive(sdev, data, buf)
+    }
+}
+
+impl<T: Driver + 'static> driver::Adapter for Adapter<T> {
+    type IdInfo = T::IdInfo;
+
+    fn of_id_table() -> Option<of::IdTable<Self::IdInfo>> {
+        T::OF_ID_TABLE
+    }
+
+    fn acpi_id_table() -> Option<acpi::IdTable<Self::IdInfo>> {
+        T::ACPI_ID_TABLE
+    }
+}
+
+/// Declares a kernel module that exposes a single serial device bus device driver.
+///
+/// # Examples
+///
+/// ```ignore
+/// kernel::module_serdev_device_driver! {
+///     type: MyDriver,
+///     name: "Module name",
+///     authors: ["Author name"],
+///     description: "Description",
+///     license: "GPL v2",
+/// }
+/// ```
+#[macro_export]
+macro_rules! module_serdev_device_driver {
+    ($($f:tt)*) => {
+        $crate::module_driver!(<T>, $crate::serdev::Adapter<T>, { $($f)* });
+    };
+}
+
+/// The serial device bus device driver trait.
+///
+/// Drivers must implement this trait in order to get a serial device bus device driver registered.
+///
+/// # Examples
+///
+///```
+/// # use kernel::{
+///     acpi,
+///     bindings,
+///     device::{
+///         Bound,
+///         Core, //
+///     },
+///     of,
+///     serdev, //
+/// };
+///
+/// struct MyDriver;
+///
+/// kernel::of_device_table!(
+///     OF_TABLE,
+///     MODULE_OF_TABLE,
+///     <MyDriver as serdev::Driver>::IdInfo,
+///     [
+///         (of::DeviceId::new(c"test,device"), ())
+///     ]
+/// );
+///
+/// kernel::acpi_device_table!(
+///     ACPI_TABLE,
+///     MODULE_ACPI_TABLE,
+///     <MyDriver as serdev::Driver>::IdInfo,
+///     [
+///         (acpi::DeviceId::new(c"LNUXBEEF"), ())
+///     ]
+/// );
+///
+/// #[vtable]
+/// impl serdev::Driver for MyDriver {
+///     type IdInfo = ();
+///     const OF_ID_TABLE: Option<of::IdTable<Self::IdInfo>> = Some(&OF_TABLE);
+///     const ACPI_ID_TABLE: Option<acpi::IdTable<Self::IdInfo>> = Some(&ACPI_TABLE);
+///
+///     fn probe(
+///         sdev: &serdev::Device<Core>,
+///         _id_info: Option<&Self::IdInfo>,
+///     ) -> impl PinInit<Self, Error> {
+///         sdev.set_baudrate(115200);
+///         sdev.write_all(b"Hello\n", serdev::Timeout::Max)?;
+///         Ok(MyDriver)
+///     }
+/// }
+///```
+#[vtable]
+pub trait Driver: Send {
+    /// The type holding driver private data about each device id supported by the driver.
+    // TODO: Use associated_type_defaults once stabilized:
+    //
+    // ```
+    // type IdInfo: 'static = ();
+    // ```
+    type IdInfo: 'static;
+
+    /// The table of OF device ids supported by the driver.
+    const OF_ID_TABLE: Option<of::IdTable<Self::IdInfo>> = None;
+
+    /// The table of ACPI device ids supported by the driver.
+    const ACPI_ID_TABLE: Option<acpi::IdTable<Self::IdInfo>> = None;
+
+    /// Serial device bus device driver probe.
+    ///
+    /// Called when a new serial device bus device is added or discovered.
+    /// Implementers should attempt to initialize the device here.
+    fn probe(
+        sdev: &Device<device::Core>,
+        id_info: Option<&Self::IdInfo>,
+    ) -> impl PinInit<Self, Error>;
+
+    /// Serial device bus device driver unbind.
+    ///
+    /// Called when a [`Device`] is unbound from its bound [`Driver`]. Implementing this callback
+    /// is optional.
+    ///
+    /// This callback serves as a place for drivers to perform teardown operations that require a
+    /// `&Device<Core>` or `&Device<Bound>` reference. For instance.
+    ///
+    /// Otherwise, release operations for driver resources should be performed in `Self::drop`.
+    fn unbind(sdev: &Device<device::Core>, this: Pin<&Self>) {
+        let _ = (sdev, this);
+    }
+
+    /// Serial device bus device data receive callback.
+    ///
+    /// Called when data got received from device.
+    ///
+    /// Returns the number of bytes accepted.
+    fn receive(sdev: &Device<device::Bound>, this: Pin<&Self>, data: &[u8]) -> usize {
+        let _ = (sdev, this, data);
+        build_error!(VTABLE_DEFAULT_ERROR)
+    }
+}
+
+/// The serial device bus device representation.
+///
+/// This structure represents the Rust abstraction for a C `struct serdev_device`. The
+/// implementation abstracts the usage of an already existing C `struct serdev_device` within Rust
+/// code that we get passed from the C side.
+///
+/// # Invariants
+///
+/// A [`Device`] instance represents a valid `struct serdev_device` created by the C portion of
+/// the kernel.
+#[repr(transparent)]
+pub struct Device<Ctx: device::DeviceContext = device::Normal>(
+    Opaque<bindings::serdev_device>,
+    PhantomData<Ctx>,
+);
+
+impl<Ctx: device::DeviceContext> Device<Ctx> {
+    fn as_raw(&self) -> *mut bindings::serdev_device {
+        self.0.get()
+    }
+}
+
+impl Device<device::Bound> {
+    /// Set the baudrate in bits per second.
+    ///
+    /// Common baudrates are 115200, 9600, 19200, 57600, 4800.
+    ///
+    /// Use [`Device::write_flush`] before calling this if you have written data prior to this call.
+    pub fn set_baudrate(&self, speed: u32) -> Result<(), u32> {
+        // SAFETY: `self.as_raw()` is guaranteed to be a pointer to a valid `serdev_device`.
+        let ret = unsafe { bindings::serdev_device_set_baudrate(self.as_raw(), speed) };
+        if ret == speed {
+            Ok(())
+        } else {
+            Err(ret)
+        }
+    }
+
+    /// Set if flow control should be enabled.
+    ///
+    /// Use [`Device::write_flush`] before calling this if you have written data prior to this call.
+    pub fn set_flow_control(&self, enable: bool) {
+        // SAFETY: `self.as_raw()` is guaranteed to be a pointer to a valid `serdev_device`.
+        unsafe { bindings::serdev_device_set_flow_control(self.as_raw(), enable) };
+    }
+
+    /// Set parity to use.
+    ///
+    /// Use [`Device::write_flush`] before calling this if you have written data prior to this call.
+    pub fn set_parity(&self, parity: Parity) -> Result {
+        // SAFETY: `self.as_raw()` is guaranteed to be a pointer to a valid `serdev_device`.
+        to_result(unsafe { bindings::serdev_device_set_parity(self.as_raw(), parity as u32) })
+    }
+
+    /// Write data to the serial device until the controller has accepted all the data or has
+    /// been interrupted by a timeout or signal.
+    ///
+    /// Note that any accepted data has only been buffered by the controller. Use
+    /// [`Device::wait_until_sent`] to make sure the controller write buffer has actually been
+    /// emptied.
+    ///
+    /// Returns the number of bytes written (less than `data.len()` if interrupted).
+    /// [`kernel::error::code::ETIMEDOUT`] or [`kernel::error::code::ERESTARTSYS`] if interrupted
+    /// before any bytes were written.
+    pub fn write_all(&self, data: &[u8], timeout: Timeout) -> Result<usize> {
+        // SAFETY:
+        // - `self.as_raw()` is guaranteed to be a pointer to a valid `serdev_device`.
+        // - `data.as_ptr()` is guaranteed to be a valid array pointer with the size of
+        //   `data.len()`.
+        let ret = unsafe {
+            bindings::serdev_device_write(
+                self.as_raw(),
+                data.as_ptr(),
+                data.len(),
+                timeout.into_jiffies(),
+            )
+        };
+        // CAST: negative return values are guaranteed to be between `-MAX_ERRNO` and `-1`,
+        // which always fit into a `i32`.
+        to_result(ret as i32).map(|()| ret.unsigned_abs())
+    }
+
+    /// Write data to the serial device.
+    ///
+    /// If you want to write until the controller has accepted all the data, use
+    /// [`Device::write_all`].
+    ///
+    /// Note that any accepted data has only been buffered by the controller. Use
+    /// [ Device::wait_until_sent`] to make sure the controller write buffer has actually been
+    /// emptied.
+    ///
+    /// Returns the number of bytes written (less than `data.len()` if not enough room in the
+    /// write buffer).
+    pub fn write(&self, data: &[u8]) -> Result<u32> {
+        // SAFETY:
+        // - `self.as_raw()` is guaranteed to be a pointer to a valid `serdev_device`.
+        // - `data.as_ptr()` is guaranteed to be a valid array pointer with the size of
+        //   `data.len()`.
+        let ret =
+            unsafe { bindings::serdev_device_write_buf(self.as_raw(), data.as_ptr(), data.len()) };
+
+        to_result(ret as i32).map(|()| ret.unsigned_abs())
+    }
+
+    /// Send data to the serial device immediately.
+    ///
+    /// Note that this doesn't guarantee that the data has been transmitted.
+    /// Use [`Device::wait_until_sent`] for this purpose.
+    pub fn write_flush(&self) {
+        // SAFETY: `self.as_raw()` is guaranteed to be a pointer to a valid `serdev_device`.
+        unsafe { bindings::serdev_device_write_flush(self.as_raw()) };
+    }
+
+    /// Wait for the data to be sent.
+    ///
+    /// After this function, the write buffer of the controller should be empty.
+    pub fn wait_until_sent(&self, timeout: Timeout) {
+        // SAFETY: `self.as_raw()` is guaranteed to be a pointer to a valid `serdev_device`.
+        unsafe { bindings::serdev_device_wait_until_sent(self.as_raw(), timeout.into_jiffies()) };
+    }
+}
+
+// SAFETY: `serdev::Device` is a transparent wrapper of `struct serdev_device`.
+// The offset is guaranteed to point to a valid device field inside `serdev::Device`.
+unsafe impl<Ctx: device::DeviceContext> device::AsBusDevice<Ctx> for Device<Ctx> {
+    const OFFSET: usize = offset_of!(bindings::serdev_device, dev);
+}
+
+// SAFETY: `Device` is a transparent wrapper of a type that doesn't depend on `Device`'s generic
+// argument.
+kernel::impl_device_context_deref!(unsafe { Device });
+kernel::impl_device_context_into_aref!(Device);
+
+// SAFETY: Instances of `Device` are always reference-counted.
+unsafe impl AlwaysRefCounted for Device {
+    fn inc_ref(&self) {
+        self.as_ref().inc_ref();
+    }
+
+    unsafe fn dec_ref(obj: NonNull<Self>) {
+        // SAFETY: The safety requirements guarantee that the refcount is non-zero.
+        unsafe { bindings::serdev_device_put(obj.cast().as_ptr()) }
+    }
+}
+
+impl<Ctx: device::DeviceContext> AsRef<device::Device<Ctx>> for Device<Ctx> {
+    fn as_ref(&self) -> &device::Device<Ctx> {
+        // SAFETY: By the type invariant of `Self`, `self.as_raw()` is a pointer to a valid
+        // `struct serdev_device`.
+        let dev = unsafe { &raw mut (*self.as_raw()).dev };
+
+        // SAFETY: `dev` points to a valid `struct device`.
+        unsafe { device::Device::from_raw(dev) }
+    }
+}
+
+// SAFETY: A `Device` is always reference-counted and can be released from any thread.
+unsafe impl Send for Device {}
+
+// SAFETY: `Device` can be shared among threads because all methods of `Device`
+// (i.e. `Device<Normal>) are thread safe.
+unsafe impl Sync for Device {}

-- 
2.52.0

[PATCH v5 4/4] samples: rust: add Rust serial device bus sample device driver

[Markus Probst]

Add a sample Rust serial device bus device driver illustrating the usage
of the serial device bus abstractions.

This drivers probes through either a match of device / driver name or a
match within the OF ID table.

Signed-off-by: Markus Probst <markus.probst@posteo.de>
---
 samples/rust/Kconfig               | 11 +++++
 samples/rust/Makefile              |  1 +
 samples/rust/rust_driver_serdev.rs | 86 ++++++++++++++++++++++++++++++++++++++
 3 files changed, 98 insertions(+)

diff --git a/samples/rust/Kconfig b/samples/rust/Kconfig
index c49ab9106345..31d62533ef25 100644
--- a/samples/rust/Kconfig
+++ b/samples/rust/Kconfig
@@ -161,6 +161,17 @@ config SAMPLE_RUST_DRIVER_AUXILIARY
 
 	  If unsure, say N.
 
+config SAMPLE_RUST_DRIVER_SERDEV
+	tristate "Serial Device Bus Device Driver"
+	select RUST_SERIAL_DEV_BUS_ABSTRACTIONS
+	help
+	  This option builds the Rust serial device bus driver sample.
+
+	  To compile this as a module, choose M here:
+	  the module will be called rust_driver_serdev.
+
+	  If unsure, say N.
+
 config SAMPLE_RUST_SOC
 	tristate "SoC Driver"
 	select SOC_BUS
diff --git a/samples/rust/Makefile b/samples/rust/Makefile
index 6c0aaa58cccc..b986b681cde5 100644
--- a/samples/rust/Makefile
+++ b/samples/rust/Makefile
@@ -14,6 +14,7 @@ obj-$(CONFIG_SAMPLE_RUST_DRIVER_PLATFORM)	+= rust_driver_platform.o
 obj-$(CONFIG_SAMPLE_RUST_DRIVER_USB)		+= rust_driver_usb.o
 obj-$(CONFIG_SAMPLE_RUST_DRIVER_FAUX)		+= rust_driver_faux.o
 obj-$(CONFIG_SAMPLE_RUST_DRIVER_AUXILIARY)	+= rust_driver_auxiliary.o
+obj-$(CONFIG_SAMPLE_RUST_DRIVER_SERDEV)		+= rust_driver_serdev.o
 obj-$(CONFIG_SAMPLE_RUST_CONFIGFS)		+= rust_configfs.o
 obj-$(CONFIG_SAMPLE_RUST_SOC)			+= rust_soc.o
 
diff --git a/samples/rust/rust_driver_serdev.rs b/samples/rust/rust_driver_serdev.rs
new file mode 100644
index 000000000000..8cf3fb451b22
--- /dev/null
+++ b/samples/rust/rust_driver_serdev.rs
@@ -0,0 +1,86 @@
+// SPDX-License-Identifier: GPL-2.0
+
+//! Rust Serial device bus device driver sample.
+
+use kernel::{
+    acpi,
+    device::{
+        Bound,
+        Core, //
+    },
+    of,
+    prelude::*,
+    serdev,
+    sync::aref::ARef, //
+};
+
+struct SampleDriver {
+    sdev: ARef<serdev::Device>,
+}
+
+kernel::of_device_table!(
+    OF_TABLE,
+    MODULE_OF_TABLE,
+    <SampleDriver as serdev::Driver>::IdInfo,
+    [(of::DeviceId::new(c"test,rust_driver_serdev"), ())]
+);
+
+kernel::acpi_device_table!(
+    ACPI_TABLE,
+    MODULE_ACPI_TABLE,
+    <SampleDriver as serdev::Driver>::IdInfo,
+    [(acpi::DeviceId::new(c"LNUXBEEF"), ())]
+);
+
+#[vtable]
+impl serdev::Driver for SampleDriver {
+    type IdInfo = ();
+    const OF_ID_TABLE: Option<of::IdTable<Self::IdInfo>> = Some(&OF_TABLE);
+    const ACPI_ID_TABLE: Option<acpi::IdTable<Self::IdInfo>> = Some(&ACPI_TABLE);
+
+    fn probe(
+        sdev: &serdev::Device<Core>,
+        _info: Option<&Self::IdInfo>,
+    ) -> impl PinInit<Self, Error> {
+        let dev = sdev.as_ref();
+
+        dev_dbg!(dev, "Probe Rust Serial device bus device driver sample.\n");
+
+        if sdev
+            .set_baudrate(
+                dev.fwnode()
+                    .and_then(|fwnode| fwnode.property_read(c"baudrate").optional())
+                    .unwrap_or(115200),
+            )
+            .is_err()
+        {
+            return Err(EINVAL);
+        }
+        sdev.set_flow_control(false);
+        sdev.set_parity(serdev::Parity::None)?;
+
+        Ok(Self { sdev: sdev.into() })
+    }
+
+    fn receive(sdev: &serdev::Device<Bound>, _this: Pin<&Self>, data: &[u8]) -> usize {
+        let _ = sdev.write_all(data, serdev::Timeout::Max);
+        data.len()
+    }
+}
+
+impl Drop for SampleDriver {
+    fn drop(&mut self) {
+        dev_dbg!(
+            self.sdev.as_ref(),
+            "Remove Rust Serial device bus device driver sample.\n"
+        );
+    }
+}
+
+kernel::module_serdev_device_driver! {
+    type: SampleDriver,
+    name: "rust_driver_serdev",
+    authors: ["Markus Probst"],
+    description: "Rust Serial device bus device driver",
+    license: "GPL v2",
+}

-- 
2.52.0

Claude review: rust: add basic serial device bus abstractions

[Claude Code Review Bot]

Overall Series Review

Subject: rust: add basic serial device bus abstractions
Author: Markus Probst <markus.probst@posteo.de>
Patches: 5
Reviewed: 2026-04-23T09:21:38.011857

---

This is a v5 series adding Rust serdev (serial device bus) abstractions to the kernel, consisting of 4 patches: a preparatory devres change, a C-side struct modification, the main Rust abstraction, and a sample driver.

The overall design follows established kernel Rust patterns well. The `Driver` trait, `Adapter` registration mechanism, and `Device` wrapper are consistent with how other bus abstractions (platform, USB, auxiliary) work. The race condition between `devm_serdev_device_open` and probe completion is handled correctly using a `Completion` barrier. The code is generally well-structured and the safety comments are mostly thorough.

Key concerns: (1) the `rust_private_data` field in `struct serdev_device` adds overhead to all serdev devices whether or not a Rust driver is in use; (2) the `devres::register` lifetime extension in patch 1 has a subtle safety argument that deserves careful review from the devres maintainers; (3) there are a few documentation typos/broken links that should be fixed.

---
Generated by Claude Code Patch Reviewer

Claude review: rust: devres: return reference in `devres::register`

[Claude Code Review Bot]

Patch Review

This changes `devres::register` from returning `Result` to `Result<&'a T>`, tying the returned reference's lifetime to the `&'a Device<Bound>` parameter.

The SAFETY comment is the critical part:
```rust
+        // SAFETY: `dev` is valid for the lifetime of 'a. As long as there is a reference to
+        // `Device<Bound>`, it is guaranteed that the device is not unbound and data has not been
+        // dropped. Thus `data_ptr` is also valid for the lifetime of 'a.
+        .map(|()| unsafe { &*data_ptr })
```

This is correct given the semantics of `Device<Bound>` -- the Bound context guarantees the device hasn't been unbound, and devres resources are freed at unbind time. However, the reasoning depends entirely on the invariant that `Device<Bound>` references cannot outlive the bound state. This is an important invariant for the devres subsystem maintainers to validate.

The mechanical updates to `cpufreq.rs` and `drm/driver.rs` (adding `?; Ok(())`) are straightforward and correct -- existing callers that don't use the returned reference simply discard it.

The doc example should also be updated to show the new return type:
```rust
-///     devres::register(dev, Registration::new(), GFP_KERNEL)
+///     devres::register(dev, Registration::new(), GFP_KERNEL)?;
+///     Ok(())
```
This is done correctly.

Already has an `Acked-by` from Viresh Kumar, which is good since it touches cpufreq.

---
Generated by Claude Code Patch Reviewer

Claude review: serdev: add rust private data to serdev_device

[Claude Code Review Bot]

Patch Review

Adds `void *rust_private_data` to `struct serdev_device`:

```c
+	void *rust_private_data;
```

This is the most debatable design choice in the series. It adds a pointer to every `serdev_device` instance, even when no Rust driver is present. The alternative would be using a side data structure (e.g., an IDR/XArray lookup), but that would add complexity and overhead in the receive path.

The doc update fixes the missing colons on `@write_comp` and `@write_lock`:
```c
- * @write_comp	Completion used by serdev_device_write() internally
- * @write_lock	Lock to serialize access when writing data
+ * @write_comp:	       Completion used by serdev_device_write() internally
+ * @write_lock:	       Lock to serialize access when writing data
```
That's a nice cleanup, but the re-alignment to very wide column positions (tabs to column ~24) changes the style relative to the other fields. Might attract style comments from the serdev maintainer.

The field is not initialized anywhere, but `serdev_device_alloc` uses `kzalloc` for the controller, and the device is allocated via `device_register`, so the field should be zero-initialized. Still, it would be more defensive to document or assert that assumption.

This patch is missing a `Signed-off-by` or `Acked-by` from Rob Herring (the serdev maintainer), which is important since it modifies a core C header he maintains.

---
Generated by Claude Code Patch Reviewer

Claude review: rust: add basic serial device bus abstractions

[Claude Code Review Bot]

Patch Review

This is the main patch adding `rust/kernel/serdev.rs` (536 lines).

**Typo in `Parity` enum:**
```rust
+    /// Even partiy.
+    Even = bindings::serdev_parity_SERDEV_PARITY_EVEN,
```
"partiy" should be "parity". The v5 changelog says "fix typo in documentation", but this one appears to still be present.

**Broken doc link in `write` method:**
```rust
+    /// [ Device::wait_until_sent`] to make sure the controller write buffer has actually been
```
This is malformed -- should be `` [`Device::wait_until_sent`] `` (the opening backtick is missing and there's a spurious space after `[`). Compare with the correct version in `write_all`:
```rust
+    /// [`Device::wait_until_sent`] to make sure the controller write buffer has actually been
```

**`Timeout` enum documentation:**
```rust
+/// Timeout in Jiffies.
+pub enum Timeout {
```
The doc says "Timeout in Jiffies" but the enum also supports `Milliseconds` and `Max` variants. This should be something like "Timeout specification for serial device operations."

**`Timeout::Max` returning 0:**
```rust
+            Self::Max => 0,
```
This is correct. The C `serdev_device_write` documentation says "timeout in jiffies, or 0 to wait indefinitely", so `Max => 0` correctly maps to indefinite wait.

**Race condition handling in `probe_callback`/`receive_buf_callback`:**

The design opens the device *before* completing probe:
```rust
+            to_result(unsafe {
+                bindings::devm_serdev_device_open(sdev.as_ref().as_raw(), sdev.as_raw())
+            })?;
+
+            let data = T::probe(sdev, id_info);
```

This means `receive_buf_callback` can fire before driver data is set. The `Completion` + `UnsafeCell<bool>` pattern handles this correctly:
- The receive callback waits on `probe_complete`
- After completion, it checks the `error` flag
- If probe failed, it consumes all data silently (`return length`)

The synchronization is sound because `complete_all()`/`wait_for_completion()` provide the necessary memory barriers for the `UnsafeCell<bool>` access. The receive callback documentation says "may sleep", so blocking on the completion is safe.

One concern: if `T::probe` returns an error-producing initializer, `set_drvdata` will return `Err(...)`. The code correctly handles this by still calling `complete_all()` after setting the error flag. But if the error happens *before* `set_drvdata` (e.g., `devres::register` or `devm_serdev_device_open` fails), the early `?` return would skip `complete_all()`. This is safe only because client_ops are set before `devm_serdev_device_open`, and if the device isn't opened, the receive callback won't be called. The ordering of operations is correct but fragile -- a future refactor that reorders these calls could introduce a deadlock.

**`set_baudrate` return type:**
```rust
+    pub fn set_baudrate(&self, speed: u32) -> Result<(), u32> {
```
Returning `Result<(), u32>` (where the error value is the actual baudrate achieved) is an unusual but defensible API choice. However, it breaks from the kernel Rust convention where `Result` typically means `Result<T, kernel::error::Error>`. Users who want to know the actual baudrate set on failure must handle the `u32` error, which is a different error type than the rest of the serdev API.

**`write_all` return value cast:**
```rust
+        to_result(ret as i32).map(|()| ret.unsigned_abs())
```
Where `ret` is `isize` (from `ssize_t`). The comment correctly notes that negative values fit in `i32`. For positive values, `ret as i32` could overflow if writing >2GB, but this is unrealistic for serial devices.

**Missing `#[cfg]` guard:** The `serdev.c` helper file is compiled unconditionally (added to `helpers.c`). It should arguably be guarded by `#ifdef CONFIG_RUST_SERIAL_DEV_BUS_ABSTRACTIONS` to avoid compiling unused helpers, matching the pattern used for the Rust module in `lib.rs`:
```rust
+#[cfg(CONFIG_RUST_SERIAL_DEV_BUS_ABSTRACTIONS)]
+pub mod serdev;
```

**Trailing `//` comments:**
```rust
+    types::{
+        AlwaysRefCounted,
+        Opaque, //
+    }, //
```
These trailing `//` comments appear to be rustfmt artifacts to force vertical formatting. This is a known pattern in kernel Rust code but could be noted as intentional.

---
Generated by Claude Code Patch Reviewer

Claude review: samples: rust: add Rust serial device bus sample device driver

[Claude Code Review Bot]

Patch Review

Clean and functional sample driver. A few observations:

**Echoing data in receive callback:**
```rust
+    fn receive(sdev: &serdev::Device<Bound>, _this: Pin<&Self>, data: &[u8]) -> usize {
+        let _ = sdev.write_all(data, serdev::Timeout::Max);
+        data.len()
+    }
```
The `write_all` with `Timeout::Max` could block indefinitely if the device can't accept data. For a sample this is acceptable, but a brief comment noting this is intentional would be helpful. Also, `let _ =` silently discards write errors, which is fine for a sample.

**Baudrate handling:**
```rust
+        if sdev
+            .set_baudrate(
+                dev.fwnode()
+                    .and_then(|fwnode| fwnode.property_read(c"baudrate").optional())
+                    .unwrap_or(115200),
+            )
+            .is_err()
+        {
+            return Err(EINVAL);
+        }
```
Reading a `baudrate` property from firmware and falling back to 115200 is a reasonable pattern. The `set_baudrate` error (which contains the actual baudrate set) is discarded and replaced with `EINVAL`, which is appropriate.

**Kconfig entry properly uses `select`:**
```
+	select RUST_SERIAL_DEV_BUS_ABSTRACTIONS
```
This ensures the abstraction is compiled when the sample is enabled. Correct.

The sample effectively demonstrates OF and ACPI device table setup, probe/remove lifecycle, baudrate/flow-control/parity configuration, and the receive callback -- good coverage for a sample driver.

---
Generated by Claude Code Patch Reviewer

Claude review: samples: rust: add Rust serial device bus sample device driver

[Claude Code Review Bot]

Patch Review

The sample is clean and demonstrates the key API surface well.

**One issue with the sample probe:**
```rust
fn probe(
    sdev: &serdev::Device<Core>,
    _info: Option<&Self::IdInfo>,
) -> impl PinInit<Self, Error> {
    ...
    if sdev.set_baudrate(...).is_err() {
        return Err(EINVAL);
    }
```
The `set_baudrate` method is on `Device<Bound>`, but `sdev` is `&Device<Core>`. This works via the deref coercion chain, which is correct but could be surprising to readers. A brief comment in the abstraction's module-level docs explaining the device context hierarchy and auto-deref would help (though this is a broader Rust-for-Linux documentation issue, not specific to this patch).

**The receive callback echoes data back:**
```rust
fn receive(sdev: &serdev::Device<Bound>, _this: Pin<&Self>, data: &[u8]) -> usize {
    let _ = sdev.write_all(data, serdev::Timeout::Max);
    data.len()
}
```
This is fine for a sample. The `let _ =` discards the write result, which is acceptable for demonstration purposes. Note that this always returns `data.len()` (claiming all data was consumed) regardless of whether `write_all` succeeded, which is the right behavior for a simple echo.

**The `SampleDriver` stores an `ARef<serdev::Device>` but only uses it in `Drop`** for a debug message. This is fine as a sample demonstrating how to keep a reference to the device.

No blocking issues with this patch.

---
Generated by Claude Code Patch Reviewer

Claude review: samples: rust: add Rust serial device bus sample device driver

[Claude Code Review Bot]

Patch Review

Clean, well-structured sample driver. It demonstrates:
- OF and ACPI device table registration
- Probe with optional baudrate property from firmware
- Receive callback that echoes data back
- Drop for cleanup logging

```rust
+    fn receive(sdev: &serdev::Device<Bound>, _this: Pin<&Self>, data: &[u8]) -> usize {
+        let _ = sdev.write_all(data, serdev::Timeout::Max);
+        data.len()
+    }
```

The `let _ =` silently discards write errors during echo, which is fine for a sample. The function always returns `data.len()` claiming all bytes were consumed regardless of write success — acceptable for a demo.

**Minor:** The Kconfig entry doesn't have a `depends on SAMPLES` guard, but that's consistent with other sample drivers in the same file.

No significant issues with this patch.

---
Generated by Claude Code Patch Reviewer