Re: [PATCH v2 3/4] rust: add basic serial device bus abstractions

[Markus Probst <markus.probst@posteo.de>]

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On Fri, 2026-03-06 at 19:35 +0000, Markus Probst wrote:
> 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>
> ---
>  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           | 533 ++++++++++++++++++++++++++++++++++++++++
>  5 files changed, 559 insertions(+)
> 
> 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 3da92f18f4ee..1d337e112922 100644
> --- a/rust/kernel/lib.rs
> +++ b/rust/kernel/lib.rs
> @@ -140,6 +140,8 @@
>  pub mod scatterlist;
>  pub mod security;
>  pub mod seq_file;
> +#[cfg(CONFIG_SERIAL_DEV_BUS)]
> +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..cba5fb6245f7
> --- /dev/null
> +++ b/rust/kernel/serdev.rs
> @@ -0,0 +1,533 @@
> +// 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, ForeignOwnable, 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()).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);
> +
> +        // SAFETY:
> +        // - `sdev.as_raw()` is guaranteed to be a pointer to a valid `serdev_device`.
> +        // - `remove_callback` is only ever called after a successful call to
> +        //   `probe_callback`, hence it's guaranteed that `private_data` is a valid pointer to
> +        //   `PrivateData` and is stored in a `Pin<KBox<PrivateData>>`.
> +        // - `private_data` will not be accessed after this function.
> +        drop(unsafe { <Pin<KBox<PrivateData>>>::from_foreign((*sdev.as_raw()).private_data) });
It seems I forgot to remove this drop statement. Please ignore it.

Thanks
- Markus Probst

> +    }
> +
> +    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: `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:
> +        // - 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()).private_data.cast::<PrivateData>() };
> +
> +        private_data.probe_complete.complete_all();
> +
> +        // SAFETY: No one has exclusive access to `private_data.error`.
> +        if unsafe { *private_data.error.get() } {
> +            return length;
> +        }
> +
> +        // 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 {}

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