[PATCH v10 3/7] rust: pwm: Add complete abstraction layer

[Michal Wilczynski]

Introduce a comprehensive abstraction layer for the PWM subsystem to
enable writing drivers in Rust.

Because `Device`, `Chip`, and `PwmOps` all refer to each other, they
form a single, indivisible unit with circular dependencies. They are
introduced together in this single commit to create a complete,
compilable abstraction layer.

The main components are:
 - Data Wrappers: Safe, idiomatic wrappers for core C types like
   `pwm_device`, and `pwm_chip`.

 - PwmOps Trait: An interface that drivers can implement to provide
   their hardware-specific logic, mirroring the C `pwm_ops` interface.

 - FFI VTable and Adapter: A bridge to connect the high-level PwmOps trait
   to the C kernel's pwm_ops vtable.

 - Allocation and Lifetime Management: A high-level `Chip::new()`
   API to safely allocate a chip and a `Registration` guard that integrates
   with `devres` to manage the chip's registration with the PWM core.
   An `AlwaysRefCounted` implementation and a custom release handler
   prevent memory leaks by managing the chip's lifetime and freeing
   driver data correctly.

Reviewed-by: Danilo Krummrich <dakr@xxxxxxxxxx>
Signed-off-by: Michal Wilczynski <m.wilczynski@xxxxxxxxxxx>
---
 rust/kernel/pwm.rs | 667 ++++++++++++++++++++++++++++++++++++++++++++++++++++-
 1 file changed, 665 insertions(+), 2 deletions(-)

diff --git a/rust/kernel/pwm.rs b/rust/kernel/pwm.rs
index 3fad101406eac728d9b12083fad7abf7b7f89b25..0e1896f70c2fb8ec6b40106c96018299fcf7157f 100644
--- a/rust/kernel/pwm.rs
+++ b/rust/kernel/pwm.rs
@@ -8,10 +8,14 @@
 
 use crate::{
     bindings,
+    container_of,
+    device::{self, Bound},
+    devres,
+    error::{self, to_result},
     prelude::*,
-    types::Opaque,
+    types::{ARef, AlwaysRefCounted, ForeignOwnable, Opaque},
 };
-use core::convert::TryFrom;
+use core::{convert::TryFrom, marker::PhantomData, ptr::NonNull};
 
 /// PWM polarity. Mirrors [`enum pwm_polarity`](srctree/include/linux/pwm.h).
 #[derive(Copy, Clone, Debug, PartialEq, Eq)]
@@ -135,3 +139,662 @@ pub fn enabled(&self) -> bool {
         self.0.enabled
     }
 }
+
+/// Describes the outcome of a `round_waveform` operation.
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+pub enum RoundingOutcome {
+    /// The requested waveform was achievable exactly or by rounding values down.
+    ExactOrRoundedDown,
+
+    /// The requested waveform could only be achieved by rounding up.
+    RoundedUp,
+}
+
+/// Wrapper for a PWM device [`struct pwm_device`](srctree/include/linux/pwm.h).
+#[repr(transparent)]
+pub struct Device(Opaque<bindings::pwm_device>);
+
+impl Device {
+    /// Creates a reference to a [`Device`] from a valid C pointer.
+    ///
+    /// # Safety
+    ///
+    /// The caller must ensure that `ptr` is valid and remains valid for the lifetime of the
+    /// returned [`Device`] reference.
+    pub(crate) unsafe fn as_ref<'a>(ptr: *mut bindings::pwm_device) -> &'a Self {
+        // SAFETY: The safety requirements guarantee the validity of the dereference, while the
+        // `Device` type being transparent makes the cast ok.
+        unsafe { &*ptr.cast::<Self>() }
+    }
+
+    /// Returns a raw pointer to the underlying `pwm_device`.
+    fn as_raw(&self) -> *mut bindings::pwm_device {
+        self.0.get()
+    }
+
+    /// Gets the hardware PWM index for this device within its chip.
+    pub fn hwpwm(&self) -> u32 {
+        // SAFETY: `self.as_raw()` provides a valid pointer for `self`'s lifetime.
+        unsafe { (*self.as_raw()).hwpwm }
+    }
+
+    /// Gets a reference to the parent `Chip` that this device belongs to.
+    pub fn chip<T: ForeignOwnable>(&self) -> &Chip<T> {
+        // SAFETY: `self.as_raw()` provides a valid pointer. (*self.as_raw()).chip
+        // is assumed to be a valid pointer to `pwm_chip` managed by the kernel.
+        // Chip::as_ref's safety conditions must be met.
+        unsafe { Chip::<T>::as_ref((*self.as_raw()).chip) }
+    }
+
+    /// Gets the label for this PWM device, if any.
+    pub fn label(&self) -> Option<&CStr> {
+        // SAFETY: self.as_raw() provides a valid pointer.
+        let label_ptr = unsafe { (*self.as_raw()).label };
+        if label_ptr.is_null() {
+            None
+        } else {
+            // SAFETY: label_ptr is non-null and points to a C string
+            // managed by the kernel, valid for the lifetime of the PWM device.
+            Some(unsafe { CStr::from_char_ptr(label_ptr) })
+        }
+    }
+
+    /// Gets a copy of the board-dependent arguments for this PWM device.
+    pub fn args(&self) -> Args {
+        // SAFETY: self.as_raw() gives a valid pointer to `pwm_device`.
+        // The `args` field is a valid `pwm_args` struct embedded within `pwm_device`.
+        // `Args::from_c_ptr`'s safety conditions are met by providing this pointer.
+        unsafe { Args::from_c_ptr(&(*self.as_raw()).args) }
+    }
+
+    /// Gets a copy of the current state of this PWM device.
+    pub fn state(&self) -> State {
+        // SAFETY: `self.as_raw()` gives a valid pointer. `(*self.as_raw()).state`
+        // is a valid `pwm_state` struct. `State::from_c` copies this data.
+        State::from_c(unsafe { (*self.as_raw()).state })
+    }
+
+    /// Sets the PWM waveform configuration and enables the PWM signal.
+    pub fn set_waveform(&self, wf: &Waveform, exact: bool) -> Result {
+        let c_wf = bindings::pwm_waveform::from(*wf);
+
+        // SAFETY: `self.as_raw()` provides a valid `*mut pwm_device` pointer.
+        // `&c_wf` is a valid pointer to a `pwm_waveform` struct. The C function
+        // handles all necessary internal locking.
+        let ret = unsafe { bindings::pwm_set_waveform_might_sleep(self.as_raw(), &c_wf, exact) };
+        to_result(ret)
+    }
+
+    /// Queries the hardware for the configuration it would apply for a given
+    /// request.
+    pub fn round_waveform(&self, wf: &mut Waveform) -> Result<RoundingOutcome> {
+        let mut c_wf = bindings::pwm_waveform::from(*wf);
+
+        // SAFETY: `self.as_raw()` provides a valid `*mut pwm_device` pointer.
+        // `&mut c_wf` is a valid pointer to a mutable `pwm_waveform` struct that
+        // the C function will update.
+        let ret = unsafe { bindings::pwm_round_waveform_might_sleep(self.as_raw(), &mut c_wf) };
+
+        to_result(ret)?;
+
+        *wf = Waveform::from(c_wf);
+
+        if ret == 1 {
+            Ok(RoundingOutcome::RoundedUp)
+        } else {
+            Ok(RoundingOutcome::ExactOrRoundedDown)
+        }
+    }
+
+    /// Reads the current waveform configuration directly from the hardware.
+    pub fn get_waveform(&self) -> Result<Waveform> {
+        let mut c_wf = bindings::pwm_waveform::default();
+
+        // SAFETY: `self.as_raw()` is a valid pointer. We provide a valid pointer
+        // to a stack-allocated `pwm_waveform` struct for the kernel to fill.
+        let ret = unsafe { bindings::pwm_get_waveform_might_sleep(self.as_raw(), &mut c_wf) };
+
+        to_result(ret)?;
+
+        Ok(Waveform::from(c_wf))
+    }
+}
+
+/// Trait defining the operations for a PWM driver.
+pub trait PwmOps: 'static + Sized {
+    /// The type of the owned driver data (e.g., `Pin<KBox<...>>`).
+    type DrvData: 'static + ForeignOwnable;
+    /// The driver-specific hardware representation of a waveform.
+    ///
+    /// This type must be [`Copy`], [`Default`], and fit within `PWM_WFHWSIZE`.
+    type WfHw: Copy + Default;
+
+    /// Optional hook for when a PWM device is requested.
+    fn request(
+        _chip: &Chip<Self::DrvData>,
+        _pwm: &Device,
+        _parent_dev: &device::Device<Bound>,
+    ) -> Result {
+        Ok(())
+    }
+
+    /// Optional hook for capturing a PWM signal.
+    fn capture(
+        _chip: &Chip<Self::DrvData>,
+        _pwm: &Device,
+        _result: &mut bindings::pwm_capture,
+        _timeout: usize,
+        _parent_dev: &device::Device<Bound>,
+    ) -> Result {
+        Err(ENOTSUPP)
+    }
+
+    /// Convert a generic waveform to the hardware-specific representation.
+    /// This is typically a pure calculation and does not perform I/O.
+    fn round_waveform_tohw(
+        _chip: &Chip<Self::DrvData>,
+        _pwm: &Device,
+        _wf: &Waveform,
+    ) -> Result<(c_int, Self::WfHw)> {
+        Err(ENOTSUPP)
+    }
+
+    /// Convert a hardware-specific representation back to a generic waveform.
+    /// This is typically a pure calculation and does not perform I/O.
+    fn round_waveform_fromhw(
+        _chip: &Chip<Self::DrvData>,
+        _pwm: &Device,
+        _wfhw: &Self::WfHw,
+        _wf: &mut Waveform,
+    ) -> Result<c_int> {
+        Err(ENOTSUPP)
+    }
+
+    /// Read the current hardware configuration into the hardware-specific representation.
+    fn read_waveform(
+        _chip: &Chip<Self::DrvData>,
+        _pwm: &Device,
+        _parent_dev: &device::Device<Bound>,
+    ) -> Result<Self::WfHw> {
+        Err(ENOTSUPP)
+    }
+
+    /// Write a hardware-specific waveform configuration to the hardware.
+    fn write_waveform(
+        _chip: &Chip<Self::DrvData>,
+        _pwm: &Device,
+        _wfhw: &Self::WfHw,
+        _parent_dev: &device::Device<Bound>,
+    ) -> Result {
+        Err(ENOTSUPP)
+    }
+}
+/// Bridges Rust `PwmOps` to the C `pwm_ops` vtable.
+struct Adapter<T: PwmOps> {
+    _p: PhantomData<T>,
+}
+
+impl<T: PwmOps> Adapter<T> {
+    const VTABLE: PwmOpsVTable = create_pwm_ops::<T>();
+
+    /// # Safety
+    ///
+    /// `wfhw_ptr` must be valid for writes of `size_of::<T::WfHw>()` bytes.
+    unsafe fn serialize_wfhw(wfhw: &T::WfHw, wfhw_ptr: *mut c_void) -> Result {
+        let size = core::mem::size_of::<T::WfHw>();
+        if size > bindings::PWM_WFHWSIZE as usize {
+            return Err(EINVAL);
+        }
+
+        // SAFETY: The caller ensures `wfhw_ptr` is valid for `size` bytes.
+        unsafe {
+            core::ptr::copy_nonoverlapping(
+                core::ptr::from_ref::<T::WfHw>(wfhw).cast::<u8>(),
+                wfhw_ptr.cast::<u8>(),
+                size,
+            );
+        }
+
+        Ok(())
+    }
+
+    /// # Safety
+    ///
+    /// `wfhw_ptr` must be valid for reads of `size_of::<T::WfHw>()` bytes.
+    unsafe fn deserialize_wfhw(wfhw_ptr: *const c_void) -> Result<T::WfHw> {
+        let size = core::mem::size_of::<T::WfHw>();
+        if size > bindings::PWM_WFHWSIZE as usize {
+            return Err(EINVAL);
+        }
+
+        let mut wfhw = T::WfHw::default();
+        // SAFETY: The caller ensures `wfhw_ptr` is valid for `size` bytes.
+        unsafe {
+            core::ptr::copy_nonoverlapping(
+                wfhw_ptr.cast::<u8>(),
+                core::ptr::from_mut::<T::WfHw>(&mut wfhw).cast::<u8>(),
+                size,
+            );
+        }
+
+        Ok(wfhw)
+    }
+
+    /// # Safety
+    ///
+    /// `dev` must be a valid pointer to a `bindings::device` embedded within a
+    /// `bindings::pwm_chip`. This function is called by the device core when the
+    /// last reference to the device is dropped.
+    unsafe extern "C" fn release_callback(dev: *mut bindings::device) {
+        // SAFETY: The function's contract guarantees that `dev` points to a `device`
+        // field embedded within a valid `pwm_chip`. `container_of!` can therefore
+        // safely calculate the address of the containing struct.
+        let c_chip_ptr = unsafe { container_of!(dev, bindings::pwm_chip, dev) };
+
+        // SAFETY: `c_chip_ptr` is a valid pointer to a `pwm_chip` as established
+        // above. Calling this FFI function is safe.
+        let drvdata_ptr = unsafe { bindings::pwmchip_get_drvdata(c_chip_ptr) };
+
+        // SAFETY: `drvdata_ptr` was stored by `Chip::new` from an owned `T::DrvData`
+        // and is guaranteed to be valid if non-null. `from_foreign` can safely
+        // reclaim ownership to allow Rust to drop and free the data.
+        let _owned_drvdata = unsafe { T::DrvData::from_foreign(drvdata_ptr.cast()) };
+
+        // Now, call the original release function to free the `pwm_chip` itself.
+        // SAFETY: `dev` is the valid pointer passed into this callback, which is
+        // the expected argument for `pwmchip_release`.
+        unsafe { bindings::pwmchip_release(dev); }
+    }
+
+    /// # Safety
+    ///
+    /// Pointers from C must be valid.
+    unsafe extern "C" fn request_callback(
+        c: *mut bindings::pwm_chip,
+        p: *mut bindings::pwm_device,
+    ) -> c_int {
+        // SAFETY: PWM core guarentees `c` and `p` are valid pointers.
+        let (chip, pwm) = unsafe { (Chip::<T::DrvData>::as_ref(c), Device::as_ref(p)) };
+
+        // SAFETY: The PWM core guarantees the parent device exists and is bound during callbacks.
+        let bound_parent = unsafe { chip.bound_parent_device() };
+        match T::request(chip, pwm, bound_parent) {
+            Ok(()) => 0,
+            Err(e) => e.to_errno(),
+        }
+    }
+
+    /// # Safety
+    ///
+    /// Pointers from C must be valid.
+    unsafe extern "C" fn capture_callback(
+        c: *mut bindings::pwm_chip,
+        p: *mut bindings::pwm_device,
+        res: *mut bindings::pwm_capture,
+        timeout: usize,
+    ) -> c_int {
+        // SAFETY: Relies on the function's contract that `c` and `p` are valid pointers.
+        let (chip, pwm, result) =
+            unsafe { (Chip::<T::DrvData>::as_ref(c), Device::as_ref(p), &mut *res) };
+
+        // SAFETY: The PWM core guarantees the parent device exists and is bound during callbacks.
+        let bound_parent = unsafe { chip.bound_parent_device() };
+        match T::capture(chip, pwm, result, timeout, bound_parent) {
+            Ok(()) => 0,
+            Err(e) => e.to_errno(),
+        }
+    }
+
+    /// # Safety
+    ///
+    /// Pointers from C must be valid.
+    unsafe extern "C" fn round_waveform_tohw_callback(
+        c: *mut bindings::pwm_chip,
+        p: *mut bindings::pwm_device,
+        w: *const bindings::pwm_waveform,
+        wh: *mut c_void,
+    ) -> c_int {
+        // SAFETY: Relies on the function's contract that `c` and `p` are valid pointers.
+        let (chip, pwm, wf) = unsafe {
+            (
+                Chip::<T::DrvData>::as_ref(c),
+                Device::as_ref(p),
+                Waveform::from(*w),
+            )
+        };
+        match T::round_waveform_tohw(chip, pwm, &wf) {
+            Ok((status, wfhw)) => {
+                // SAFETY: `wh` is valid per this function's safety contract.
+                if unsafe { Self::serialize_wfhw(&wfhw, wh) }.is_err() {
+                    return EINVAL.to_errno();
+                }
+                status
+            }
+            Err(e) => e.to_errno(),
+        }
+    }
+
+    /// # Safety
+    ///
+    /// Pointers from C must be valid.
+    unsafe extern "C" fn round_waveform_fromhw_callback(
+        c: *mut bindings::pwm_chip,
+        p: *mut bindings::pwm_device,
+        wh: *const c_void,
+        w: *mut bindings::pwm_waveform,
+    ) -> c_int {
+        // SAFETY: Relies on the function's contract that `c` and `p` are valid pointers.
+        let (chip, pwm) = unsafe { (Chip::<T::DrvData>::as_ref(c), Device::as_ref(p)) };
+        // SAFETY: `deserialize_wfhw`'s safety contract is met by this function's contract.
+        let wfhw = match unsafe { Self::deserialize_wfhw(wh) } {
+            Ok(v) => v,
+            Err(e) => return e.to_errno(),
+        };
+
+        let mut rust_wf = Waveform::default();
+        match T::round_waveform_fromhw(chip, pwm, &wfhw, &mut rust_wf) {
+            Ok(ret) => {
+                // SAFETY: `w` is guaranteed valid by the C caller.
+                unsafe {
+                    *w = rust_wf.into();
+                };
+                ret
+            }
+            Err(e) => e.to_errno(),
+        }
+    }
+
+    /// # Safety
+    ///
+    /// Pointers from C must be valid.
+    unsafe extern "C" fn read_waveform_callback(
+        c: *mut bindings::pwm_chip,
+        p: *mut bindings::pwm_device,
+        wh: *mut c_void,
+    ) -> c_int {
+        // SAFETY: Relies on the function's contract that `c` and `p` are valid pointers.
+        let (chip, pwm) = unsafe { (Chip::<T::DrvData>::as_ref(c), Device::as_ref(p)) };
+
+        // SAFETY: The PWM core guarantees the parent device exists and is bound during callbacks.
+        let bound_parent = unsafe { chip.bound_parent_device() };
+        match T::read_waveform(chip, pwm, bound_parent) {
+            // SAFETY: `wh` is valid per this function's safety contract.
+            Ok(wfhw) => match unsafe { Self::serialize_wfhw(&wfhw, wh) } {
+                Ok(()) => 0,
+                Err(e) => e.to_errno(),
+            },
+            Err(e) => e.to_errno(),
+        }
+    }
+
+    /// # Safety
+    ///
+    /// Pointers from C must be valid.
+    unsafe extern "C" fn write_waveform_callback(
+        c: *mut bindings::pwm_chip,
+        p: *mut bindings::pwm_device,
+        wh: *const c_void,
+    ) -> c_int {
+        // SAFETY: Relies on the function's contract that `c` and `p` are valid pointers.
+        let (chip, pwm) = unsafe { (Chip::<T::DrvData>::as_ref(c), Device::as_ref(p)) };
+
+        // SAFETY: The PWM core guarantees the parent device exists and is bound during callbacks.
+        let bound_parent = unsafe { chip.bound_parent_device() };
+
+        // SAFETY: `wh` is valid per this function's safety contract.
+        let wfhw = match unsafe { Self::deserialize_wfhw(wh) } {
+            Ok(v) => v,
+            Err(e) => return e.to_errno(),
+        };
+        match T::write_waveform(chip, pwm, &wfhw, bound_parent) {
+            Ok(()) => 0,
+            Err(e) => e.to_errno(),
+        }
+    }
+}
+
+/// VTable structure wrapper for PWM operations.
+/// Mirrors [`struct pwm_ops`](srctree/include/linux/pwm.h).
+#[repr(transparent)]
+pub struct PwmOpsVTable(bindings::pwm_ops);
+
+// SAFETY: PwmOpsVTable is Send. The vtable contains only function pointers
+// and a size, which are simple data types that can be safely moved across
+// threads. The thread-safety of calling these functions is handled by the
+// kernel's locking mechanisms.
+unsafe impl Send for PwmOpsVTable {}
+
+// SAFETY: PwmOpsVTable is Sync. The vtable is immutable after it is created,
+// so it can be safely referenced and accessed concurrently by multiple threads
+// e.g. to read the function pointers.
+unsafe impl Sync for PwmOpsVTable {}
+
+impl PwmOpsVTable {
+    /// Returns a raw pointer to the underlying `pwm_ops` struct.
+    pub(crate) fn as_raw(&self) -> *const bindings::pwm_ops {
+        &self.0
+    }
+}
+
+/// Creates a PWM operations vtable for a type `T` that implements `PwmOps`.
+///
+/// This is used to bridge Rust trait implementations to the C `struct pwm_ops`
+/// expected by the kernel.
+pub const fn create_pwm_ops<T: PwmOps>() -> PwmOpsVTable {
+    // SAFETY: `core::mem::zeroed()` is unsafe. For `pwm_ops`, all fields are
+    // `Option<extern "C" fn(...)>` or data, so a zeroed pattern (None/0) is valid initially.
+    let mut ops: bindings::pwm_ops = unsafe { core::mem::zeroed() };
+
+    ops.request = Some(Adapter::<T>::request_callback);
+    ops.capture = Some(Adapter::<T>::capture_callback);
+
+    ops.round_waveform_tohw = Some(Adapter::<T>::round_waveform_tohw_callback);
+    ops.round_waveform_fromhw = Some(Adapter::<T>::round_waveform_fromhw_callback);
+    ops.read_waveform = Some(Adapter::<T>::read_waveform_callback);
+    ops.write_waveform = Some(Adapter::<T>::write_waveform_callback);
+    ops.sizeof_wfhw = core::mem::size_of::<T::WfHw>();
+
+    PwmOpsVTable(ops)
+}
+
+/// Wrapper for a PWM chip/controller ([`struct pwm_chip`](srctree/include/linux/pwm.h)).
+#[repr(transparent)]
+pub struct Chip<T: ForeignOwnable>(Opaque<bindings::pwm_chip>, PhantomData<T>);
+
+impl<T: ForeignOwnable> Chip<T> {
+    /// Creates a reference to a [`Chip`] from a valid pointer.
+    ///
+    /// # Safety
+    ///
+    /// The caller must ensure that `ptr` is valid and remains valid for the lifetime of the
+    /// returned [`Chip`] reference.
+    pub(crate) unsafe fn as_ref<'a>(ptr: *mut bindings::pwm_chip) -> &'a Self {
+        // SAFETY: The safety requirements guarantee the validity of the dereference, while the
+        // `Chip` type being transparent makes the cast ok.
+        unsafe { &*ptr.cast::<Self>() }
+    }
+
+    /// Returns a raw pointer to the underlying `pwm_chip`.
+    pub(crate) fn as_raw(&self) -> *mut bindings::pwm_chip {
+        self.0.get()
+    }
+
+    /// Gets the number of PWM channels (hardware PWMs) on this chip.
+    pub fn npwm(&self) -> u32 {
+        // SAFETY: `self.as_raw()` provides a valid pointer for `self`'s lifetime.
+        unsafe { (*self.as_raw()).npwm }
+    }
+
+    /// Returns `true` if the chip supports atomic operations for configuration.
+    pub fn is_atomic(&self) -> bool {
+        // SAFETY: `self.as_raw()` provides a valid pointer for `self`'s lifetime.
+        unsafe { (*self.as_raw()).atomic }
+    }
+
+    /// Returns a reference to the embedded `struct device` abstraction.
+    pub fn device(&self) -> &device::Device {
+        // SAFETY: `self.as_raw()` provides a valid pointer to `bindings::pwm_chip`.
+        // The `dev` field is an instance of `bindings::device` embedded within `pwm_chip`.
+        // Taking a pointer to this embedded field is valid.
+        // `device::Device` is `#[repr(transparent)]`.
+        // The lifetime of the returned reference is tied to `self`.
+        unsafe { device::Device::as_ref(&raw mut (*self.as_raw()).dev) }
+    }
+
+    /// Returns a reference to the parent device of this PWM chip's device.
+    ///
+    /// # Safety
+    ///
+    /// The caller must guarantee that the parent device exists and is bound.
+    /// This is guaranteed by the PWM core during `PwmOps` callbacks.
+    unsafe fn bound_parent_device(&self) -> &device::Device<Bound> {
+        // SAFETY: Per the function's safety contract, the parent device exists.
+        let parent = unsafe { self.device().parent().unwrap_unchecked() };
+
+        // SAFETY: Per the function's safety contract, the parent device is bound.
+        // The pointer is cast from `&Device` to `&Device<Bound>`.
+        unsafe { &*core::ptr::from_ref(parent).cast::<device::Device<Bound>>() }
+    }
+}
+
+impl<T: 'static + ForeignOwnable> Chip<T> {
+    /// Allocates and wraps a PWM chip using `bindings::pwmchip_alloc`.
+    ///
+    /// Returns an [`ARef<Chip>`] managing the chip's lifetime via refcounting
+    /// on its embedded `struct device`.
+    pub fn new<O: PwmOps<DrvData = T>>(
+        parent_dev: &device::Device,
+        npwm: u32,
+        sizeof_priv: usize,
+        drvdata: T,
+    ) -> Result<ARef<Self>> {
+        // SAFETY: `parent_device_for_dev_field.as_raw()` is valid.
+        // `bindings::pwmchip_alloc` returns a valid `*mut bindings::pwm_chip` (refcount 1)
+        // or an ERR_PTR.
+        let c_chip_ptr_raw =
+            unsafe { bindings::pwmchip_alloc(parent_dev.as_raw(), npwm, sizeof_priv) };
+
+        let c_chip_ptr: *mut bindings::pwm_chip = error::from_err_ptr(c_chip_ptr_raw)?;
+
+        // Set the custom release function on the embedded device. This is the crucial step
+        // to ensure `drvdata` is freed when the chip's refcount reaches zero, regardless
+        // of whether `Registration::register` was called.
+        // SAFETY: `c_chip_ptr` points to a valid chip.
+        unsafe { (*c_chip_ptr).dev.release = Some(Adapter::<O>::release_callback); }
+
+        // SAFETY: `c_chip_ptr` points to a valid chip from `pwmchip_alloc`.
+        // The `Adapter`'s `VTABLE` has a 'static lifetime, so the pointer
+        // returned by `as_raw()` is always valid.
+        unsafe { (*c_chip_ptr).ops = Adapter::<O>::VTABLE.as_raw(); }
+
+        // Cast the `*mut bindings::pwm_chip` to `*mut Chip`. This is valid because
+        // `Chip` is `repr(transparent)` over `Opaque<bindings::pwm_chip>`, and
+        // `Opaque<T>` is `repr(transparent)` over `T`.
+        let chip_ptr_as_self = c_chip_ptr.cast::<Self>();
+
+        // SAFETY: The pointer is valid, so we can create a temporary ref to set data.
+        let chip_ref = unsafe { &*chip_ptr_as_self };
+        // SAFETY: `chip_ref` points to a valid chip from `pwmchip_alloc` and `drvdata` is a valid,
+        // owned pointer from `ForeignOwnable` to be stored in the chip's private data.
+        unsafe { bindings::pwmchip_set_drvdata(chip_ref.as_raw(), drvdata.into_foreign().cast()) }
+
+        // SAFETY: `chip_ptr_as_self` points to a valid `Chip` (layout-compatible with
+        // `bindings::pwm_chip`) whose embedded device has refcount 1.
+        // `ARef::from_raw` takes this pointer and manages it via `AlwaysRefCounted`.
+        Ok(unsafe { ARef::from_raw(NonNull::new_unchecked(chip_ptr_as_self)) })
+    }
+
+    /// Gets the *typed* driver-specific data associated with this chip's embedded device.
+    pub fn drvdata(&self) -> T::Borrowed<'_> {
+        // SAFETY: `self.as_raw()` gives a valid pwm_chip pointer.
+        // `bindings::pwmchip_get_drvdata` is the C function to retrieve driver data.
+        let ptr = unsafe { bindings::pwmchip_get_drvdata(self.as_raw()) };
+
+        // SAFETY: The only way to create a chip is through Chip::new, which initializes
+        // this pointer.
+        unsafe { T::borrow(ptr.cast()) }
+    }
+}
+
+// SAFETY: Implements refcounting for `Chip` using the embedded `struct device`.
+unsafe impl<T: ForeignOwnable> AlwaysRefCounted for Chip<T> {
+    #[inline]
+    fn inc_ref(&self) {
+        // SAFETY: `self.0.get()` points to a valid `pwm_chip` because `self` exists.
+        // The embedded `dev` is valid. `get_device` increments its refcount.
+        unsafe { bindings::get_device(&raw mut (*self.0.get()).dev); }
+    }
+
+    #[inline]
+    unsafe fn dec_ref(obj: NonNull<Chip<T>>) {
+        let c_chip_ptr = obj.cast::<bindings::pwm_chip>().as_ptr();
+
+        // SAFETY: `obj` is a valid pointer to a `Chip` (and thus `bindings::pwm_chip`)
+        // with a non-zero refcount. `put_device` handles decrement and final release.
+        unsafe { bindings::put_device(&raw mut (*c_chip_ptr).dev); }
+    }
+}
+
+// SAFETY: `Chip` is a wrapper around `*mut bindings::pwm_chip`. The underlying C
+// structure's state is managed and synchronized by the kernel's device model
+// and PWM core locking mechanisms. Therefore, it is safe to move the `Chip`
+// wrapper (and the pointer it contains) across threads.
+unsafe impl<T: ForeignOwnable + Send> Send for Chip<T> {}
+
+// SAFETY: It is safe for multiple threads to have shared access (`&Chip`) because
+// the `Chip` data is immutable from the Rust side without holding the appropriate
+// kernel locks, which the C core is responsible for. Any interior mutability is
+// handled and synchronized by the C kernel code.
+unsafe impl<T: ForeignOwnable + Sync> Sync for Chip<T> {}
+
+/// A resource guard that ensures `pwmchip_remove` is called on drop.
+///
+/// This struct is intended to be managed by the `devres` framework by transferring its ownership
+/// via [`Devres::register`]. This ties the lifetime of the PWM chip registration
+/// to the lifetime of the underlying device.
+pub struct Registration<T: ForeignOwnable> {
+    chip: ARef<Chip<T>>,
+}
+
+impl<T: 'static + ForeignOwnable + Send + Sync> Registration<T> {
+    /// Registers a PWM chip with the PWM subsystem.
+    ///
+    /// Transfers its ownership to the `devres` framework, which ties its lifetime
+    /// to the parent device.
+    /// On unbind of the parent device, the `devres` entry will be dropped, automatically
+    /// calling `pwmchip_remove`. This function should be called from the driver's `probe`.
+    pub fn register(
+        dev: &device::Device<Bound>,
+        chip: ARef<Chip<T>>,
+    ) -> Result {
+	let chip_parent = chip.device().parent().ok_or(EINVAL)?;
+        if dev.as_raw() != chip_parent.as_raw() {
+            return Err(EINVAL);
+        }
+
+        let c_chip_ptr = chip.as_raw();
+
+        // SAFETY: `c_chip_ptr` points to a valid chip with its ops initialized.
+        // `__pwmchip_add` is the C function to register the chip with the PWM core.
+        unsafe {
+            to_result(bindings::__pwmchip_add(c_chip_ptr, core::ptr::null_mut()))?;
+        }
+
+        let registration = Registration { chip };
+
+        devres::register(dev, registration, GFP_KERNEL)
+    }
+}
+
+impl<T: ForeignOwnable> Drop for Registration<T> {
+    fn drop(&mut self) {
+        let chip_raw = self.chip.as_raw();
+
+        // SAFETY: `chip_raw` points to a chip that was successfully registered.
+        // `bindings::pwmchip_remove` is the correct C function to unregister it.
+        // This `drop` implementation is called automatically by `devres` on driver unbind.
+        unsafe {
+            bindings::pwmchip_remove(chip_raw);
+        }
+    }
+}

-- 
2.34.1