The `pwm::Registration::register` function provides no guarantee that the
function isn't called twice with the same pwm chip, which is considered
unsafe.

Add the code responsible for the registration into `pwm::Chip::new`. The
registration will happen before the driver gets access to the refcounted
pwm chip and can therefore guarantee that the registration isn't called
twice on the same pwm chip.

Signed-off-by: Markus Probst <markus.probst@posteo.de>
---
This patch provides the additional guarantee that the pwm chip doesn't
get registered twice.

The following changes were made:
- change the visibility of `pwm::Registration` to private
- remove the `pwm::Registration::register` function
- add code for registering the pwm chip in `pwm::Chip::new`
- add Send + Sync bounds to `PwmOps`

Note that I wasn't able to test this patch, due to the lack of hardware.
---
 drivers/pwm/pwm_th1520.rs |  4 +---
 rust/kernel/pwm.rs        | 53 ++++++++++++++++++-----------------------------
 2 files changed, 21 insertions(+), 36 deletions(-)

diff --git a/drivers/pwm/pwm_th1520.rs b/drivers/pwm/pwm_th1520.rs
index 955c359b07fb..1919b5a1f69e 100644
--- a/drivers/pwm/pwm_th1520.rs
+++ b/drivers/pwm/pwm_th1520.rs
@@ -363,7 +363,7 @@ fn probe(
             return Err(EINVAL);
         }
 
-        let chip = pwm::Chip::new(
+        pwm::Chip::new(
             dev,
             TH1520_MAX_PWM_NUM,
             try_pin_init!(Th1520PwmDriverData {
@@ -372,8 +372,6 @@ fn probe(
             }),
         )?;
 
-        pwm::Registration::register(dev, chip)?;
-
         Ok(KBox::new(Th1520PwmPlatformDriver, GFP_KERNEL)?.into())
     }
 }
diff --git a/rust/kernel/pwm.rs b/rust/kernel/pwm.rs
index cb00f8a8765c..c5d03ee8bc95 100644
--- a/rust/kernel/pwm.rs
+++ b/rust/kernel/pwm.rs
@@ -173,7 +173,7 @@ pub struct RoundedWaveform<WfHw> {
 }
 
 /// Trait defining the operations for a PWM driver.
-pub trait PwmOps: 'static + Sized {
+pub trait PwmOps: 'static + Send + Sync + Sized {
     /// The driver-specific hardware representation of a waveform.
     ///
     /// This type must be [`Copy`], [`Default`], and fit within `PWM_WFHWSIZE`.
@@ -585,7 +585,7 @@ unsafe fn bound_parent_device(&self) -> &device::Device<Bound> {
     /// Returns an [`ARef<Chip>`] managing the chip's lifetime via refcounting
     /// on its embedded `struct device`.
     pub fn new(
-        parent_dev: &device::Device,
+        parent_dev: &device::Device<Bound>,
         num_channels: u32,
         data: impl pin_init::PinInit<T, Error>,
     ) -> Result<ARef<Self>> {
@@ -623,7 +623,21 @@ pub fn new(
         // 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)) })
+        let chip = unsafe { ARef::from_raw(NonNull::new_unchecked(chip_ptr_as_self)) };
+
+        // 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: ARef::clone(&chip),
+        };
+
+        devres::register(parent_dev, registration, GFP_KERNEL)?;
+
+        Ok(chip)
     }
 }
 
@@ -654,50 +668,23 @@ unsafe fn dec_ref(obj: NonNull<Chip<T>>) {
 // 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: PwmOps + Send> Send for Chip<T> {}
+unsafe impl<T: PwmOps> 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: PwmOps + Sync> Sync for Chip<T> {}
+unsafe impl<T: PwmOps> 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: PwmOps> {
+struct Registration<T: PwmOps> {
     chip: ARef<Chip<T>>,
 }
 
-impl<T: 'static + PwmOps + 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: PwmOps> Drop for Registration<T> {
     fn drop(&mut self) {
         let chip_raw = self.chip.as_raw();

---
base-commit: fae00ea9f00367771003ace78f29549dead58fc7
change-id: 20251127-pwm_safe_register-e49b0a261101

Hi Markus,
On 11/27/25 18:15, Markus Probst wrote:
> The `pwm::Registration::register` function provides no guarantee that the
> function isn't called twice with the same pwm chip, which is considered
> unsafe.
> 
> Add the code responsible for the registration into `pwm::Chip::new`. The
> registration will happen before the driver gets access to the refcounted
> pwm chip and can therefore guarantee that the registration isn't called
> twice on the same pwm chip.

I agree that the double registration safety issue needs to be fixed.

However, merging the allocation and registration steps into Chip::new
restricts the API significantly.

There is a specific reason the C API separates pwmchip_alloc and
pwmchip_add. Drivers might need to configure the chip e.g. setting the
atomic flag, or acquire resources that require a pointer to the chip
(like requesting an IRQ) after allocation but before the device is
registered and visible to the subsystem.

I suggest we solve this using the Builder Pattern instead. This would
allow us to allocate, configure, and finally register the chip in
distinct safe steps.

Best regards,
-- 
Michal Wilczynski <m.wilczynski@samsung.com>

> 
> Signed-off-by: Markus Probst <markus.probst@posteo.de>
> ---
> This patch provides the additional guarantee that the pwm chip doesn't
> get registered twice.
> 
> The following changes were made:
> - change the visibility of `pwm::Registration` to private
> - remove the `pwm::Registration::register` function
> - add code for registering the pwm chip in `pwm::Chip::new`
> - add Send + Sync bounds to `PwmOps`
> 
> Note that I wasn't able to test this patch, due to the lack of hardware.
> ---
>  drivers/pwm/pwm_th1520.rs |  4 +---
>  rust/kernel/pwm.rs        | 53 ++++++++++++++++++-----------------------------
>  2 files changed, 21 insertions(+), 36 deletions(-)
> 
> diff --git a/drivers/pwm/pwm_th1520.rs b/drivers/pwm/pwm_th1520.rs
> index 955c359b07fb..1919b5a1f69e 100644
> --- a/drivers/pwm/pwm_th1520.rs
> +++ b/drivers/pwm/pwm_th1520.rs
> @@ -363,7 +363,7 @@ fn probe(
>              return Err(EINVAL);
>          }
>  
> -        let chip = pwm::Chip::new(
> +        pwm::Chip::new(
>              dev,
>              TH1520_MAX_PWM_NUM,
>              try_pin_init!(Th1520PwmDriverData {
> @@ -372,8 +372,6 @@ fn probe(
>              }),
>          )?;
>  
> -        pwm::Registration::register(dev, chip)?;
> -
>          Ok(KBox::new(Th1520PwmPlatformDriver, GFP_KERNEL)?.into())
>      }
>  }
> diff --git a/rust/kernel/pwm.rs b/rust/kernel/pwm.rs
> index cb00f8a8765c..c5d03ee8bc95 100644
> --- a/rust/kernel/pwm.rs
> +++ b/rust/kernel/pwm.rs
> @@ -173,7 +173,7 @@ pub struct RoundedWaveform<WfHw> {
>  }
>  
>  /// Trait defining the operations for a PWM driver.
> -pub trait PwmOps: 'static + Sized {
> +pub trait PwmOps: 'static + Send + Sync + Sized {
>      /// The driver-specific hardware representation of a waveform.
>      ///
>      /// This type must be [`Copy`], [`Default`], and fit within `PWM_WFHWSIZE`.
> @@ -585,7 +585,7 @@ unsafe fn bound_parent_device(&self) -> &device::Device<Bound> {
>      /// Returns an [`ARef<Chip>`] managing the chip's lifetime via refcounting
>      /// on its embedded `struct device`.
>      pub fn new(
> -        parent_dev: &device::Device,
> +        parent_dev: &device::Device<Bound>,
>          num_channels: u32,
>          data: impl pin_init::PinInit<T, Error>,
>      ) -> Result<ARef<Self>> {
> @@ -623,7 +623,21 @@ pub fn new(
>          // 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)) })
> +        let chip = unsafe { ARef::from_raw(NonNull::new_unchecked(chip_ptr_as_self)) };
> +
> +        // 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: ARef::clone(&chip),
> +        };
> +
> +        devres::register(parent_dev, registration, GFP_KERNEL)?;
> +
> +        Ok(chip)
>      }
>  }
>  
> @@ -654,50 +668,23 @@ unsafe fn dec_ref(obj: NonNull<Chip<T>>) {
>  // 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: PwmOps + Send> Send for Chip<T> {}
> +unsafe impl<T: PwmOps> 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: PwmOps + Sync> Sync for Chip<T> {}
> +unsafe impl<T: PwmOps> 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: PwmOps> {
> +struct Registration<T: PwmOps> {
>      chip: ARef<Chip<T>>,
>  }
>  
> -impl<T: 'static + PwmOps + 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: PwmOps> Drop for Registration<T> {
>      fn drop(&mut self) {
>          let chip_raw = self.chip.as_raw();
> 
> ---
> base-commit: fae00ea9f00367771003ace78f29549dead58fc7
> change-id: 20251127-pwm_safe_register-e49b0a261101
> 
>

> On 27 Nov 2025, at 14:15, Markus Probst <markus.probst@posteo.de> wrote:
> 
> The `pwm::Registration::register` function provides no guarantee that the
> function isn't called twice with the same pwm chip, which is considered
> unsafe.
> 
> Add the code responsible for the registration into `pwm::Chip::new`. The
> registration will happen before the driver gets access to the refcounted
> pwm chip and can therefore guarantee that the registration isn't called
> twice on the same pwm chip.
> 
> Signed-off-by: Markus Probst <markus.probst@posteo.de>
> ---
> This patch provides the additional guarantee that the pwm chip doesn't
> get registered twice.
> 
> The following changes were made:
> - change the visibility of `pwm::Registration` to private
> - remove the `pwm::Registration::register` function
> - add code for registering the pwm chip in `pwm::Chip::new`
> - add Send + Sync bounds to `PwmOps`
> 
> Note that I wasn't able to test this patch, due to the lack of hardware.
> ---
> drivers/pwm/pwm_th1520.rs |  4 +---
> rust/kernel/pwm.rs        | 53 ++++++++++++++++++-----------------------------
> 2 files changed, 21 insertions(+), 36 deletions(-)
> 
> diff --git a/drivers/pwm/pwm_th1520.rs b/drivers/pwm/pwm_th1520.rs
> index 955c359b07fb..1919b5a1f69e 100644
> --- a/drivers/pwm/pwm_th1520.rs
> +++ b/drivers/pwm/pwm_th1520.rs
> @@ -363,7 +363,7 @@ fn probe(
>             return Err(EINVAL);
>         }
> 
> -        let chip = pwm::Chip::new(
> +        pwm::Chip::new(
>             dev,
>             TH1520_MAX_PWM_NUM,
>             try_pin_init!(Th1520PwmDriverData {
> @@ -372,8 +372,6 @@ fn probe(
>             }),
>         )?;
> 
> -        pwm::Registration::register(dev, chip)?;
> -
>         Ok(KBox::new(Th1520PwmPlatformDriver, GFP_KERNEL)?.into())
>     }
> }
> diff --git a/rust/kernel/pwm.rs b/rust/kernel/pwm.rs
> index cb00f8a8765c..c5d03ee8bc95 100644
> --- a/rust/kernel/pwm.rs
> +++ b/rust/kernel/pwm.rs
> @@ -173,7 +173,7 @@ pub struct RoundedWaveform<WfHw> {
> }
> 
> /// Trait defining the operations for a PWM driver.
> -pub trait PwmOps: 'static + Sized {
> +pub trait PwmOps: 'static + Send + Sync + Sized {
>     /// The driver-specific hardware representation of a waveform.
>     ///
>     /// This type must be [`Copy`], [`Default`], and fit within `PWM_WFHWSIZE`.
> @@ -585,7 +585,7 @@ unsafe fn bound_parent_device(&self) -> &device::Device<Bound> {
>     /// Returns an [`ARef<Chip>`] managing the chip's lifetime via refcounting
>     /// on its embedded `struct device`.
>     pub fn new(
> -        parent_dev: &device::Device,
> +        parent_dev: &device::Device<Bound>,
>         num_channels: u32,
>         data: impl pin_init::PinInit<T, Error>,
>     ) -> Result<ARef<Self>> {
> @@ -623,7 +623,21 @@ pub fn new(
>         // 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)) })
> +        let chip = unsafe { ARef::from_raw(NonNull::new_unchecked(chip_ptr_as_self)) };
> +
> +        // 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: ARef::clone(&chip),
> +        };
> +
> +        devres::register(parent_dev, registration, GFP_KERNEL)?;
> +
> +        Ok(chip)
>     }
> }
> 
> @@ -654,50 +668,23 @@ unsafe fn dec_ref(obj: NonNull<Chip<T>>) {
> // 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: PwmOps + Send> Send for Chip<T> {}
> +unsafe impl<T: PwmOps> 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: PwmOps + Sync> Sync for Chip<T> {}
> +unsafe impl<T: PwmOps> 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: PwmOps> {
> +struct Registration<T: PwmOps> {
>     chip: ARef<Chip<T>>,
> }
> 
> -impl<T: 'static + PwmOps + 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: PwmOps> Drop for Registration<T> {
>     fn drop(&mut self) {
>         let chip_raw = self.chip.as_raw();
> 
> ---
> base-commit: fae00ea9f00367771003ace78f29549dead58fc7
> change-id: 20251127-pwm_safe_register-e49b0a261101
> 
> 

Reviewed-by: Daniel Almeida <daniel.almeida@collabora.com>

On Thu, Nov 27, 2025 at 05:15:06PM +0000, Markus Probst wrote:
> The `pwm::Registration::register` function provides no guarantee that the
> function isn't called twice with the same pwm chip, which is considered
> unsafe.
> 
> Add the code responsible for the registration into `pwm::Chip::new`. The
> registration will happen before the driver gets access to the refcounted
> pwm chip and can therefore guarantee that the registration isn't called
> twice on the same pwm chip.
> 
> Signed-off-by: Markus Probst <markus.probst@posteo.de>
> ---
> This patch provides the additional guarantee that the pwm chip doesn't
> get registered twice.
> 
> The following changes were made:
> - change the visibility of `pwm::Registration` to private
> - remove the `pwm::Registration::register` function
> - add code for registering the pwm chip in `pwm::Chip::new`
> - add Send + Sync bounds to `PwmOps`
> 
> Note that I wasn't able to test this patch, due to the lack of hardware.

Overall looks reasonable, but I have one question:

> @@ -654,50 +668,23 @@ unsafe fn dec_ref(obj: NonNull<Chip<T>>) {
>  // 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: PwmOps + Send> Send for Chip<T> {}
> +unsafe impl<T: PwmOps> 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: PwmOps + Sync> Sync for Chip<T> {}
> +unsafe impl<T: PwmOps> Sync for Chip<T> {}

Why was this changed?

Alice

On Fri, 2025-11-28 at 09:28 +0000, Alice Ryhl wrote:
> On Thu, Nov 27, 2025 at 05:15:06PM +0000, Markus Probst wrote:
> > The `pwm::Registration::register` function provides no guarantee that the
> > function isn't called twice with the same pwm chip, which is considered
> > unsafe.
> > 
> > Add the code responsible for the registration into `pwm::Chip::new`. The
> > registration will happen before the driver gets access to the refcounted
> > pwm chip and can therefore guarantee that the registration isn't called
> > twice on the same pwm chip.
> > 
> > Signed-off-by: Markus Probst <markus.probst@posteo.de>
> > ---
> > This patch provides the additional guarantee that the pwm chip doesn't
> > get registered twice.
> > 
> > The following changes were made:
> > - change the visibility of `pwm::Registration` to private
> > - remove the `pwm::Registration::register` function
> > - add code for registering the pwm chip in `pwm::Chip::new`
> > - add Send + Sync bounds to `PwmOps`
> > 
> > Note that I wasn't able to test this patch, due to the lack of hardware.
> 
> Overall looks reasonable, but I have one question:
> 
> > @@ -654,50 +668,23 @@ unsafe fn dec_ref(obj: NonNull<Chip<T>>) {
> >  // 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: PwmOps + Send> Send for Chip<T> {}
> > +unsafe impl<T: PwmOps> 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: PwmOps + Sync> Sync for Chip<T> {}
> > +unsafe impl<T: PwmOps> Sync for Chip<T> {}
> 
> Why was this changed?

Registration::register required PwmOps to be Send + Sync.
Prior to this change, Chip::new didn't require it for PwmOps. Meaning
it was possible to allocate a new Chip with a PwmOps that is not Send +
Sync. As there was no use for it and it isn't possible anymore to
allocate a new Chip without registering it, I added Send + Sync as
trait dependency (see 1. hunk of rust/kernel/pwm.rs in the patch).

Because PwmOps now implied Send + Sync, it wasn't necessary anymore to
have the additional bounds there.

Thanks
- Markus Probst

> 
> Alice

On Fri, Nov 28, 2025 at 12:25:04PM +0000, Markus Probst wrote:
> On Fri, 2025-11-28 at 09:28 +0000, Alice Ryhl wrote:
> > On Thu, Nov 27, 2025 at 05:15:06PM +0000, Markus Probst wrote:
> > > The `pwm::Registration::register` function provides no guarantee that the
> > > function isn't called twice with the same pwm chip, which is considered
> > > unsafe.
> > > 
> > > Add the code responsible for the registration into `pwm::Chip::new`. The
> > > registration will happen before the driver gets access to the refcounted
> > > pwm chip and can therefore guarantee that the registration isn't called
> > > twice on the same pwm chip.
> > > 
> > > Signed-off-by: Markus Probst <markus.probst@posteo.de>
> > > ---
> > > This patch provides the additional guarantee that the pwm chip doesn't
> > > get registered twice.
> > > 
> > > The following changes were made:
> > > - change the visibility of `pwm::Registration` to private
> > > - remove the `pwm::Registration::register` function
> > > - add code for registering the pwm chip in `pwm::Chip::new`
> > > - add Send + Sync bounds to `PwmOps`
> > > 
> > > Note that I wasn't able to test this patch, due to the lack of hardware.
> > 
> > Overall looks reasonable, but I have one question:
> > 
> > > @@ -654,50 +668,23 @@ unsafe fn dec_ref(obj: NonNull<Chip<T>>) {
> > >  // 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: PwmOps + Send> Send for Chip<T> {}
> > > +unsafe impl<T: PwmOps> 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: PwmOps + Sync> Sync for Chip<T> {}
> > > +unsafe impl<T: PwmOps> Sync for Chip<T> {}
> > 
> > Why was this changed?
> 
> Registration::register required PwmOps to be Send + Sync.
> Prior to this change, Chip::new didn't require it for PwmOps. Meaning
> it was possible to allocate a new Chip with a PwmOps that is not Send +
> Sync. As there was no use for it and it isn't possible anymore to
> allocate a new Chip without registering it, I added Send + Sync as
> trait dependency (see 1. hunk of rust/kernel/pwm.rs in the patch).
> 
> Because PwmOps now implied Send + Sync, it wasn't necessary anymore to
> have the additional bounds there.

Ah ok, I missed that. That's fine then.

Alice

On Thu, 2025-11-27 at 17:15 +0000, Markus Probst wrote:
> The `pwm::Registration::register` function provides no guarantee that the
> function isn't called twice with the same pwm chip, which is considered
> unsafe.
> 
> Add the code responsible for the registration into `pwm::Chip::new`. The
> registration will happen before the driver gets access to the refcounted
> pwm chip and can therefore guarantee that the registration isn't called
> twice on the same pwm chip.
> 
> Signed-off-by: Markus Probst <markus.probst@posteo.de>
> ---
> This patch provides the additional guarantee that the pwm chip doesn't
> get registered twice.
> 
> The following changes were made:
> - change the visibility of `pwm::Registration` to private
> - remove the `pwm::Registration::register` function
> - add code for registering the pwm chip in `pwm::Chip::new`
> - add Send + Sync bounds to `PwmOps`
> 
> Note that I wasn't able to test this patch, due to the lack of hardware.
> ---
>  drivers/pwm/pwm_th1520.rs |  4 +---
>  rust/kernel/pwm.rs        | 53 ++++++++++++++++++-----------------------------
>  2 files changed, 21 insertions(+), 36 deletions(-)
> 
> diff --git a/drivers/pwm/pwm_th1520.rs b/drivers/pwm/pwm_th1520.rs
> index 955c359b07fb..1919b5a1f69e 100644
> --- a/drivers/pwm/pwm_th1520.rs
> +++ b/drivers/pwm/pwm_th1520.rs
> @@ -363,7 +363,7 @@ fn probe(
>              return Err(EINVAL);
>          }
>  
> -        let chip = pwm::Chip::new(
> +        pwm::Chip::new(
>              dev,
>              TH1520_MAX_PWM_NUM,
>              try_pin_init!(Th1520PwmDriverData {
> @@ -372,8 +372,6 @@ fn probe(
>              }),
>          )?;
>  
> -        pwm::Registration::register(dev, chip)?;
> -
>          Ok(KBox::new(Th1520PwmPlatformDriver, GFP_KERNEL)?.into())
>      }
>  }
> diff --git a/rust/kernel/pwm.rs b/rust/kernel/pwm.rs
> index cb00f8a8765c..c5d03ee8bc95 100644
> --- a/rust/kernel/pwm.rs
> +++ b/rust/kernel/pwm.rs
> @@ -173,7 +173,7 @@ pub struct RoundedWaveform<WfHw> {
>  }
>  
>  /// Trait defining the operations for a PWM driver.
> -pub trait PwmOps: 'static + Sized {
> +pub trait PwmOps: 'static + Send + Sync + Sized {
>      /// The driver-specific hardware representation of a waveform.
>      ///
>      /// This type must be [`Copy`], [`Default`], and fit within `PWM_WFHWSIZE`.
> @@ -585,7 +585,7 @@ unsafe fn bound_parent_device(&self) -> &device::Device<Bound> {
>      /// Returns an [`ARef<Chip>`] managing the chip's lifetime via refcounting
>      /// on its embedded `struct device`.
>      pub fn new(
> -        parent_dev: &device::Device,
> +        parent_dev: &device::Device<Bound>,
>          num_channels: u32,
>          data: impl pin_init::PinInit<T, Error>,
>      ) -> Result<ARef<Self>> {
> @@ -623,7 +623,21 @@ pub fn new(
>          // 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)) })
> +        let chip = unsafe { ARef::from_raw(NonNull::new_unchecked(chip_ptr_as_self)) };
> +
> +        // 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: ARef::clone(&chip),
> +        };
> +
> +        devres::register(parent_dev, registration, GFP_KERNEL)?;
> +
> +        Ok(chip)
>      }
>  }
>  
> @@ -654,50 +668,23 @@ unsafe fn dec_ref(obj: NonNull<Chip<T>>) {
>  // 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: PwmOps + Send> Send for Chip<T> {}
> +unsafe impl<T: PwmOps> 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: PwmOps + Sync> Sync for Chip<T> {}
> +unsafe impl<T: PwmOps> 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: PwmOps> {
> +struct Registration<T: PwmOps> {
>      chip: ARef<Chip<T>>,
>  }
>  
> -impl<T: 'static + PwmOps + 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: PwmOps> Drop for Registration<T> {
>      fn drop(&mut self) {
>          let chip_raw = self.chip.as_raw();
> 
> ---
> base-commit: fae00ea9f00367771003ace78f29549dead58fc7
> change-id: 20251127-pwm_safe_register-e49b0a261101

Just noticed 5 sec too late, that I forgot the "rust: pwm: " commit
message prefix.

Thanks
- Markus Probst