Re: [PATCH v2 1/3] rust: add runtime PM support

["Onur Özkan" <work@onurozkan.dev>]

On Wed, 29 Jul 2026 11:07:12 +0200
Beata Michalska <beata.michalska@arm.com> wrote:

> On Mon, Jul 27, 2026 at 05:56:20PM +0300, Onur Özkan wrote:
> > On Tue, 21 Jul 2026 17:34:02 +0200
> > Beata Michalska <beata.michalska@arm.com> wrote:
> > 
> > > Add a Rust abstraction for Linux runtime PM, allowing Rust drivers to
> > > register runtime PM callbacks and use scoped runtime PM requests while
> > > retaining the behavior and guarantees of the underlying C PM core.
> > > 
> > > Runtime PM is managed through a registration object tied to the device
> > > lifetime. The registration stores the callback payload and 'ensures' that
> > > the registration, its `PMContext`, and the payload do not outlive the
> > > device. All that while `PMContext` holds a lifetime-annotated device reference
> > > so that runtime PM requests are made only while the device remains bound.
> > > 
> > > Drivers define runtime PM callbacks through the `PMOps` trait. The
> > > generated `dev_pm_ops` callbacks take the stored payload, pass ownership
> > > to the corresponding Rust callback, and store the returned payload for
> > > the next invocation. Each callback must return a valid payload,
> > > regardless of whether the power transition succeeds or not.
> > > 
> > > `PMContext` provides scoped helpers for common runtime PM operations:
> > > 
> > > - `ResumeScope` resumes the device for the lifetime of the scope.
> > > - `AwakeScope`  resumes the device and holds a runtime PM usage reference.
> > > - `RetainScope` increments the runtime PM usage count without resuming the
> > >   		device.
> > > 
> > > Each scope performs the corresponding inverse operation when dropped,
> > > according to the selected driver-defined profile.
> > > 
> > > The abstraction does not change the semantics of the C runtime PM API.
> > > Asynchronous requests may only queue work, non-waiting requests may fail
> > > instead of sleeping, and callbacks remain subject to the existing driver
> > > core rules.
> > > 
> > > The current implementation requires the PM registration and `PMOps`
> > > implementation to use the same type because callback dispatch interprets
> > > the stored registration data using that relationship. This requirement is
> > > not yet fully enforced by the type system.
> > > 
> > > Signed-off-by: Beata Michalska <beata.michalska@arm.com>
> > > ---
> > >  drivers/base/base.h             |    3 +
> > >  rust/bindings/bindings_helper.h |    1 +
> > >  rust/helpers/helpers.c          |    1 +
> > >  rust/helpers/pm_runtime.c       |   44 ++
> > >  rust/kernel/lib.rs              |    1 +
> > >  rust/kernel/pm.rs               | 1020 +++++++++++++++++++++++++++++++
> > >  6 files changed, 1070 insertions(+)
> > >  create mode 100644 rust/helpers/pm_runtime.c
> > >  create mode 100644 rust/kernel/pm.rs
> > > 
> > > diff --git a/drivers/base/base.h b/drivers/base/base.h
> > > index a19f4cda2c83..c49303e4a86a 100644
> > > --- a/drivers/base/base.h
> > > +++ b/drivers/base/base.h
> > > @@ -119,6 +119,9 @@ struct device_private {
> > >  	const struct device_driver *async_driver;
> > >  	char *deferred_probe_reason;
> > >  	struct device *device;
> > > +#ifdef CONFIG_RUST
> > > +	void *rust_private;
> > > +#endif
> > >  	u8 dead:1;
> > >  };
> > >  #define to_device_private_parent(obj)	\
> > > diff --git a/rust/bindings/bindings_helper.h b/rust/bindings/bindings_helper.h
> > > index 1124785e210b..78a9153c6a01 100644
> > > --- a/rust/bindings/bindings_helper.h
> > > +++ b/rust/bindings/bindings_helper.h
> > > @@ -77,6 +77,7 @@
> > >  #include <linux/pid_namespace.h>
> > >  #include <linux/platform_device.h>
> > >  #include <linux/pm_opp.h>
> > > +#include <linux/pm_runtime.h>
> > >  #include <linux/poll.h>
> > >  #include <linux/property.h>
> > >  #include <linux/pwm.h>
> > > diff --git a/rust/helpers/helpers.c b/rust/helpers/helpers.c
> > > index 4488a87223b9..14d93a5d8b1f 100644
> > > --- a/rust/helpers/helpers.c
> > > +++ b/rust/helpers/helpers.c
> > > @@ -76,6 +76,7 @@
> > >  #include "pci.c"
> > >  #include "pid_namespace.c"
> > >  #include "platform.c"
> > > +#include "pm_runtime.c"
> > >  #include "poll.c"
> > >  #include "processor.c"
> > >  #include "property.c"
> > > diff --git a/rust/helpers/pm_runtime.c b/rust/helpers/pm_runtime.c
> > > new file mode 100644
> > > index 000000000000..d0d71fcb0097
> > > --- /dev/null
> > > +++ b/rust/helpers/pm_runtime.c
> > > @@ -0,0 +1,44 @@
> > > +// SPDX-License-Identifier: GPL-2.0
> > > +
> > > +#include <linux/pm_runtime.h>
> > > +
> > > +__rust_helper void rust_helper_pm_runtime_get_noresume(struct device *dev)
> > > +{
> > > +	pm_runtime_get_noresume(dev);
> > > +}
> > > +
> > > +__rust_helper void rust_helper_pm_runtime_put_noidle(struct device *dev)
> > > +{
> > > +	pm_runtime_put_noidle(dev);
> > > +}
> > > +
> > > +__rust_helper void rust_helper_pm_runtime_mark_last_busy(struct device *dev)
> > > +{
> > > +	pm_runtime_mark_last_busy(dev) ;
> > > +}
> > > +
> > > +__rust_helper bool rust_helper_pm_runtime_active(struct device *dev)
> > > +{
> > > +	return pm_runtime_active(dev);

[...]

> > > +        // The `Device<Bound>` reference provides that guarantee.
> > > +        unsafe {
> > > +            bindings::pm_runtime_mark_last_busy(dev.as_raw());
> > > +        }
> > > +    }
> > > +
> > > +    #[inline]
> > > +    fn suspend(dev: &device::Device<device::Bound>, mode: Mode) -> Result {
> > > +        // SAFETY: `dev.as_raw()` must provide a valid pointer to
> > > +        // `struct device` for the duration of the call.
> > > +        // The `Device<Bound>` reference provides that guarantee.
> > > +        to_result(unsafe { bindings::__pm_runtime_suspend(dev.as_raw(), mode.into()) })
> > > +    }
> > > +
> > > +    #[inline]
> > > +    fn get_if_active(dev: &device::Device<device::Bound>) -> Result {
> > > +        // SAFETY: `dev.as_raw()` must provide a valid pointer to
> > > +        // `struct device` for the duration of the call.
> > > +        // The `Device<Bound>` reference provides that guarantee.
> > > +        match unsafe { bindings::pm_runtime_get_if_active(dev.as_raw()) } {
> > > +            ret if ret < 0 => Err(Error::from_errno(ret)),
> > > +            0 => Err(EAGAIN),
> > > +            _ => Ok(()),
> > > +        }
> > > +    }
> > > +
> > > +    #[inline]
> > > +    fn runtime_enable(dev: &device::Device<device::Bound>) {
> > > +        // SAFETY: `dev.as_raw()` must provide a valid pointer to
> > > +        // `struct device` for the duration of the call.
> > > +        // The `Device<Bound>` reference provides that guarantee.
> > > +        unsafe { bindings::pm_runtime_enable(dev.as_raw()) }
> > > +    }
> > > +
> > > +    #[inline]
> > > +    fn runtime_disable(dev: &device::Device<device::Bound>) {
> > > +        // SAFETY: `dev.as_raw()` must provide a valid pointer to
> > > +        // `struct device` for the duration of the call.
> > > +        // The `Device<Bound>` reference provides that guarantee.
> > > +        unsafe { bindings::__pm_runtime_disable(dev.as_raw(), true) };
> > > +    }
> > 
> > What happens when you call these twice i.e. runtime_enable() when it's already
> > enabled or runtime_disable() when it's already disabled?
> > 
> For pm_runtime_enable  - this will drop a warning.
> For pm_runtime_disable - the disable_depth counter will be increased. This
> implies that for every extra disable call - enable one needs to be issued.
> All handled by PM core.

I see. I was about to suggest typestate pattern here as we can easily avoid
these issues, but if you want to rely on PM core that's also fine I guess.

> > > +}
> > > +
> > > +#[cfg(not(CONFIG_PM))]
> > > +impl Request {
> > > +    #[inline]
> > > +    fn active(dev: &device::Device<device::Bound>) -> bool {
> > > +        true
> > > +    }
> > > +
> > > +    #[inline]
> > > +    fn suspended(dev: &device::Device<device::Bound>) -> bool {
> > > +        false
> > > +    }
> > > +
> > > +    #[inline]
> > > +    fn resume(_dev: &device::Device<device::Bound>, _mode: Mode) -> Result {
> > > +        Ok(())
> > > +    }
> > > +
> > > +    #[inline]
> > > +    fn idle(_dev: &device::Device<device::Bound>, _mode: Mode) -> Result {
> > > +        Err(ENOSYS)
> > > +    }
> > > +

[...]

> > > +    /// Returns the runtime PM context associated with this registration.
> > > +    pub fn ctx(&self) -> &PMContext<'a, T> {
> > > +        &self.ctx
> > > +    }
> > > +}
> > > +
> > > +impl<'a, T: PMOps> Drop for Registration<'a, T> {
> > > +    fn drop(&mut self) {
> > > +        // SAFETY: `self.ctx.inner.dev` is the device this registration was
> > > +        // created for. Runtime PM is disabled first, and `pm_runtime_barrier`
> > > +        // waits for pending runtime PM work/callbacks before the callback data
> > > +        // is removed below.
> > > +
> > > +        unsafe {
> > > +            bindings::__pm_runtime_disable(self.ctx.inner.dev.as_raw(), true);
> > 
> > Same here, is this safe to call even if it's not enabled? Registration can drop
> > even when pm runtime is not enabled, right?
> True but that again is handled by PM core.

You should at least document that.

> > 
> > > +            bindings::pm_runtime_barrier(self.ctx.inner.dev.as_raw());
> > > +        }
> > 
> > You should not combine multiple unsafe calls in single unsafe block I think.
> Why?

To keep the scope of each unsafe block as small as possible.

> Do not see much point in separating those.
> 
> ---
> BR
> Beata
> > 
> > > +        // SAFETY: The pointer, if non-null, was stored by `Registration::new`
> > > +        // using `Pin<KBox<RegistrationData<T>>>::into_foreign`. Runtime PM has
> > > +        // been disabled and drained above, so generated callbacks can no longer
> > > +        // borrow this data. Clearing `rust_private` prevents later lookup, and
> > > +        // `from_foreign` reconstructs the owning allocation so it is dropped.
> > > +        unsafe {
> > > +            let ptr = (*(*self.ctx.inner.dev.as_raw()).p).rust_private;
> > > +
> > > +            if !ptr.is_null() {
> > > +                (*(*self.ctx.inner.dev.as_raw()).p).rust_private = core::ptr::null_mut();
> > > +                Pin::<KBox<RegistrationData<'_, T>>>::from_foreign(ptr);
> > > +            }
> > > +        }
> > > +    }
> > > +}
> > > -- 
> > > 2.43.0
> > >