Re: [RFC PATCH V2 1/8] rust: Add initial bindings for OPP framework

[Alice Ryhl <aliceryhl@google.com>]

On Fri, Jun 7, 2024 at 11:12 AM Viresh Kumar <viresh.kumar@linaro.org> wrote:
>
> This commit adds initial Rust bindings for the Operating performance
> points (OPP) core. This adds bindings for `struct dev_pm_opp` and
> `struct dev_pm_opp_data` to begin with.
>
> Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>

> +//! Operating performance points.
> +//!
> +//! This module provides bindings for interacting with the OPP subsystem.
> +//!
> +//! C header: [`include/linux/pm_opp.h`](../../../../../../include/linux/pm_opp.h)

Please use srctree links instead.

C header: [`include/linux/pm_opp.h`](srctree/include/linux/pm_opp.h)

> +impl OPP {
> +    /// Creates a reference to a [`OPP`] from a valid pointer.
> +    ///
> +    /// # Safety
> +    ///
> +    /// The caller must ensure that `ptr` is valid and remains valid for the lifetime of the
> +    /// returned [`OPP`] reference.
> +    pub unsafe fn from_ptr_owned(ptr: *mut bindings::dev_pm_opp) -> Result<ARef<Self>> {
> +        let ptr = ptr::NonNull::new(ptr).ok_or(ENODEV)?;
> +
> +        // SAFETY: The safety requirements guarantee the validity of the pointer.
> +        //
> +        // INVARIANT: The refcount is already incremented by the C API that returned the pointer,
> +        // and we pass ownership of the refcount to the new `ARef<OPP>`.
> +        Ok(unsafe { ARef::from_raw(ptr.cast()) })
> +    }
> +
> +    /// Creates a reference to a [`OPP`] from a valid pointer.
> +    ///
> +    /// # Safety
> +    ///
> +    /// The caller must ensure that `ptr` is valid and remains valid for the lifetime of the
> +    /// returned [`OPP`] reference.
> +    pub unsafe fn from_ptr(ptr: *mut bindings::dev_pm_opp) -> Result<ARef<Self>> {
> +        let opp = unsafe { Self::from_ptr_owned(ptr) }?;
> +
> +        // Take an extra reference to the OPP since the caller didn't take it.
> +        opp.inc_ref();
> +
> +        Ok(opp)
> +    }

I would recommend a slightly different approach here. You can provide
a method called `from_raw_opp` that takes a *mut bindings::dev_pm_opp
and returns a &Self. The ARef type provides a method that converts
&Self to ARef<Self> by taking a refcount. This way, users would also
be able to call OPP methods without giving Rust any refcounts. You can
compare to my file patchset, where I am going to rename the equivalent
method to `from_raw_file` in the next version.

As for `from_ptr_owned`, I would probably rename it to
`from_raw_opp_owned` or similar. It's often nice to use a more
descriptive name than just "ptr".

> +    fn as_mut_ptr(&self) -> *mut bindings::dev_pm_opp {
> +        self.0.get()
> +    }

I think most existing examples call this `as_raw` and mark it `#[inline]`.

> +    /// Adds an OPP dynamically.
> +    pub fn add(dev: ARef<Device>, mut data: Data) -> Result<()> {
> +        // SAFETY: The requirements are satisfied by the existence of `Device` and its safety
> +        // requirements.
> +        to_result(unsafe { bindings::dev_pm_opp_add_dynamic(dev.as_raw(), &mut data.0) })
> +    }
> +
> +    /// Removes a dynamically added OPP.
> +    pub fn remove(dev: ARef<Device>, freq: u64) {
> +        // SAFETY: The requirements are satisfied by the existence of `Device` and its safety
> +        // requirements.
> +        unsafe { bindings::dev_pm_opp_remove(dev.as_raw(), freq) };
> +    }

Is it intentional that these methods take ownership of a refcount to
the device that it then drops after calling the C function?

Also, why are these methods defined on OPP when they appear to be
methods on Device and don't take any OPP argument?

Alice