linux-kernel - Re: [PATCH V10 13/15] rust: cpufreq: Extend abstractions for driver registration

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Message-ID: <Z_9ysHFmvZvaoe8H@pollux>
Date: Wed, 16 Apr 2025 11:04:48 +0200
From: Danilo Krummrich <dakr@...nel.org>
To: Viresh Kumar <viresh.kumar@...aro.org>
Cc: "Rafael J. Wysocki" <rafael@...nel.org>,
	Miguel Ojeda <miguel.ojeda.sandonis@...il.com>,
	Danilo Krummrich <dakr@...hat.com>, Miguel Ojeda <ojeda@...nel.org>,
	Alex Gaynor <alex.gaynor@...il.com>,
	Boqun Feng <boqun.feng@...il.com>, Gary Guo <gary@...yguo.net>,
	Björn Roy Baron <bjorn3_gh@...tonmail.com>,
	Benno Lossin <benno.lossin@...ton.me>,
	Andreas Hindborg <a.hindborg@...nel.org>,
	Alice Ryhl <aliceryhl@...gle.com>, Trevor Gross <tmgross@...ch.edu>,
	linux-pm@...r.kernel.org,
	Vincent Guittot <vincent.guittot@...aro.org>,
	Stephen Boyd <sboyd@...nel.org>, Nishanth Menon <nm@...com>,
	rust-for-linux@...r.kernel.org,
	Manos Pitsidianakis <manos.pitsidianakis@...aro.org>,
	Alex Bennée <alex.bennee@...aro.org>,
	Joakim Bech <joakim.bech@...aro.org>, Rob Herring <robh@...nel.org>,
	Yury Norov <yury.norov@...il.com>, Burak Emir <bqe@...gle.com>,
	Rasmus Villemoes <linux@...musvillemoes.dk>,
	Russell King <linux@...linux.org.uk>, linux-clk@...r.kernel.org,
	Michael Turquette <mturquette@...libre.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH V10 13/15] rust: cpufreq: Extend abstractions for driver
 registration

On Wed, Apr 16, 2025 at 12:09:30PM +0530, Viresh Kumar wrote:
> +    /// Registers a CPU frequency driver with the cpufreq core.
> +    pub fn new() -> Result<Self> {
> +        let drv: *const bindings::cpufreq_driver = &Self::VTABLE;
> +        let drv = drv.cast_mut();
> +
> +        // SAFETY: It is safe to register the driver with the cpufreq core in the kernel C code.
> +        to_result(unsafe { bindings::cpufreq_register_driver(drv) })?;

You need to justify why drv is a valid pointer to be passed to
cpufreq_register_driver(), i.e. something like

	// SAFETY:
	// - `drv` comes from Self::VTABLE and hence is a valid pointer to a `struct cpufreq_driver`,
	// - `cpufreq_register_driver()` never attempts to modify the data `drv` points to

> +
> +        Ok(Self(
> +            NonNull::new(drv.cast()).ok_or(AllocError)?,

We know `drv` can't be NULL, hence it's better to use NonNull::new_unchecked().

> +            PhantomData,
> +        ))
> +    }
> +
> +    /// Same as [`Registration::new`], but does not return a [`Registration`] instance.
> +    ///
> +    /// Instead the [`Registration`] is owned by [`Devres`] and will be revoked / dropped, once the
> +    /// device is detached.
> +    pub fn new_foreign_owned(dev: &Device) -> Result<()> {
> +        Devres::new_foreign_owned(dev, Self::new()?, GFP_KERNEL)?;

If you remove the question mark operator and the semicolon, you can remove the
below.

> +        Ok(())
> +    }
> +}

<snip>

> +impl<T: Driver> Drop for Registration<T> {
> +    // Removes the `Registration` from the kernel, if it has initialized successfully earlier.
> +    fn drop(&mut self) {
> +        // SAFETY: The driver was earlier registered from `new`.

Should be similar to the safety comment in Self::new().

> +        unsafe { bindings::cpufreq_unregister_driver(self.0.as_ptr()) };
> +    }
> +}

With those fixed,

	Reviewed-by: Danilo Krummrich <dakr@...nel.org>