linux-kernel - Re: [PATCH V9 09/17] rust: cpu: Add from

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Message-ID: <Z_lZWPr1vBCrYbkA@cassiopeiae>
Date: Fri, 11 Apr 2025 20:03:04 +0200
From: Danilo Krummrich <dakr@...nel.org>
To: Yury Norov <yury.norov@...il.com>
Cc: Viresh Kumar <viresh.kumar@...aro.org>,
	"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>,
	Thomas Gleixner <tglx@...utronix.de>,
	Peter Zijlstra <peterz@...radead.org>, 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>,
	Erik Schilling <erik.schilling@...aro.org>,
	Alex Bennée <alex.bennee@...aro.org>,
	Joakim Bech <joakim.bech@...aro.org>, Rob Herring <robh@...nel.org>,
	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 V9 09/17] rust: cpu: Add from_cpu()

On Fri, Apr 11, 2025 at 12:18:26PM -0400, Yury Norov wrote:
> On Fri, Apr 11, 2025 at 04:25:08PM +0530, Viresh Kumar wrote:
> > +/// Creates a new instance of CPU's device.
> > +///
> > +/// # Safety
> > +///
> > +/// Reference counting is not implemented for the CPU device in the C code. When a CPU is
> > +/// hot-unplugged, the corresponding CPU device is unregistered, but its associated memory
> > +/// is not freed.
> > +///
> > +/// Callers must ensure that the CPU device is not used after it has been unregistered.
> > +/// This can be achieved, for example, by registering a CPU hotplug notifier and removing
> > +/// any references to the CPU device within the notifier's callback.
> > +pub unsafe fn from_cpu(cpu: u32) -> Result<&'static Device> {
> > +    // SAFETY: The pointer returned by `get_cpu_device()`, if not `NULL`, is a valid pointer to
> > +    // a `struct device` and is never freed by the C code.
> > +    let ptr = unsafe { bindings::get_cpu_device(cpu) };

You don't need to justify the return value here, you need to justify why cpu is
a valid value to call get_cpu_device() with.

> > +    if ptr.is_null() {
> > +        return Err(ENODEV);
> > +    }
> > +
> > +    // SAFETY: The pointer returned by `get_cpu_device()`, if not `NULL`, is a valid pointer to
> > +    // a `struct device` and is never freed by the C code.

Here, the comment is good.

> If you think that writing the same thing twice would help in SAFETY
> department, I'd recommend you to write it for a couple more times.

Fixing the above, should also fix the duplication. :)

> > +    Ok(unsafe { Device::as_ref(ptr) })