linux-kernel - Re: [RFC PATCH V3 8/8] cpufreq: Add Rust based cpufreq-dt driver

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Message-ID: <Zo6oqfFX-TNIeaIC@pollux>
Date: Wed, 10 Jul 2024 17:28:41 +0200
From: Danilo Krummrich <dakr@...hat.com>
To: Viresh Kumar <viresh.kumar@...aro.org>
Cc: "Rafael J. Wysocki" <rafael@...nel.org>,
	Miguel Ojeda <miguel.ojeda.sandonis@...il.com>,
	Miguel Ojeda <ojeda@...nel.org>,
	Alex Gaynor <alex.gaynor@...il.com>,
	Wedson Almeida Filho <wedsonaf@...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@...sung.com>,
	Alice Ryhl <aliceryhl@...gle.com>, 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>,
	linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH V3 8/8] cpufreq: Add Rust based cpufreq-dt driver

On Wed, Jul 10, 2024 at 02:26:52PM +0530, Viresh Kumar wrote:
> On 05-07-24, 13:32, Danilo Krummrich wrote:
> > On 7/3/24 09:14, Viresh Kumar wrote:
> > > +    fn probe(_dev: &mut platform::Device, _id_info: Option<&Self::IdInfo>) -> Result<Self::Data> {
> > > +        let drv = Arc::new(
> > > +            cpufreq::Registration::<CPUFreqDTDriver>::register(
> > > +                c_str!("cpufreq-dt"),
> > > +                (),
> > > +                cpufreq::flags::NEED_INITIAL_FREQ_CHECK | cpufreq::flags::IS_COOLING_DEV,
> > > +                true,
> > > +            )?,
> > > +            GFP_KERNEL,
> > > +        )?;
> > 
> > Putting the `cpufreq::Registration` into `Arc<DeviceData>` is unsafe from a
> > lifetime point of view. Nothing prevents this `Arc` to out-live the
> > `platform::Driver`.
> 
> Hmm, the platform driver layer (in Rust) should guarantee that the
> data will be freed from the driver removal path. Isn't it ?

No, the platform driver layer will only guarantee that it decreses the reference
count of the `Arc` by one, that doesn't guarantee a free. If something else
still holds a reference to the `Arc` it will keep the `Registration` alive,
unless it's wrapped by `Devres`.

> 
> > Instead, you should wrap `cpufreq::Registration` into `Devres`. See
> > `drm::drv::Registration` for an example [1].
> > 
> > [1] https://gitlab.freedesktop.org/drm/nova/-/blob/nova-next/rust/kernel/drm/drv.rs?ref_type=heads#L173
> 
> I can convert to that too, will do it anyway.
> 
> -- 
> viresh
>