linux-kernel - Re: [PATCH v3 1/3] rust: clk: use the type-state pattern

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Message-Id: <E7286D12-0BD9-4726-B072-FE5A040312B1@collabora.com>
Date: Tue, 3 Feb 2026 10:33:34 -0300
From: Daniel Almeida <daniel.almeida@...labora.com>
To: Boris Brezillon <boris.brezillon@...labora.com>
Cc: Alice Ryhl <aliceryhl@...gle.com>,
 Maxime Ripard <mripard@...nel.org>,
 "Rafael J. Wysocki" <rafael@...nel.org>,
 Viresh Kumar <viresh.kumar@...aro.org>,
 Danilo Krummrich <dakr@...nel.org>,
 Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
 Thomas Zimmermann <tzimmermann@...e.de>,
 David Airlie <airlied@...il.com>,
 Simona Vetter <simona@...ll.ch>,
 Drew Fustini <fustini@...nel.org>,
 Guo Ren <guoren@...nel.org>,
 Fu Wei <wefu@...hat.com>,
 Uwe Kleine-König <ukleinek@...nel.org>,
 Michael Turquette <mturquette@...libre.com>,
 Stephen Boyd <sboyd@...nel.org>,
 Miguel Ojeda <ojeda@...nel.org>,
 Boqun Feng <boqun.feng@...il.com>,
 Gary Guo <gary@...yguo.net>,
 Björn Roy Baron <bjorn3_gh@...tonmail.com>,
 Benno Lossin <lossin@...nel.org>,
 Andreas Hindborg <a.hindborg@...nel.org>,
 Trevor Gross <tmgross@...ch.edu>,
 linux-pm@...r.kernel.org,
 linux-kernel@...r.kernel.org,
 dri-devel@...ts.freedesktop.org,
 linux-riscv@...ts.infradead.org,
 linux-pwm@...r.kernel.org,
 linux-clk@...r.kernel.org,
 rust-for-linux@...r.kernel.org
Subject: Re: [PATCH v3 1/3] rust: clk: use the type-state pattern

Hi Boris,

> On 3 Feb 2026, at 07:39, Boris Brezillon <boris.brezillon@...labora.com> wrote:
> 
> On Mon, 19 Jan 2026 12:35:21 +0000
> Alice Ryhl <aliceryhl@...gle.com> wrote:
> 
>> On Mon, Jan 19, 2026 at 11:45:57AM +0100, Maxime Ripard wrote:
>>> On Thu, Jan 08, 2026 at 11:14:37AM -0300, Daniel Almeida wrote:  
>>>>> For example, it's quite typical to have (at least) one clock for the bus
>>>>> interface that drives the register, and one that drives the main
>>>>> component logic. The former needs to be enabled only when you're
>>>>> accessing the registers (and can be abstracted with
>>>>> regmap_mmio_attach_clk for example), and the latter needs to be enabled
>>>>> only when the device actually starts operating.
>>>>> 
>>>>> You have a similar thing for the prepare vs enable thing. The difference
>>>>> between the two is that enable can be called into atomic context but
>>>>> prepare can't.
>>>>> 
>>>>> So for drivers that would care about this, you would create your device
>>>>> with an unprepared clock, and then at various times during the driver
>>>>> lifetime, you would mutate that state.  
>> 
>> The case where you're doing it only while accessing registers is
>> interesting, because that means the Enable bit may be owned by a local
>> variable. We may imagine an:
>> 
>>    let enabled = self.prepared_clk.enable_scoped();
>>    ... use registers
>>    drop(enabled);
>> 
>> Now ... this doesn't quite work with the current API - the current
>> Enabled stated owns both a prepare and enable count, but the above keeps
>> the prepare count in `self` and the enabled count in a local variable.
>> But it could be done with a fourth state, or by a closure method:
>> 
>>    self.prepared_clk.with_enabled(|| {
>>        ... use registers
>>    });
>> 
>> All of this would work with an immutable variable of type Clk<Prepared>.
> 
> Hm, maybe it'd make sense to implement Clone so we can have a temporary
> clk variable that has its own prepare/enable refs and releases them
> as it goes out of scope. This implies wrapping *mut bindings::clk in an
> Arc<> because bindings::clk is not ARef, but should be relatively easy
> to do. Posting the quick experiment I did with this approach, in case
> you're interested [1]
> 
> [1]https://gitlab.freedesktop.org/bbrezillon/linux/-/commit/d5d04da4f4f6192b6e6760d5f861c69596c7d837

The problem with what you have suggested is that the previous state is not
consumed if you can clone it, and consuming the previous state is a pretty key
element in ensuring you cannot misuse it. For example, here:

let enabled_clk = prepared_clk.clone().enable()?;
// do stuff
// enabled_clk goes out of scope and releases the enable
// ref it had

prepared_clk is still alive. Now, this may not be the end of the world in this
particular case, but for API consistency, I'd say we should probably avoid this
behavior.

I see that Alice suggested a closure approach. IMHO, we should use that
instead.

— Daniel