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

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Message-Id: <DFSKZW10CRAW.1547Q71WU0B0U@garyguo.net>
Date: Mon, 19 Jan 2026 12:57:19 +0000
From: "Gary Guo" <gary@...yguo.net>
To: "Alice Ryhl" <aliceryhl@...gle.com>, "Maxime Ripard"
 <mripard@...nel.org>
Cc: "Daniel Almeida" <daniel.almeida@...labora.com>, "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

On Mon Jan 19, 2026 at 12:35 PM GMT, Alice Ryhl 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>.
>
>> > > AFAIU, encoding the state of the clock into the Clk type (and thus
>> > > forcing the structure that holds it) prevents that mutation. If not, we
>> > > should make it clearer (by expanding the doc maybe?) how such a pattern
>> > > can be supported.
>> > > 
>> > > Maxime
>> > 
>> > IIUC, your main point seems to be about mutating the state at runtime? This is
>> > possible with this code. You can just have an enum, for example:
>> > 
>> > enum MyClocks {
>> >     Unprepared(Clk<Unprepared>),
>> >     Prepared(Clk<Prepared>),
>> >     Enabled(Clk<Enabled>), 
>> > }
>
> I believe you need an extra state if the state is not bound to the scope
> of a function:
>
> enum MyClocks {
>     Unprepared(Clk<Unprepared>),
>     Prepared(Clk<Prepared>),
>     Enabled(Clk<Enabled>), 
>     Transitioning,
> }
>
> since mem::replace() needs a new value before you can take ownership of
> the existing Clk value.

We can provide `replace_with` in the kernel, it's fine as we don't have
unwinding panics.

Best,
Gary