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

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Message-ID: <20260203113902.501e5803@fedora>
Date: Tue, 3 Feb 2026 11:39:02 +0100
From: Boris Brezillon <boris.brezillon@...labora.com>
To: Alice Ryhl <aliceryhl@...gle.com>
Cc: Maxime Ripard <mripard@...nel.org>, 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, 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