Message-Id: <DG5DCYIUHCF5.3JL8I7GQ8REI8@garyguo.net>
Date: Tue, 03 Feb 2026 13:42:54 +0000
From: "Gary Guo" <gary@...yguo.net>
To: "Daniel Almeida" <daniel.almeida@...labora.com>, "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

On Tue Feb 3, 2026 at 1:33 PM GMT, Daniel Almeida wrote:
> 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.

Is this an issue though? You cannot mistakenly own `Clk<Enabled>` while the clk
is not enabled, (and similarly for `Prepared`), and that should be sufficient.

Having `Clk<Prepared>` makes no guarantee on if the clk is enabled or not anyway
as you can have another user do `Clk<Unprepared>::get().enable()`.

The only guarantee is that the state the clk have is going to be greater than or
equal to the type state, so allowing cloning an intermediate state is no
problem.

Best,
Gary

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

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