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

Daniel Almeida
2026-02-03T13:34:26Z

Hi Boris,

> On 3 Feb 2026, at 07:39, Boris Brezillon <boris.brezillon@collabora.com> wrote:
>
> On Mon, 19 Jan 2026 12:35:21 +0000
> Alice Ryhl <aliceryhl@google.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