linux-kernel - Re: [PATCH v3] rust: regulator: add a bare minimum regulator abstraction

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Message-Id: <D9W2GYMMVKLV.WXCSH0FYL4QC@kernel.org>
Date: Wed, 14 May 2025 19:41:05 +0200
From: "Benno Lossin" <lossin@...nel.org>
To: "Daniel Almeida" <daniel.almeida@...labora.com>
Cc: "Mark Brown" <broonie@...nel.org>, "Miguel Ojeda" <ojeda@...nel.org>,
 "Alex Gaynor" <alex.gaynor@...il.com>, "Boqun Feng" <boqun.feng@...il.com>,
 "Gary Guo" <gary@...yguo.net>, Björn Roy Baron
 <bjorn3_gh@...tonmail.com>, "Benno Lossin" <benno.lossin@...ton.me>,
 "Andreas Hindborg" <a.hindborg@...nel.org>, "Alice Ryhl"
 <aliceryhl@...gle.com>, "Trevor Gross" <tmgross@...ch.edu>, "Danilo
 Krummrich" <dakr@...nel.org>, "Boris Brezillon"
 <boris.brezillon@...labora.com>, "Sebastian Reichel"
 <sebastian.reichel@...labora.com>, "Liam Girdwood" <lgirdwood@...il.com>,
 <linux-kernel@...r.kernel.org>, <rust-for-linux@...r.kernel.org>
Subject: Re: [PATCH v3] rust: regulator: add a bare minimum regulator
 abstraction

On Wed May 14, 2025 at 6:19 PM CEST, Daniel Almeida wrote:
>> On 14 May 2025, at 13:05, Benno Lossin <lossin@...nel.org> wrote:
>> On Wed May 14, 2025 at 5:50 PM CEST, Mark Brown wrote:
>>> On Wed, May 14, 2025 at 05:38:40PM +0200, Benno Lossin wrote:
>>>> On Wed May 14, 2025 at 4:40 PM CEST, Daniel Almeida wrote:
>>>>> By the way, IIUC, regulator_disable() does not disable a regulator necessarily.
>>>>> It just tells the system that you don't care about it being enabled anymore. It can
>>>>> still remain on if there are other users.
>>> 
>>>> Hmm, so a `struct regulator` might already be enabled and calling
>>>> `regulator_enable` doesn't do anything?
>>> 
>>> It takes a reference to the regulator.  This may or may not result in a
>>> change in an underlying physical regulator.
>> 
>> Gotcha. So calling `regulator_enable` twice on the same regulator is
>> fine?
>> 
>> If that is the case -- and after re-reading the functions exposed on
>> both types `EnabledRegulator` and `Regulator` -- I am confused why we
>> even need two different type states? Both expose the same functions
>> (except `enable` and `disable`) and I don't otherwise see the purpose of
>> having two types.
>> 
>> ---
>> Cheers,
>> Benno
>> 
>
>
> As Mark said:
>
>> IIUC the point is to allow Rust's type system to keep track of the
>> reference on the regulator, otherwise the user code has to keep track of
>> the number of enables it's done like it currently does in C code.
>
> So this all started because keeping track of the enables was rather clunky. See
> v1 [0].
>
> [0] https://lore.kernel.org/rust-for-linux/20250219162517.278362-1-daniel.almeida@collabora.com/

Ahh thanks for the pointer this makes much more sense now. Yeah that's a
reason to have two types.

Please document this properly. A good reference is
`rust/kernel/fs/file.rs`. It also deals with different refcounts and
ownership, so that might give you some idea for how to write the
comments around changes in ownership. Also have a general comment about
the two different refcounts in a `struct regulator` on the two regulator
wrappers.

---
Cheers,
Benno