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

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Message-ID: <20260203204354.7032d958@fedora>
Date: Tue, 3 Feb 2026 20:43:54 +0100
From: Boris Brezillon <boris.brezillon@...labora.com>
To: Daniel Almeida <daniel.almeida@...labora.com>
Cc: Gary Guo <gary@...yguo.net>, 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>, 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, 3 Feb 2026 16:26:22 -0300
Daniel Almeida <daniel.almeida@...labora.com> wrote:

> > 
> > I think it's fine to have all of these:
> > * `Clone` impl
> > * `enable` which consumes `Clk<Prepared>` by value and spit out `Clk<Enabled>`
> > * `with_enabled` that gives `&Clk<Enabled>`
> > 
> > This way, if you only want to enable in short time, you can do `with_enabled`.
> > If the closure callback wants to keep clock enabled for longer, it can just do
> > `.clone()` inside the closure and obtain an owned `Clk<Enabled>`.
> > 
> > If the user just have a reference and want to enable the callback they can do
> > `prepared_clk.clone().enable()` which gives an owned `Clk<Enabled>`. Thoughts?
> > 
> > Best,
> > Gary  
> 
> 
> I’m ok with what you proposed above. The only problem is that implementing
> clone() is done through an Arc<*mut bindings::clk>  in Boris’ current
> design,

It's actually Arc<RawClk> with

    struct RawClk(*mut bindings::clk);

    impl Drop for RawClk {
        fn drop(&mut self) {
            // SAFETY: By the type invariants, self.as_raw() is a valid argument for // [`clk_put`].
            unsafe { bindings::clk_put(self.0) };
        }
    }

This is because struct clk is not refcounted, so cloning
implies wrapping this object in an Arc, and only calling
clk_put() when the Arc refcnt reaches zero.

> so this requires an extra allocation.

That's true. But the memory overhead should be pretty negligible,
and I don't think the extra indirection makes any noticeable
difference for an actual clk implementation (one that's not a NOP),
since we have indirections all over the place already (clk -> clk_hw,
clk_ops, ...). So I think I'd value ease of use over this small
perfs/mem-usage hit.