linux-kernel - Re: [PATCH 4/4] leds: add driver for synology atmega1608 controlled LEDs

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Message-Id: <DDDSCBNFRLG9.26UA3ZEOA9LJH@kernel.org>
Date: Thu, 09 Oct 2025 14:20:45 +0200
From: "Danilo Krummrich" <dakr@...nel.org>
To: "Markus Probst" <markus.probst@...teo.de>
Cc: "Lee Jones" <lee@...nel.org>, "Pavel Machek" <pavel@...nel.org>, "Miguel
 Ojeda" <ojeda@...nel.org>, "Alex Gaynor" <alex.gaynor@...il.com>, "Igor
 Korotin" <igor.korotin.linux@...il.com>, "Lorenzo Stoakes"
 <lorenzo.stoakes@...cle.com>, "Vlastimil Babka" <vbabka@...e.cz>, "Liam R.
 Howlett" <Liam.Howlett@...cle.com>, "Uladzislau Rezki" <urezki@...il.com>,
 "Boqun Feng" <boqun.feng@...il.com>, "Gary Guo" <gary@...yguo.net>,
 <bjorn3_gh@...tonmail.com>, "Benno Lossin" <lossin@...nel.org>, "Andreas
 Hindborg" <a.hindborg@...nel.org>, "Alice Ryhl" <aliceryhl@...gle.com>,
 "Trevor Gross" <tmgross@...ch.edu>, "Daniel Almeida"
 <daniel.almeida@...labora.com>, <linux-leds@...r.kernel.org>,
 <rust-for-linux@...r.kernel.org>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 4/4] leds: add driver for synology atmega1608 controlled
 LEDs

(Not a full review (let's work out the dependencies first), but there's one
thing that stood out to me.)

On Wed Oct 8, 2025 at 8:10 PM CEST, Markus Probst wrote:
> +struct Atmega1608Led {
> +    addr: Atmega1608LedAddress,
> +    id: Atmega1608LedId,
> +
> +    client: ARef<I2cClient>,
> +
> +    mode_lock: Arc<Mutex<()>>,

Mutex<()> raises an eyebrow, since a mutex that doesn't protect anything is
pointless. So, I assume it is protecting some data, but in an unsound way.

> +impl Atmega1608Led {
> +    fn update_mode(&self, mode: Atmega1608LedMode) -> Result<Atmega1608LedMode> {
> +        let _guard = self.mode_lock.lock();

What exactly does the mutex protect in the code below?

> +
> +        let mut current = self
> +            .client
> +            .read_byte_data(self.addr as u8)
> +            .inspect_err(|err| {
> +                dev_err!(
> +                    self.client.as_ref(),
> +                    "failed to read {:#2x}: {err:?}\n",
> +                    self.addr as u8
> +                );
> +            })?;
> +
> +        current =
> +            (current & !self.id.mask()) | (((mode as u8) << self.id.shift()) & self.id.mask());
> +
> +        self.client
> +            .write_byte_data(self.addr as u8, current)
> +            .inspect_err(|err| {
> +                dev_err!(
> +                    self.client.as_ref(),
> +                    "failed to write {:#2x}: {err:?}",
> +                    self.addr as u8
> +                );
> +            })?;
> +
> +        Ok(mode)
> +    }
> +}