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

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Message-Id: <DDDSSGI3H08V.1L6YQN2Q5C8TE@kernel.org>
Date: Thu, 09 Oct 2025 14:41:50 +0200
From: "Danilo Krummrich" <dakr@...nel.org>
To: "Markus Probst" <markus.probst@...teo.de>, "Alexandre Courbot"
 <acourbot@...dia.com>
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

(Cc: Alex)

On Thu Oct 9, 2025 at 2:30 PM CEST, Markus Probst wrote:
> On Thu, 2025-10-09 at 14:20 +0200, Danilo Krummrich wrote:
>> (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?
> Otherwise there would be a race condition. Each register has 8 bits,
> each led has 2 bits. If the led mode is updated at the same time with
> another one in the same register, it could lead to the first action
> being overwritten by the second.
> Meaning if two actions run at the same time:
> - led0 reads from the register
> - led1 reads from the register
> - led0 writes to the register
> - led1 writes to the register (the changes for led0 have been
> overwritten here, as it did read the register before led0 has written
> to it)

Ok, so you need exclusive access to a register. I think this is something the
register abstraction I also mentioned in [1] could support.

@Alex: Have you thought about this already?

[1] https://lore.kernel.org/rust-for-linux/DDDS2V0V2NVJ.16ZKXCKUA1HUV@kernel.org/