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

["Danilo Krummrich" <dakr@kernel.org>]

(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/