Re: Problem with resetting LED in led_classdev_unregister in case of USB LED device removal

[Heiner Kallweit <hkallweit1@gmail.com>]

Am 19.01.2016 um 01:11 schrieb Milo Kim:
> On 01/19/2016 05:52 AM, Heiner Kallweit wrote:
>> Setting such a flag from the driver might cause significant effort in different layers.
>> When we talk about thingm as an example, it uses the hid subsystem with the usbhid low level driver.
>> We would need a callback in the usbhid driver (to be notified when the device is unplugged)
>> and a way to propagate this event to the hid core.
>>
>> Maybe simpler: We could ignore ENODEV errors if a function is called from led_classdev_unregister.
>> This way we wouldn't have to touch drivers. I think of something like this:
> 
> Well, simple solution is good but I'm thinking about more generic handling.
> 
> 
> LED subsystem                HID LED driver
> -------------                --------------
>                     Create a LED device
> 
> Registers an event notifier
> 
>                     Device is unplugged,
>                     notify an event to LED
>                     subsystem
> 
> Notification callback sets
> a flag which means HW is removed
> 
> Set-brightness scheduler work
> function checks this flag and
> ignore the brightness update
> 
Most likely not the LED subsystem but the respective driver would have to register the notifier as only the driver
knows what kind of subsystem (usb, ..) is used.

I agree that this would be somewhat cleaner. However I have my doubts that the relatively small benefit of getting read of one
a little annoying error message justifies the required efforts.
> 
> blocking_notifier_chain_register() and blocking_notifier_call_chain() helpers can be used for this implementation.
> 
> However, I'm not sure how much latency will exist between step 3 (device is unplugged) and step 5 (check the flag and ignore brightness-set).
> 
At a first glance I dont't think this is an issue because the notifier chain is blocking.

> Best regards,
> Milo
> 
Regards, Heiner