LED: inform userspace about brightness change

LED: inform userspace about brightness change

[Pali Rohár]

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Hello!

What is preferred way from kernel driver to inform userspace 
about LED device brightness change? In /sys/class/leds/<device>/ 
is file brightness which can be used to read current brightness 
or set write new value. But sometimes also firmware (or device) 
can change value on its own and thus userspace software which 
provides slider for setting brightness will be incorrect.

For example on Dell laptops there is driver which support setting 
and reading keyboard backlight (via leds kernel interface). But 
firmware/BIOS can change brightness (when something happen) and 
it can inform running system about this change via ACPI/WMI event 
interface. But how to use kernel led interface to inform userpace 
about this change? It is possible?

-- 
Pali Rohár
pali.rohar@gmail.com

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Re: LED: inform userspace about brightness change

[Bryan Wu]

On Wed, Dec 24, 2014 at 12:06 PM, Pali Rohár <pali.rohar@gmail.com> wrote:
> Hello!
>
> What is preferred way from kernel driver to inform userspace
> about LED device brightness change? In /sys/class/leds/<device>/
> is file brightness which can be used to read current brightness
> or set write new value. But sometimes also firmware (or device)
> can change value on its own and thus userspace software which
> provides slider for setting brightness will be incorrect.
>
> For example on Dell laptops there is driver which support setting
> and reading keyboard backlight (via leds kernel interface). But
> firmware/BIOS can change brightness (when something happen) and
> it can inform running system about this change via ACPI/WMI event
> interface. But how to use kernel led interface to inform userpace
> about this change? It is possible?
>

Sorry for the delay, actually I don't find any possible existing way
to send brightness change event from LED subsystem to user space.

I think we can add an udev event in set_brightness API, then when we
change the brightness, it will send out udev event to user space.

Thanks,
-Bryan