linux-kernel - LED: inform userspace about brightness change

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Message-Id: <201412242106.09959@pali>
Date:	Wed, 24 Dec 2014 21:06:09 +0100
From:	Pali Rohár <pali.rohar@...il.com>
To:	Bryan Wu <cooloney@...il.com>, Richard Purdie <rpurdie@...ys.net>,
	Darren Hart <dvhart@...radead.org>, linux-leds@...r.kernel.org,
	platform-driver-x86@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: LED: inform userspace about brightness change

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@...il.com

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