linux-kernel - Re: LED: inform userspace about brightness change

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Date:	Tue, 6 Jan 2015 09:55:41 -0800
From:	Bryan Wu <cooloney@...il.com>
To:	Pali Rohár <pali.rohar@...il.com>
Cc:	Richard Purdie <rpurdie@...ys.net>,
	Darren Hart <dvhart@...radead.org>,
	Linux LED Subsystem <linux-leds@...r.kernel.org>,
	platform-driver-x86@...r.kernel.org,
	lkml <linux-kernel@...r.kernel.org>
Subject: Re: LED: inform userspace about brightness change

On Wed, Dec 24, 2014 at 12:06 PM, Pali Rohár <pali.rohar@...il.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
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