linux-kernel - Building BarGraph with LED subsystem

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Message-ID: <1b29a178dd288b0f17608b5815ca77a5.squirrel@mungewell.org>
Date:	Fri, 16 Mar 2012 11:24:47 -0400
From:	simon@...gewell.org
To:	linux-input@...r.kernel.org
Cc:	linux-kernel@...r.kernel.org
Subject: Building BarGraph with LED subsystem

Hi all,
I have a couple of questions about the (user-land) use of the LED subsystem.

I recently offered a patch for controlling the LEDs of the G27 gaming
wheel, on which the LEDs are arranged in a bar-graph of 5 to show
simulation RPM. The suggestion was to use the LED subsystem, which this
patch implements.

http://www.spinics.net/lists/linux-input/msg19747.html

Thinking further about how to about how to control this I was wondering
whether a 'threshold' trigger would be useful. This would allow the use of
value (uint?) to be compare against a threshold and the LED turned on/off
automatically.

The main benefit of doing this as a 'trigger' is that we could 'link' (or
'sync') multiple LEDs to use the same value input, thus making the
user-land stuff trivial.

Concept - ledtrig-thres would provide the controls:
   threshold - value to compare against.
   value - incoming value, if >= threshold LED is lit, if < threshold LED
is dark.
   link - 'none' (standalone comparison), or the path to the value of
another LED's value to use (ie '/sys/leds/my_device::2/value')

Comments? Is this all too complex?

Also I have a question about the default permissions on the LED controls;
Is it possible to register LEDs so that users have write permission? At
present the controls are only accessible as root.

Many thanks,
Simon.

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