linux-kernel - Re: Building BarGraph with LED subsystem

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Message-ID: <20323.25792.171143.809563@quad.stoffel.home>
Date:	Fri, 16 Mar 2012 12:05:20 -0400
From:	"John Stoffel" <john@...ffel.org>
To:	simon@...gewell.org
Cc:	linux-input@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: Building BarGraph with LED subsystem

simon> I have a couple of questions about the (user-land) use of the
simon> LED subsystem.

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

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

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

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

While making the kernel more complex... esp for a feature which is of
such limited use.  Do it all in userland, it's easier, you can use
floating point math, etc.  And to test changes, you don't have to
recompile a module and unload/reload it, etc.

Keeping as much of this in userland is better.

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

udev is the answer here.  You probably want to make it so that the
LEDs are open to the user logged into the system console or currently
active Xsession.

John
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