Date:	Wed, 21 May 2008 09:49:42 -0400
From:	Dmitry Torokhov <dmitry.torokhov@...il.com>
To:	Jean Delvare <khali@...ux-fr.org>
Cc:	Jonathan Cameron <Jonathan.Cameron@...il.com>,
	linux-kernel@...r.kernel.org,
	spi-devel-general@...ts.sourceforge.net,
	LM Sensors <lm-sensors@...sensors.org>
Subject: Re: Accelerometer, Gyros and ADC's etc within the kernel.

On Tue, May 20, 2008 at 01:28:17PM +0200, Jean Delvare wrote:
> Hi Jonathan,
> 
> On Tue, 20 May 2008 11:04:01 +0100, Jonathan Cameron wrote:
> > This email is basically a request for opinions on how and where such sensors
> > should be integrated into the kernel.
> > 
> > To set the scene...
> > 
> > Increasing numbers of embedded devices are being supplied attached MEMS
> > devices (www.xbow.com imote2 etc). Along with more traditional sensors such
> > as ADC's not being used for hardware monitoring, these do not really 
> > seem to
> > fit with in an particular subsystem of the kernel.  A previous 
> > discussion on
> > lkml in 2006 considered the accelerometers to be found within some laptop
> > hard drives, but I haven't been able to track down any more general 
> > discussions
> > of such non hardware monitoring sensors.
> > 
> > The obvious possibilities are:
> > 
> > * To place the various drivers within the spi / i2c etc subsystems as 
> > relevant.
> 
> Bad idea. Grouping drivers by connectivity is almost always the wrong
> thing to do, as it means that different persons will maintain them and
> they have all chances to diverge. You really want to group drivers by
> functionality. On top of that, I am busy enough maintaining the i2c
> core and bus drivers without having more i2c device drivers added to my
> plate. These days I am trying to _empty_ drivers/i2c/chips, if anything.
> 
> > * To place within the hwmon subsystem as this is probably closest.
> > (there is already at least one straight ADC driver in hwmon)
> 
> Probably not the wisest choice, if nothing else, because the hwmon
> maintainer is already overloaded. And I don't think that these devices
> have much in common with the traditional hardware monitoring components
> anyway.
> 
> I'm not sure what "straight ADC driver" you are referring to, but
> anything which measures a voltage and exports the reading to user-space
> is, well, a voltage sensor, and thus fix within hwmon. If the same chip
> is used for a higher-level, dedicated function then we would probably
> have a separate driver for it, outside of hwmon.
> 
> > * To create a new subsystem, or perhaps merely sysfs class to contain these
> >   elements.
> 
> Would be OK. Or:
> 
> * Place these within the input subsystem. You might want to discuss
> this with the input subsystem maintainer Dmitry Torokhov (Cc'd). The
> Wii remote is essentially a joystick, and joysticks belong to the input
> subsystem.
> 

I don't think that input subsystem woudl be the best choice. While we
do support the event-driven mechanism for delivering information to
userspace input is mostly oriented for human interface devices, not
general data acquisition. If anything input_dev is too fat. Another
thing is that input layer anonymizes input devices, makes them
capability-oriented. I.e. we dont really care what model of joystick
we have, we want something that reports ABS_X, ABS_Y, BTN_FIRE and
userpsace can use it whether it a simple analog joystick or something
fancy and USB connected. For your purposes you probably do want to know
what the device is and what exactly being sampled/measured.

I'd go with a lean and mean new subsytem.

-- 
Dmitry
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