linux-kernel - Re: [PATCH v2 2/4] iio: add LM3533 ambient light sensor driver

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Date:	Fri, 18 May 2012 19:57:42 +0200
From:	Johan Hovold <jhovold@...il.com>
To:	Jonathan Cameron <jic23@...nel.org>
Cc:	Johan Hovold <jhovold@...il.com>,
	Jonathan Cameron <jic23@....ac.uk>,
	Rob Landley <rob@...dley.net>,
	Richard Purdie <rpurdie@...ys.net>,
	Samuel Ortiz <sameo@...ux.intel.com>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Florian Tobias Schandinat <FlorianSchandinat@....de>,
	Arnd Bergmann <arnd@...db.de>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Mark Brown <broonie@...nsource.wolfsonmicro.com>,
	linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-iio@...r.kernel.org, devel@...verdev.osuosl.org,
	linux-fbdev@...r.kernel.org
Subject: Re: [PATCH v2 2/4] iio: add LM3533 ambient light sensor driver

On Fri, May 18, 2012 at 06:34:01PM +0100, Jonathan Cameron wrote:
> On 05/18/2012 01:27 PM, Johan Hovold wrote:

[...]

> > I really think that this should remain a device specific attribute as I
> > originally suggested. It's an integration parameter that needs to be set
> > precisely depending on the actual hardware setup (which analog light
> > sensor and other external components). 
> Then it shouldn't be exposed to userspace.  If there is reason to vary
> it from userspace then it is a calibration parameter and should be
> treated like the other ones we have, if not it should be done from
> dt or platform data.
> > 
> > The lm3533 also supports two types of light sensors: pwm- and analog-
> > output ones. The resistor select settings only applies when in analog
> > mode as the input is always high impedance otherwise. Thus a generic
> > attribute (such as calibscale or hardware gain) shouldn't be used as it
> > will have no effect whatsoever in PWM-mode.
> > 
> > I'm thus back at my original proposal, albeit with a different name (I
> > think a lot of this discussion could have been avoided had I not
> > misnamed the parameter "gain"): 
> > 
> > What:		/sys/bus/iio/devices/iio:deviceX/r_select
> > Description:
> > 		Set the ALS internal pull-down resistor for analog input mode
> > 		(1..127), such that,
> > 
> > 		R_als = 200000 / r_select	(ohm)
> > 
> > 		This setting is ignored in PWM-mode (input is always high
> > 		impedance in PWM-mode).
> > 
> > I don't think much is gained from using ohm as the unit: it just adds
> > complexity and the selected resistor setting will likely not match the
> > input value anyway. It's better that the chip integrators have full
> > control over which resistor setting is actually used so that it matches
> > external components.
> This smacks of something that should never be exposed to users.
> I'd hide it away in platform data.

Fair enough. I'll drop the sysfs param and submit a patch for mfd-next
which adds r_select to the platform data.

Thanks,
Johan
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