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

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Date:	Sat, 19 May 2012 09:04:04 +0100
From:	Jonathan Cameron <jic23@...nel.org>
To:	Johan Hovold <jhovold@...il.com>
CC:	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 05/18/2012 06:57 PM, Johan Hovold wrote:
> 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.
> 
cool. I'll review the rest of the patch with the assumption you'll do this.

Jonathan
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