linux-kernel - Re: [PATCHv2 1/3] iio: Add Nuvoton NAU7802 ADC driver

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Message-ID: <51C6FE00.8010900@metafoo.de>
Date:	Sun, 23 Jun 2013 15:54:08 +0200
From:	Lars-Peter Clausen <lars@...afoo.de>
To:	Alexandre Belloni <alexandre.belloni@...e-electrons.com>
CC:	Jonathan Cameron <jic23@...nel.org>,
	Shawn Guo <shawn.guo@...aro.org>,
	Grant Likely <grant.likely@...retlab.ca>, jimwall@...om,
	brian@...stalfontz.com,
	Maxime Ripard <maxime.ripard@...e-electrons.com>,
	linux-iio@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-doc@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
	devicetree-discuss@...ts.ozlabs.org,
	Jonathan Cameron <jic23@....ac.uk>,
	Rob Landley <rob@...dley.net>,
	Rob Herring <rob.herring@...xeda.com>
Subject: Re: [PATCHv2 1/3] iio: Add Nuvoton NAU7802 ADC driver

On 06/22/2013 03:28 PM, Alexandre Belloni wrote:
> On 22/06/2013 15:20, Lars-Peter Clausen wrote:
>> On 06/22/2013 03:07 PM, Alexandre Belloni wrote:
>>> On 22/06/2013 14:02, Lars-Peter Clausen wrote:
>>>> On 06/22/2013 01:55 PM, Jonathan Cameron wrote:
>>>>> On 06/20/2013 07:57 PM, Alexandre Belloni wrote:
>>>>>> The Nuvoton NAU7802 ADC is a 24-bit 2-channels I2C ADC, with adjustable
>>>>>> gain and sampling rates.
>>>>>>
>>>>> Sorry, somewhat low on time today so only a quick review.
>>>>>
>>>>> 1) Missing userspace ABI documentation.  Also, perhaps min_conversions is
>>>>>    a little vague?  Not that I have a better idea!
>>>> I really don't like the name min_conversions either. Isn't this effectively
>>>> a decimation filter?
>>> Yeah, it could be seen like that but it is only relevant and only
>>> happens when switching between channels. I'm open to any ideas.
>>>
>> I see. Is there anything about this in the datasheet on how many conversions
>> you usually need? Is this really something you need to change at runtime or
>> does moving this to platform data work?
>>
>>
> 
> There is actually nothing in the datasheet. The default value (6
> conversions) was found experimentally. What I did was saturating the ADC
> with the higher value on one channel and the lower value on the other
> one and I tried to find when reading both channel sequentially was
> resulting in a correct value.
> 
> You may not need to change it at runtime. And that value mainly depend
> on the precision versus speed balance you want to achieve. If you know
> that the values on both channels will not be to far apart, then you may
> not need to wait at all.
> 
> Would you think that is something I should hide in the DT ? Or maybe I
> can drop that knob for now and see if it is needed in the future.
> 

It is always a good idea to be conservative when introducing new ABI, so if
you think we can get away with hardcoding this in the driver I think that's
a good idea.

- Lars
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