linux-kernel - Re: [PATCH v2 2/4] iio: adc: add support for Allwinner SoCs ADC

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Message-ID: <583cc331-69f9-bea4-486b-323c10c65447@kernel.org>
Date:	Wed, 20 Jul 2016 15:59:20 +0100
From:	Jonathan Cameron <jic23@...nel.org>
To:	Quentin Schulz <quentin.schulz@...e-electrons.com>,
	jdelvare@...e.com, linux@...ck-us.net, knaack.h@....de,
	lars@...afoo.de, pmeerw@...erw.net,
	maxime.ripard@...e-electrons.com, wens@...e.org,
	lee.jones@...aro.org
Cc:	linux-kernel@...r.kernel.org, linux-hwmon@...r.kernel.org,
	linux-iio@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
	thomas.petazzoni@...e-electrons.com,
	antoine.tenart@...e-electrons.com
Subject: Re: [PATCH v2 2/4] iio: adc: add support for Allwinner SoCs ADC

On 20/07/16 13:37, Quentin Schulz wrote:
> On 18/07/2016 15:18, Jonathan Cameron wrote:
> [...]
>>> +
>>> +	if (!wait_for_completion_timeout(&info->completion,
>>> +					 msecs_to_jiffies(100))) {
>>> +		ret = -ETIMEDOUT;
>>> +		goto out;
>>> +	}
>>> +
>>> +	if (info->flags & SUNXI_GPADC_ARCH_SUN4I)
>>> +		*val = info->temp_data * 133 - 257000;
>> Why report as processed?  I'd just report them as raw with the scale
>> and offset provided.  It's not a big thing, but if we can leave it so
>> that the conversion only occurs when desired, why not?
>>
>> For in kernel users, this all happen 'automagically' anyway ;)
>>
> 
> Mmmmh, in the code above we apply the scale on the raw value and then
> the offset. While in iio_convert_raw_to_processed
> (http://lxr.free-electrons.com/source/drivers/iio/inkern.c#L507), the
> offset is applied before the scale.
> 
> The way would be to factorize the computation by scale:
> Now: *val = raw * scale + offset
> Then: *val = (raw + offset/scale) * scale
> 
> But the offset is an integer and offset/scale is therefore rounded.
> Currently, we have the following values:
> sun4i: -257000/133 = -1932.3308270676691
> sun5i: -144700/100 = -1447
> sun6i: -271000/167 = -1622.754491017964
> 
> Do we accept such rounding?
Yes - how accurate do you think a temp sensor is - really not a problem.
> 
> If not, we either stay with the processed value in read_raw or patch
> inkern to add an offset to apply after having applied the scale to the
> raw value (val2 from iio_channel_read is yet unused with
> IIO_CHAN_INFO_OFFSET for example, we could use that to specify an
> offset2 to apply after the switch(scale_type)-case).
I don't really care that much if you just want to keep them as
processed values.

J
> 
> [...]
> Quentin
>