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

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Date:	Wed, 20 Jul 2016 14:37:40 +0200
From:	Quentin Schulz <quentin.schulz@...e-electrons.com>
To:	Jonathan Cameron <jic23@...nel.org>, 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 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?

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).

[...]
Quentin



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