linux-kernel - Re: [PATCH v4] iio: adc: Add TI ADS1015 ADC driver support

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Message-ID: <20160301024217.GA12912@deathstar>
Date:	Mon, 29 Feb 2016 20:42:17 -0600
From:	Michael Welling <mwelling@...e.org>
To:	Lucas De Marchi <lucas.de.marchi@...il.com>
Cc:	Daniel Baluta <daniel.baluta@...el.com>,
	Jonathan Cameron <jic23@...nel.org>,
	Hartmut Knaack <knaack.h@....de>,
	Lars-Peter Clausen <lars@...afoo.de>,
	Peter Meerwald-Stadler <pmeerw@...erw.net>,
	lkml <linux-kernel@...r.kernel.org>, linux-iio@...r.kernel.org,
	Lucas De Marchi <lucas.demarchi@...el.com>,
	Guenter Roeck <linux@...ck-us.net>, eibach@...ys.de
Subject: Re: [PATCH v4] iio: adc: Add TI ADS1015 ADC driver support

On Mon, Feb 29, 2016 at 10:09:10PM -0300, Lucas De Marchi wrote:
> On Mon, Feb 29, 2016 at 9:50 PM, Michael Welling <mwelling@...e.org> wrote:
> > On Fri, Feb 05, 2016 at 03:17:18PM +0200, Daniel Baluta wrote:
> >> The driver has sysfs readings with runtime PM support for power saving.
> >> It also offers buffer support that can be used together with IIO software
> >> triggers.
> >>
> >
> > Daniel,
> >
> > So I noticed something yesterday while testing new boards.
> > The channels are occassionally swapping when accessing data from multiple channels.
> >
> > I wrote a simple bash script to demonstrate.
> 
> This happened to me in a previous version of the patch. I remember it
> being fixed in the last version (or at least I could not reproduce).
> I'll test again tomorrow with your script.
>

Here is what I believe is happening.

The request for a conversion on a new channel comes in while the conversion
for the previous channel is still converting. The driver waits approximately
one conversion cycle. The previous channel completes within this timeframe
and the MUX is changed and the new sample is started. The new sample is still
converting and the driver returns the value from the previous conversion.

For a test I multiplied the conv_time value by 2 in the ads1015_get_adc_result
function. This allows time for the current sample flush out and always returns
the appropriate channel's value.

Looking at the buffered mode it appears that only one channel is being accessed
at any time. This being the first one in the active_scan_mask found by
find_first_bit. So the MUX would never change is buffered mode as far I can
tell.

Don't we typically want to read all of enabled channels in buffered mode?

> Lucas De Marchi