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

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Date:	Tue, 1 Mar 2016 13:28:26 +0200
From:	Daniel Baluta <daniel.baluta@...el.com>
To:	Michael Welling <mwelling@...e.org>
Cc:	Lucas De Marchi <lucas.de.marchi@...il.com>,
	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" <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 Tue, Mar 1, 2016 at 4:42 AM, Michael Welling <mwelling@...e.org> wrote:
> 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.
>

Yes, this was the problem in the initial driver. The fix was to wait
for the conversion to complete
but it seems that it doesn't always work on your setup :(.

     if (change) {
+               conv_time = DIV_ROUND_UP(USEC_PER_SEC, ads1015_data_rate[dr]);
+               usleep_range(conv_time, conv_time + 1);
+       }
+


> 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?

For the moment we allow only a single channel to be enabled in buffer mode.