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

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Date:	Tue, 01 Mar 2016 08:35:01 +0000
From:	jic23@...23.retrosnub.co.uk
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,
	Lucas De Marchi <lucas.demarchi@...el.com>,
	Guenter Roeck <linux@...ck-us.net>, eibach@...ys.de,
	linux-iio-owner@...r.kernel.org
Subject: Re: [PATCH v4] iio: adc: Add TI ADS1015 ADC driver support

On 01.03.2016 02:42, Michael Welling 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.
> 
> 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?
In some devices that is effectively impossible (fifo's that fill only 
from the currently
selected channel). That is what the onehot validation callback is about.
In this particular case it looks like doing a multichannel read is fair 
bit more time
consuming than a single channel read and so will result in a 
considerable reduction in
throughput.  This is of course why many parts include a simple 
sequencer!

Anyhow, supporting multi channel buffered reads could be done (probably) 
but you
would want to fall back to the single channel case as it is now.

Jonathan
>> Lucas De Marchi
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