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

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Date:	Tue, 1 Mar 2016 10:02:00 -0600
From:	Michael Welling <mwelling@...e.org>
To:	jic23@...23.retrosnub.co.uk
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 Tue, Mar 01, 2016 at 08:35:01AM +0000, jic23@...23.retrosnub.co.uk wrote:
> 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!

Yes the sampling rate would effectively be divided by the number of channels
enabled.

> 
> 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.

Understood.

> 
> Jonathan
> >>Lucas De Marchi
> >--
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