linux-kernel - Re: [PATCH v2 3/3] iio: adc: ad7380: new driver for AD7380 ADCs

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Date:   Thu, 14 Dec 2023 16:03:40 +0100
From:   David Lechner <dlechner@...libre.com>
To:     Jonathan Cameron <Jonathan.Cameron@...wei.com>
Cc:     linux-iio@...r.kernel.org, devicetree@...r.kernel.org,
        Rob Herring <robh+dt@...nel.org>,
        Krzysztof Kozlowski <krzysztof.kozlowski+dt@...aro.org>,
        Conor Dooley <conor+dt@...nel.org>,
        Jonathan Cameron <jic23@...nel.org>,
        Michael Hennerich <michael.hennerich@...log.com>,
        Nuno Sá <nuno.sa@...log.com>,
        Liam Girdwood <lgirdwood@...il.com>,
        Mark Brown <broonie@...nel.org>, linux-kernel@...r.kernel.org,
        Stefan Popa <stefan.popa@...log.com>
Subject: Re: [PATCH v2 3/3] iio: adc: ad7380: new driver for AD7380 ADCs

On Thu, Dec 14, 2023 at 1:36 PM Jonathan Cameron
<Jonathan.Cameron@...wei.com> wrote:
>
> On Thu, 14 Dec 2023 11:33:51 +0100
> David Lechner <dlechner@...libre.com> wrote:
>
> > On Thu, Dec 14, 2023 at 11:14 AM Jonathan Cameron
> > <Jonathan.Cameron@...wei.com> wrote:
> > >
> > > On Wed, 13 Dec 2023 05:21:20 -0600
> > > David Lechner <dlechner@...libre.com> wrote:
> > >
> > > > This adds a new driver for the AD7380 family ADCs.
> > > >
> > > > The driver currently implements basic support for the AD7380, AD7381,
> > > > AD7383, and AD7384 2-channel differential ADCs. Support for additional
> > > > single-ended and 4-channel chips that use the same register map as well
> > > > as additional features of the chip will be added in future patches.
> > > >
> > > > Co-developed-by: Stefan Popa <stefan.popa@...log.com>
> > > > Signed-off-by: Stefan Popa <stefan.popa@...log.com>
> > > > Signed-off-by: David Lechner <dlechner@...libre.com>
> > >
> > > Just one additional comment.  I 'might' sort both this an Nuno's comment
> > > if Mark is fine with the SPI and no on else has review comments.
> > > Feel free to send a v3 though if you like ;)
> > >
> > >
> > > > +/* fully differential */
> > > > +DEFINE_AD7380_DIFFERENTIAL_2_CHANNEL(ad7380_channels, 16);
> > > > +DEFINE_AD7380_DIFFERENTIAL_2_CHANNEL(ad7381_channels, 14);
> > > > +/* pseudo differential */
> > > > +DEFINE_AD7380_DIFFERENTIAL_2_CHANNEL(ad7383_channels, 16);
> > > > +DEFINE_AD7380_DIFFERENTIAL_2_CHANNEL(ad7384_channels, 14);
> > > > +
> > > > +/* Since this is simultaneous sampling, we don't allow individual channels. */
> > > > +static const unsigned long ad7380_2_channel_scan_masks[] = {
> > > > +     GENMASK(2, 0), /* both ADC channels and soft timestamp */
> > > > +     GENMASK(1, 0), /* both ADC channels, no timestamp */
> > >
> > > https://elixir.bootlin.com/linux/v6.7-rc5/source/include/linux/iio/iio.h#L567
> > > See the comment (added recently!)
> >
> > I did see this comment but this is already sorted in order of
> > preference, so I'm not sure why you are calling it out. Just FYI, I
> > guess?
>
> No. Order of preference would be turn on the minimal if that is enough.
> First item is the highest preference (if the requested channels are a subset of
> that we don't look any further).  Here that means we always stop on the first
> entry and never look at the second.

OK, I understand what you are getting at now. I thought the preference
could be my personal preference rather than the minimal case. :-)

But as you pointed out, the timestamp is handled separately, so it
doesn't make a difference here. The main point was to ensure that both
channels are always enabled since the ADC is doing simultaneous
sampling and always reading two channels at the same time.

>
> >
> > >
> > > Also, if I remember how this works correctly there is no need to include
> > > the timestamp in the mask.  We do special handling for it to avoid having to double
> > > the number of provided masks.  The details being that it uses
> > > iio_scan_el_ts_store rather than iio_scan_el_Store.
> >
> > Indeed. I've been working ahead on adding more features and noticed
> > this. So we will need to find a way to say that we the timestamp
> > should not be allowed under certain conditions. But that will be a
> > discussion for a later series.
>
> Interesting - you have cases where it's not valid at all?
> It sometimes becomes inaccurate because we are interpolating across
> data from a fifo, but I've not seen a case where we can't provide anything
> useful.  Ah well - as you say I'll wait for that later series!
>
> Jonathan
>
> >
> > >
> > > So as you have it I think you'll always end up with the first entry
> > > and that will include a bonus bit that isn't a problem as it will match
> > > anyway.
> > >
> > > So just have the second entry and 0.
> > >
> > > Jonathan
> > >
> > > > +     0
> > > > +};
>