linux-kernel - Re: [PATCH] iio: adc: ad7192: Change "shorted" channels to differential

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Message-ID: <86b68c3a06aecfbdb58ddff77b4e5059f6acd2b5.camel@crapouillou.net>
Date:   Tue, 25 Apr 2023 11:07:11 +0200
From:   Paul Cercueil <paul@...pouillou.net>
To:     Jonathan Cameron <jic23@...nel.org>
Cc:     Lars-Peter Clausen <lars@...afoo.de>,
        Michael Hennerich <Michael.Hennerich@...log.com>,
        linux-iio@...r.kernel.org, linux-kernel@...r.kernel.org,
        Alisa Roman <alisa.roman@...log.com>,
        Fabrizio Lamarque <fl.scratchpad@...il.com>
Subject: Re: [PATCH] iio: adc: ad7192: Change "shorted" channels to
 differential

Hi Jonathan,

Le samedi 01 avril 2023 à 15:42 +0100, Jonathan Cameron a écrit :
> On Thu, 30 Mar 2023 12:21:00 +0200
> Paul Cercueil <paul@...pouillou.net> wrote:
> 
> > The AD7192 provides a specific channel configuration where both
> > negative
> > and positive inputs are connected to AIN2. This was represented in
> > the
> > ad7192 driver as a IIO channel with .channel = 2 and .extended_name
> > set
> > to "shorted".
> > 
> > The problem with this approach, is that the driver provided two IIO
> > channels with the identifier .channel = 2; one "shorted" and the
> > other
> > not. This goes against the IIO ABI, as a channel identifier should
> > be
> > unique.
> > 
> > Address this issue by changing "shorted" channels to being
> > differential
> > instead, with channel 2 vs. itself, as we're actually measuring
> > AIN2 vs.
> > itself.
> > 
> > Note that the fix tag is for the commit that moved the driver out
> > of
> > staging. The bug existed before that, but backporting would become
> > very
> > complex further down and unlikely to happen.
> > 
> > Fixes: b581f748cce0 ("staging: iio: adc: ad7192: move out of
> > staging")
> > Signed-off-by: Paul Cercueil <paul@...pouillou.net>
> > Co-developed-by: Alisa Roman <alisa.roman@...log.com>
> > Signed-off-by: Alisa Roman <alisa.roman@...log.com>
> 
> +CC Fabrizio who has a fix series under review for the same driver.
> 
> I'm going to let this one sit on the list for a little while.
> It is a breaking ABI change (that hopefully no one will notice -
> given
> the first fix from Fabrizio shows the driver crashes on probe
> currently we
> should be safe on that).
> 
> Arguably just changing the index would also have been an ABI change,
> but
> that would have gotten past any code that didn't take much notice of
> the
> channel index whereas this won't.
> 
> Anyhow, will give it a little while for comments then pick this up
> on top of Fabrizio's fixes series.  Give me a poke in 2-3 weeks if I
> seem to have lost it.

Friendly ping :)

Cheers,
-Paul

> 
> Jonathan
> 
> 
> > ---
> >  drivers/iio/adc/ad7192.c | 8 ++------
> >  1 file changed, 2 insertions(+), 6 deletions(-)
> > 
> > diff --git a/drivers/iio/adc/ad7192.c b/drivers/iio/adc/ad7192.c
> > index 55a6ab591016..99bb604b78c8 100644
> > --- a/drivers/iio/adc/ad7192.c
> > +++ b/drivers/iio/adc/ad7192.c
> > @@ -897,10 +897,6 @@ static const struct iio_info ad7195_info = {
> >         __AD719x_CHANNEL(_si, _channel1, -1, _address, NULL,
> > IIO_VOLTAGE, \
> >                 BIT(IIO_CHAN_INFO_SCALE), ad7192_calibsys_ext_info)
> >  
> > -#define AD719x_SHORTED_CHANNEL(_si, _channel1, _address) \
> > -       __AD719x_CHANNEL(_si, _channel1, -1, _address, "shorted",
> > IIO_VOLTAGE, \
> > -               BIT(IIO_CHAN_INFO_SCALE), ad7192_calibsys_ext_info)
> > -
> >  #define AD719x_TEMP_CHANNEL(_si, _address) \
> >         __AD719x_CHANNEL(_si, 0, -1, _address, NULL, IIO_TEMP, 0,
> > NULL)
> >  
> > @@ -908,7 +904,7 @@ static const struct iio_chan_spec
> > ad7192_channels[] = {
> >         AD719x_DIFF_CHANNEL(0, 1, 2, AD7192_CH_AIN1P_AIN2M),
> >         AD719x_DIFF_CHANNEL(1, 3, 4, AD7192_CH_AIN3P_AIN4M),
> >         AD719x_TEMP_CHANNEL(2, AD7192_CH_TEMP),
> > -       AD719x_SHORTED_CHANNEL(3, 2, AD7192_CH_AIN2P_AIN2M),
> > +       AD719x_DIFF_CHANNEL(3, 2, 2, AD7192_CH_AIN2P_AIN2M),
> >         AD719x_CHANNEL(4, 1, AD7192_CH_AIN1),
> >         AD719x_CHANNEL(5, 2, AD7192_CH_AIN2),
> >         AD719x_CHANNEL(6, 3, AD7192_CH_AIN3),
> > @@ -922,7 +918,7 @@ static const struct iio_chan_spec
> > ad7193_channels[] = {
> >         AD719x_DIFF_CHANNEL(2, 5, 6, AD7193_CH_AIN5P_AIN6M),
> >         AD719x_DIFF_CHANNEL(3, 7, 8, AD7193_CH_AIN7P_AIN8M),
> >         AD719x_TEMP_CHANNEL(4, AD7193_CH_TEMP),
> > -       AD719x_SHORTED_CHANNEL(5, 2, AD7193_CH_AIN2P_AIN2M),
> > +       AD719x_DIFF_CHANNEL(5, 2, 2, AD7193_CH_AIN2P_AIN2M),
> >         AD719x_CHANNEL(6, 1, AD7193_CH_AIN1),
> >         AD719x_CHANNEL(7, 2, AD7193_CH_AIN2),
> >         AD719x_CHANNEL(8, 3, AD7193_CH_AIN3),
>