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

[Jonathan Cameron <jic23@kernel.org>]

On Tue, 25 Apr 2023 11:07:11 +0200
Paul Cercueil <paul@crapouillou.net> wrote:

> 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@crapouillou.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@crapouillou.net>
> > > Co-developed-by: Alisa Roman <alisa.roman@analog.com>
> > > Signed-off-by: Alisa Roman <alisa.roman@analog.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 :)

Wise move :)

Applied to the fixes-togreg branch of iio.git and marked for stable.

Thanks,

Jonathan

> 
> 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),  
> >   
>