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

[Fabrizio Lamarque]

On Sat, Apr 1, 2023 at 4:27 PM Jonathan Cameron <jic23@xxxxxxxxxx> wrote:
>
> On Thu, 30 Mar 2023 12:21:00 +0200
> Paul Cercueil <paul@xxxxxxxxxxxxxxx> 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@xxxxxxxxxxxxxxx>
> > Co-developed-by: Alisa Roman <alisa.roman@xxxxxxxxxx>
> > Signed-off-by: Alisa Roman <alisa.roman@xxxxxxxxxx>
>
> +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.

The bug report in libiio related to this patch was from the company I work for.
I guess this confirms there are few actual users of the driver (at least
of its later revisions).

I see a somewhat limited usage of the "input2 vs input2" configuration,
it was just causing a libiio initialization issue, without impacting
sysfs operation.
The breaking ABI change would impact only users of this channel
relying on sysfs without libiio.

I tested this patch and I confirm it solves the issue with libiio.

Fabrizio

>
> 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),
>