linux-kernel - Re: [PATCH] iio:proximity:sx9324: Fix hardware gain read/write

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Date:   Mon, 21 Mar 2022 19:36:33 +0100
From:   Stephen Boyd <swboyd@...omium.org>
To:     Jonathan Cameron <jic23@...nel.org>
Cc:     Lars-Peter Clausen <lars@...afoo.de>, linux-kernel@...r.kernel.org,
        linux-iio@...r.kernel.org, Gwendal Grignou <gwendal@...omium.org>
Subject: Re: [PATCH] iio:proximity:sx9324: Fix hardware gain read/write

Quoting Jonathan Cameron (2022-03-19 08:26:41)
> On Fri, 18 Mar 2022 13:48:08 -0700
> Stephen Boyd <swboyd@...omium.org> wrote:
>
> Hi Stephen,
>
>
> > There are four possible gain values according to sx9324_gain_vals[]: 1,
> > 2, 4, and 8. When writing and reading the register the values are off by
> > one.
> > The bits should be set according to this equation:
> >
> >       ilog2(<gain>) + 1
> >
> > so that a gain of 8 is 0x3 in the register field and a gain of 4 is 0x2
> > in the register field, etc. Fix up the functions.
>
> So is the 0 value reserved?  I can't find an sx9324 datasheet but he
> 9320 is online and that seems to be the case there.  If so please state
> that in this description as well.

Yes 0 is reserved. The top of this driver's C file has the datasheet
link[1]

>
> >
> > Fixes: 4c18a890dff8 ("iio:proximity:sx9324: Add SX9324 support")
> > Cc: Gwendal Grignou <gwendal@...omium.org>
> > Signed-off-by: Stephen Boyd <swboyd@...omium.org>
> > ---
> >  drivers/iio/proximity/sx9324.c | 7 +++++--
> >  1 file changed, 5 insertions(+), 2 deletions(-)
> >
> > diff --git a/drivers/iio/proximity/sx9324.c b/drivers/iio/proximity/sx9324.c
> > index 0d9bbbb50cb4..a3c8e02f5a56 100644
> > --- a/drivers/iio/proximity/sx9324.c
> > +++ b/drivers/iio/proximity/sx9324.c
> > @@ -379,7 +379,10 @@ static int sx9324_read_gain(struct sx_common_data *data,
> >       if (ret)
> >               return ret;
> >
> > -     *val = 1 << FIELD_GET(SX9324_REG_PROX_CTRL0_GAIN_MASK, regval);
> > +     regval = FIELD_GET(SX9324_REG_PROX_CTRL0_GAIN_MASK, regval);
> > +     if (regval)
>
> If 0 is reserved then I'd return and error code here to indicate
> we don't know what the gain is rather than carrying on regardless.
> Or is this going to cause problems as it will be an ABI change (error
> return possible when it wasn't really before)?
>

That sounds OK to me. The driver is only being introduced now so we can
still fix it to reject a gain of 0. Unless 0 should mean "off", i.e.
hardware gain of 1?

[1] https://edit.wpgdadawant.com/uploads/news_file/program/2019/30184/tech_files/program_30184_suggest_other_file.pdf