linux-kernel - Re: [PATCH 1/1] iio:proximity:sx9360: Fix hardware gain read/write

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Date:   Sat, 11 Jun 2022 17:03:40 +0100
From:   Jonathan Cameron <jic23@...nel.org>
To:     Andy Shevchenko <andy.shevchenko@...il.com>
Cc:     Xiaohui Zhang <xiaohuizhang@....edu.cn>,
        Lars-Peter Clausen <lars@...afoo.de>,
        Gwendal Grignou <gwendal@...omium.org>,
        Stephen Boyd <swboyd@...omium.org>,
        Jongpil Jung <jongpil19.jung@...sung.com>,
        linux-iio <linux-iio@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/1] iio:proximity:sx9360: Fix hardware gain read/write

On Fri, 10 Jun 2022 16:37:05 +0200
Andy Shevchenko <andy.shevchenko@...il.com> wrote:

> On Fri, Jun 10, 2022 at 7:53 AM Xiaohui Zhang <xiaohuizhang@....edu.cn> wrote:
> >
> > Similar to the handling of read/write in commit 108e4d4de2b5
> > ("iio:proximity:sx9324: Fix hardware gain read/write"), we thought
> > a patch might be needed here as well.
> >
> > There are four possible gain values according to 'sx9360_gain_vals[]':
> >
> >         1, 2, 4, and 8
> >
> > The values are off by one when writing and reading the register. The
> > bits should be set according to this equation:
> >
> >         ilog2(<gain>) + 1
> >
> > so that a gain of 8 is 0x4 in the register field and a gain of 4 is 0x3
> > in the register field, etc. Note that a gain of 0 is reserved per the
> > datasheet. The default gain (SX9360_REG_PROX_CTRL0_GAIN_1) is also
> > wrong. It should be 0x1 << 3, i.e. 0x8, not 0x80 which is setting the
> > reserved bit 7.
> >
> > Fix this all up to properly handle the hardware gain and return errors
> > for invalid settings.  
> 
> ...
> 
> > +       regval = FIELD_GET(SX9360_REG_PROX_CTRL0_GAIN_MASK, regval);
> > +       if (regval)
> > +               regval--;
> > +       else if (regval == SX9360_REG_PROX_CTRL0_GAIN_RSVD ||
> > +                regval > SX9360_REG_PROX_CTRL0_GAIN_8)  
> 
> else?! Isn't it a dead code? How has it been tested?

Gah. Missed this in review of sx9324 change.  First check is
fine because GAIN_RSVD is 0 though not a lot of point in the if.

Second one is intended as hardening against malicious / broken
hardware only so you would never see that value except via emulation
or a unit test.  So test wouldn't have spotted this as far as I
can see.
Needs good old eyeballs. :)


> 
> > +               return -EINVAL;  
> 
> > +       *val = 1 << regval;  
> 
> Even in the original code this is wrong in accordance with C standard.
> It might have potentially UB. BIT(), for example, solves this issue.
> You may do what it does under the hood.
>