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

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Date:   Fri, 10 Jun 2022 16:37:05 +0200
From:   Andy Shevchenko <andy.shevchenko@...il.com>
To:     Xiaohui Zhang <xiaohuizhang@....edu.cn>
Cc:     Jonathan Cameron <jic23@...nel.org>,
        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, 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?

> +               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.

-- 
With Best Regards,
Andy Shevchenko