linux-kernel - Re: [PATCH v2 1/2] iio/adc: ingenic: Fix channel offsets in buffer

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Message-ID: <CAHp75VeLRHwcKQALwnBb-gqVeyxxH=_F40TserRXqo_kbaZzoQ@mail.gmail.com>
Date:   Mon, 22 May 2023 13:15:03 +0300
From:   Andy Shevchenko <andy.shevchenko@...il.com>
To:     Artur Rojek <contact@...ur-rojek.eu>
Cc:     Paul Cercueil <paul@...pouillou.net>,
        Jonathan Cameron <jic23@...nel.org>,
        Dmitry Torokhov <dmitry.torokhov@...il.com>,
        Chris Morgan <macromorgan@...mail.com>,
        linux-mips@...r.kernel.org, linux-iio@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-input@...r.kernel.org
Subject: Re: [PATCH v2 1/2] iio/adc: ingenic: Fix channel offsets in buffer

On Mon, May 22, 2023 at 1:59 AM Artur Rojek <contact@...ur-rojek.eu> wrote:
>
> Consumers expect the buffer to only contain enabled channels. While
> preparing the buffer, the driver makes two mistakes:
> 1) It inserts empty data for disabled channels.
> 2) Each ADC readout contains samples for two 16-bit channels. If either
>    of them is active, the whole 32-bit sample is pushed into the buffer
>    as-is.
>
> Both of those issues cause the active channels to appear at the wrong
> offsets in the buffer. Fix the above by demuxing samples for active
> channels only.
>
> This has remained unnoticed, as all the consumers so far were only using
> channels 0 and 1, leaving them unaffected by changes introduced in this
> commit.

...

> +       u16 tdat[6];
> +       u32 val;
> +
> +       memset(tdat, 0, ARRAY_SIZE(tdat));

Yeah, as LKP tells us this should be sizeof() instead of ARRAY_SIZE().

> +       for (i = 0; mask && i < ARRAY_SIZE(tdat); mask >>= 2) {
> +               if (mask & 0x3) {

(for the consistency it has to be GENMASK(), but see below)

First of all, strictly speaking we should use the full mask without
limiting it to the 0 element in the array (I'm talking about
active_scan_mask).

That said, we may actually use bit operations here in a better way, i.e.

  unsigned long mask = active_scan_mask[0] & (active_scan_mask[0] - 1);

  j = 0;
  for_each_set_bit(i, active_scan_mask, ...) {
    val = readl(...);
    /* Two channels per sample. Demux active. */
    tdat[j++] = val >> (16 * (i % 2));
  }

> +                       val = readl(adc->base + JZ_ADC_REG_ADTCH);
> +                       /* Two channels per sample. Demux active. */
> +                       if (mask & BIT(0))
> +                               tdat[i++] = val & 0xffff;
> +                       if (mask & BIT(1))
> +                               tdat[i++] = val >> 16;
> +               }
>         }

-- 
With Best Regards,
Andy Shevchenko