linux-kernel - Re: [PATCH] iio: adc: ad4000: Avoid potential double data word read

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Message-ID: <CAHp75VdKpSnwfKO4GaJmVOs8kHASsZ4V9yNcZ7EF_GVoDNmrfw@mail.gmail.com>
Date: Wed, 16 Apr 2025 08:59:50 +0300
From: Andy Shevchenko <andy.shevchenko@...il.com>
To: David Lechner <dlechner@...libre.com>
Cc: Marcelo Schmitt <marcelo.schmitt@...log.com>, linux-iio@...r.kernel.org, 
	linux-kernel@...r.kernel.org, jic23@...nel.org, lars@...afoo.de, 
	Michael.Hennerich@...log.com, nuno.sa@...log.com, andy@...nel.org, 
	marcelo.schmitt1@...il.com
Subject: Re: [PATCH] iio: adc: ad4000: Avoid potential double data word read

On Wed, Apr 16, 2025 at 12:22 AM David Lechner <dlechner@...libre.com> wrote:
> On 4/15/25 1:02 PM, Andy Shevchenko wrote:
> > On Tue, Apr 15, 2025 at 3:21 PM Marcelo Schmitt
> > <marcelo.schmitt@...log.com> wrote:

...

> >>         xfers[1].rx_buf = &st->scan.data;
> >> -       xfers[1].len = BITS_TO_BYTES(chan->scan_type.storagebits);
> >> +       xfers[1].len = chan->scan_type.realbits > 16 ? 4 : 2;
> >
> > But wouldn't be logical to have
> >
> >        xfers[1].len = BITS_TO_BYTES(chan->scan_type.realbits);
> >
> > ?
>
> No, SPI expects 1, 2 or 4 bytes, never 3. If realbits is 18, we
> need len = 4.
>
> It would have to be:
>
>         xfers[1].len = roundup_pow_of_two(BITS_TO_BYTES(chan->scan_type.realbits));
>
> But that gets too long for 1 line

Actually there are a handful of drivers including SPI core that need
that helper already, I would prefer to have a helper defined in spi.h.

, so I prefer what Marcelo wrote.
>
> Maybe an idea for another day:
>
> #define SPI_LEN_FOR_BITS(bits) roundup_pow_of_two(BITS_TO_BYTES(bits))

Right, but as static inline to have stricter types.

> There are a couple of places in spi/ that could use this and several
> iio drivers.

-- 
With Best Regards,
Andy Shevchenko