linux-kernel - Re: [PATCH v2] iio: adc: ad7944: Add support for "3-wire mode"

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Date: Mon, 18 Mar 2024 15:09:32 +0200
From: Andy Shevchenko <andy.shevchenko@...il.com>
To: Jonathan Cameron <Jonathan.Cameron@...wei.com>
Cc: David Lechner <dlechner@...libre.com>, Jonathan Cameron <jic23@...nel.org>, 
	Michael Hennerich <michael.hennerich@...log.com>, Nuno Sá <nuno.sa@...log.com>, 
	linux-iio@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] iio: adc: ad7944: Add support for "3-wire mode"

On Mon, Mar 18, 2024 at 2:41 PM Jonathan Cameron
<Jonathan.Cameron@...wei.com> wrote:
> > >  struct ad7944_adc {
> > >     struct spi_device *spi;
> > > +   enum ad7944_spi_mode spi_mode;
> > >     /* Chip-specific timing specifications. */
> > >     const struct ad7944_timing_spec *timing_spec;
> > >     /* GPIO connected to CNV pin. */
> > > @@ -58,6 +75,9 @@ struct ad7944_adc {
> > >      } sample __aligned(IIO_DMA_MINALIGN);
> > >  };
> >
> > Have you run `pahole` to see if there is a better place for a new member?
>
> I know this matters for structures where we see lots of them, but do we actually
> care for one offs?  Whilst it doesn't matter here I'd focus much more
> on readability and like parameter grouping for cases like this than wasting
> a few bytes.

This is _also_ true, but think more about cache line contamination.
Even not-so-important bytes may decrease the performance. In some
cases it's tolerable, in some it is not (high-speed ADC). In general I
assume that the developer has to understand many aspects of the
software and cache line contamination may be last but definitely not
least.

-- 
With Best Regards,
Andy Shevchenko