linux-kernel - Re: [RFC PATCH 1/2] iio: adc: ad7476: Simplify chip type detection

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Message-ID: <CAHp75VePYYH3VbOjY2r_1BuLZ-CEa+mJiS2SwSswsFhiha0VYw@mail.gmail.com>
Date: Mon, 4 Aug 2025 10:31:56 +0200
From: Andy Shevchenko <andy.shevchenko@...il.com>
To: Matti Vaittinen <mazziesaccount@...il.com>
Cc: Matti Vaittinen <matti.vaittinen@...rohmeurope.com>, Lars-Peter Clausen <lars@...afoo.de>, 
	Michael Hennerich <Michael.Hennerich@...log.com>, Jonathan Cameron <jic23@...nel.org>, 
	David Lechner <dlechner@...libre.com>, Nuno Sá <nuno.sa@...log.com>, 
	Andy Shevchenko <andy@...nel.org>, Liam Girdwood <lgirdwood@...il.com>, Mark Brown <broonie@...nel.org>, 
	linux-iio@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH 1/2] iio: adc: ad7476: Simplify chip type detection

On Mon, Aug 4, 2025 at 7:56 AM Matti Vaittinen <mazziesaccount@...il.com> wrote:
> On 02/08/2025 01:01, Andy Shevchenko wrote:
> > On Fri, Aug 1, 2025 at 12:07 PM Matti Vaittinen
> > <mazziesaccount@...il.com> wrote:
> >>
> >> The ad7476 driver uses a table of structures for defining the IC variant
> >> specific data. Table is indexed using enum values, which are picked by
> >> SPI ID.
> >>
> >> Having the table and an enum adds extra complexity. It is potentially
> >> unsafe if someone alters the enumeration values, or size of the IC data
> >> table.
> >
> > I don't see the problem here. I like the part about converting ID
> > tables to use chip_info instead of plain integers, but other than that
> > I do not see how enum is worse than the split version.
>
> The potential culprit with using the enum for array indexing is, that it
> requires the array size and enum values (used for indexing) to stay in
> sync. Eg, used enum values must be smaller than the size of the array.
> Also, the chip-info items in the array must be kept in locations which
> match the enum values.
>
> Yes, we have ways to do this, often using the last enum value as the
> size of the array,

> and/or using designated array initializers.

That's what I kept in mind and seems already the case in this driver.
That's why I doubt the brave statement in the commit message.

> It still
> requires programmer to do this correctly. Changing enum at the top of
> the file may break the array indexing (in seemingly unrelated place, at
> the bottom of the file). I agree this is pretty trivial issue, but it's
> still a thing to keep in mind.
>
> Splitting the chip-info in own structs and using direct pointer to the
> struct makes it harder to get it wrong.
>
> Finally, dropping the enum makes adding code which does decisions based
> on the chip-ID less appealing. It hopefully encourages adding _all_ IC
> specific quirks in the chip-info instead, which will keep the code path
> (IMHO) cleaner when all chip-specifics are in the chip-info.

Final argument makes sense to me.

> Anyways, Thanks for the feedback!

You're always welcome.

-- 
With Best Regards,
Andy Shevchenko