linux-kernel - Re: [PATCH v2] mux: remove the Kconfig question for the subsystem

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Message-Id: <efaf326b-3cd9-40a4-8424-b5f60270beae@app.fastmail.com>
Date:   Wed, 07 Dec 2022 19:57:31 +0100
From:   "Arnd Bergmann" <arnd@...db.de>
To:     "Randy Dunlap" <rdunlap@...radead.org>,
        "Peter Rosin" <peda@...ntia.se>,
        "Linus Torvalds" <torvalds@...ux-foundation.org>
Cc:     "Greg Kroah-Hartman" <gregkh@...uxfoundation.org>,
        "Andrew Morton" <akpm@...ux-foundation.org>,
        "Linux Kernel Mailing List" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2] mux: remove the Kconfig question for the subsystem

On Wed, Dec 7, 2022, at 18:19, Randy Dunlap wrote:
> On 12/7/22 00:41, Arnd Bergmann wrote:
>> For the other subsystems I mentioned, there are occasionally 
>> problems with missing 'select' that tend to be a pain to find,
>> compared to subsystems consistently using 'depends on', which
>> show up as link failures in randconfig builds.
>
> I find that various drivers mixing the use of "select" and
> "depends on" is problematic.

Agreed. Even just mixing 'select' with user-visible symbols
is very confusing. The two sensible ways are either using
user-visible options with 'depends on' or hidden options with
'select'.

> However, there was no answer for the original question:
> How does a user enable the 4 Kconfig symbols in drivers/mux/Kconfig
> if some other random driver has not selected MULTIPLEXER?

There is no need to enable any of them in this case, because
the mux drivers are not usable by themselves.

> I.e.:
>
> config MUX_ADG792A
> 	tristate "Analog Devices ADG792A/ADG792G Multiplexers"
>
> config MUX_ADGS1408
> 	tristate "Analog Devices ADGS1408/ADGS1409 Multiplexers"
>
> config MUX_GPIO
> 	tristate "GPIO-controlled Multiplexer"
>
> config MUX_MMIO
> 	tristate "MMIO/Regmap register bitfield-controlled Multiplexer"
>
> OK, MUX_MMIO is selected from some other drivers, but if that is not done,
> how can the first 3 be enabled by a user?

They cannot, that is the entire point of hiding the subsystem
when it is not used.

      Arnd