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

From: Randy Dunlap <rdunlap@infradead.org>
To: Arnd Bergmann <arnd@arndb.de>, Peter Rosin <peda@axentia.se>,
	Linus Torvalds <torvalds@linux-foundation.org>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
	Andrew Morton <akpm@linux-foundation.org>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH v2] mux: remove the Kconfig question for the subsystem
Date: Wed, 7 Dec 2022 09:19:15 -0800	[thread overview]
Message-ID: <41a5931e-3543-6a3d-ca85-2dd8ad581f2e@infradead.org> (raw)
In-Reply-To: <a546f2db-371e-4d2f-a0ee-c71fcae8c548@app.fastmail.com>

On 12/7/22 00:41, Arnd Bergmann wrote:
> On Tue, Dec 6, 2022, at 23:20, Randy Dunlap wrote:
>> On 7/4/17 01:22, Peter Rosin wrote:
>>> The MULTIPLEXER question in the Kconfig might be confusing and is
>>> of dubious value. Remove it. This makes consumers responsible for
>>> selecting MULTIPLEXER, which they already do.
>>>
>>> Signed-off-by: Peter Rosin <peda@axentia.se>
>>
>>
>> How does a user enable any of the 4 drivers in drivers/mux/Kconfig unless
>> some other totally unrelated driver has just happened to select MULTIPLEXER
>> so that the mux driver menu is visible to them?
> 
> We have this mechanism for a few subsystems, LEDS_CLASS/NEW_LEDS and
> CRYPTO being more common examples.
> 
> The idea clearly is that there is no need for the subsystem if no
> drivers call into it. This works if every single driver calling
> \(devm_\|\)mux_control_get also results in 'select MULTIPLEXER'
> in Kconfig, and none of them ever uses 'depends on MULTIPLEXER'.
> I think this is used correctly most of the time in mainline:
> git grep '\<\(mux/consumer.h\|MULTIPLEXER\)\>' indicates that
> PHY_J721E_WIZ and MTD_PHYSMAP_BT1_ROM may not actually need it,
> but that is fairly harmless.

Yes, sure.

> 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.

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?

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?

thanks.

-- 
~Randy