linux-kernel - Abuse of CONFIG_FOO's as feature selectors

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Date:	Wed, 22 Apr 2015 20:20:21 +0200
From:	Denys Vlasenko <dvlasenk@...hat.com>
To:	LKML <linux-kernel@...r.kernel.org>
Subject: Abuse of CONFIG_FOO's as feature selectors

Hi,

Kernel has a growing number of CONFIG items which are not
user-selectable features of their particular kernel builds,
but simply booleans controlled by other CONFIGs.
Example:

config X86
        def_bool y
        select ARCH_MIGHT_HAVE_ACPI_PDC if ACPI
        select ARCH_HAS_DEBUG_STRICT_USER_COPY_CHECKS
        select ARCH_HAS_FAST_MULTIPLIER
        select ARCH_HAS_GCOV_PROFILE_ALL
        select ARCH_MIGHT_HAVE_PC_PARPORT
        select ARCH_MIGHT_HAVE_PC_SERIO
        select HAVE_AOUT if X86_32
        select HAVE_UNSTABLE_SCHED_CLOCK
        select ARCH_SUPPORTS_NUMA_BALANCING if X86_64
        select ARCH_SUPPORTS_INT128 if X86_64
        select HAVE_IDE
        select HAVE_OPROFILE
        ...

I see how this practice originated: "select" statement
was initially added so that if feature X requires feature Y,
this can be enforced, but it was easy to use it to define
other booleans.

I have a feeling that in retrospect, it was a mistake.

It clutters .config with information which has nothing to do
with user's choice.

More importantly, now when you read some code, you don't know
whether a CONFIG_FOO you look at is user's configuration choice
or something else.

Now there are hundreds, maybe even thousands of these non-config
CONFIGs everywhere.

The same effect can be achieved, with marginally more typing,
with usual C defines in some header file:

#ifdef CONFIG_X86
# define ARCH_HAS_DEBUG_STRICT_USER_COPY_CHECKS
# define ARCH_HAS_FAST_MULTIPLIER
# define ARCH_HAS_GCOV_PROFILE_ALL
# define ARCH_MIGHT_HAVE_PC_PARPORT
# define ARCH_MIGHT_HAVE_PC_SERIO
...

Maybe we should stop doing the former and use the latter method?
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