linux-kernel - Re: [RFC PATCH 5/7] arm64: smp: use generic SMP stop common code

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Date:   Mon, 26 Aug 2019 20:58:01 +0100
From:   Cristian Marussi <cristian.marussi@....com>
To:     Christoph Hellwig <hch@...radead.org>
Cc:     linux-kernel@...r.kernel.org, linux-arch@...r.kernel.org,
        mark.rutland@....com, peterz@...radead.org,
        catalin.marinas@....com, takahiro.akashi@...aro.org,
        james.morse@....com, hidehiro.kawai.ez@...achi.com,
        tglx@...utronix.de, will@...nel.org, dave.martin@....com,
        linux-arm-kernel@...ts.infradead.org
Subject: Re: [RFC PATCH 5/7] arm64: smp: use generic SMP stop common code

Hi

On 8/26/19 4:32 PM, Christoph Hellwig wrote:
>> +config ARCH_USE_COMMON_SMP_STOP
>> +	def_bool y if SMP
> 
> The option belongs into common code and the arch code shoud only
> select it.
>

In fact that was my first approach, but then I noticed that in kernel/ topdir
there was no generic Kconfig but only subsystem specific ones:

Kconfig.freezer  Kconfig.hz       Kconfig.locks    Kconfig.preempt

while instead looking into archs top level Kconfig, beside the usual arch/Kconfig selects,
I could find this similar sort of "reversed" approach in which the arch defined and
selected a CONFIG which was indeed then used only in common code like in:

20:37 $ egrep -R ARCH_HAS_CACHE_LINE_SIZE .
./arch/arc/Kconfig:config ARCH_HAS_CACHE_LINE_SIZE
./arch/x86/Kconfig:config ARCH_HAS_CACHE_LINE_SIZE
./arch/arm64/Kconfig:config ARCH_HAS_CACHE_LINE_SIZE
./include/linux/cache.h:#ifndef CONFIG_ARCH_HAS_CACHE_LINE_SIZE

20:39 $ egrep -R ARCH_HAS_KEXEC_PURGATORY .
./arch/powerpc/Kconfig:config ARCH_HAS_KEXEC_PURGATORY
./arch/x86/Kconfig:config ARCH_HAS_KEXEC_PURGATORY
./arch/s390/Kconfig:config ARCH_HAS_KEXEC_PURGATORY
./arch/s390/purgatory/Makefile:obj-$(CONFIG_ARCH_HAS_KEXEC_PURGATORY) += kexec-purgatory.o
./arch/s390/Kbuild:obj-$(CONFIG_ARCH_HAS_KEXEC_PURGATORY) += purgatory/
./kernel/kexec_file.c:	if (!IS_ENABLED(CONFIG_ARCH_HAS_KEXEC_PURGATORY))

so I thought it was an acceptable option and I went for it, not to introduce a new kernel/Kconfig.smp
just for this new config option; but in fact I could have missed the real reason underlying these two
different choices.

Thanks

Cristian