linux-kernel - Re: [PATCH 5/7] arm64: Select ARCH_HAS_CPU

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Date:   Tue, 29 Aug 2023 15:49:44 +0100
From:   "Russell King (Oracle)" <linux@...linux.org.uk>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Mihai Carabas <mihai.carabas@...cle.com>,
        Joao Martins <joao.m.martins@...cle.com>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org,
        "H. Peter Anvin" <hpa@...or.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        "Rafael J. Wysocki" <rafael@...nel.org>,
        Daniel Lezcano <daniel.lezcano@...aro.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Kees Cook <keescook@...omium.org>,
        Petr Mladek <pmladek@...e.com>,
        Ard Biesheuvel <ardb@...nel.org>,
        Sami Tolvanen <samitolvanen@...gle.com>,
        Nicholas Piggin <npiggin@...il.com>,
        Juerg Haefliger <juerg.haefliger@...onical.com>,
        Mickaël Salaün <mic@...ikod.net>,
        Arnd Bergmann <arnd@...db.de>,
        Ankur Arora <ankur.a.arora@...cle.com>,
        linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        kvm@...r.kernel.org, linux-pm@...r.kernel.org
Subject: Re: [PATCH 5/7] arm64: Select ARCH_HAS_CPU_RELAX

On Wed, Aug 09, 2023 at 03:49:41PM +0200, Peter Zijlstra wrote:
> On Wed, Aug 09, 2023 at 02:39:39PM +0300, Mihai Carabas wrote:
> > From: Joao Martins <joao.m.martins@...cle.com>
> > 
> > cpu_relax() is necessary to allow cpuidle poll-state to be used,
> > so select it from ARM64 kconfig.
> > 
> > Signed-off-by: Joao Martins <joao.m.martins@...cle.com>
> > Signed-off-by: Mihai Carabas <mihai.carabas@...cle.com>
> > ---
> >  arch/arm64/Kconfig | 1 +
> >  1 file changed, 1 insertion(+)
> > 
> > diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
> > index 87ade6549790..7c47617b5722 100644
> > --- a/arch/arm64/Kconfig
> > +++ b/arch/arm64/Kconfig
> > @@ -105,6 +105,7 @@ config ARM64
> >  	select ARCH_WANT_LD_ORPHAN_WARN
> >  	select ARCH_WANTS_NO_INSTR
> >  	select ARCH_WANTS_THP_SWAP if ARM64_4K_PAGES
> > +	select ARCH_HAS_CPU_RELAX
> >  	select ARCH_HAS_UBSAN_SANITIZE_ALL
> >  	select ARM_AMBA
> >  	select ARM_ARCH_TIMER
> 
> Uh what ?! cpu_relax() is assumed present on all archs, no?

I think you have x86 to blame for that!

That symbol is used in drivers/acpi/processor_idle.c to setup stuff
for the cpuidle polling, and also by cpuidle's Makefile to build
poll_state.o

It isn't to do with the presence of cpu_relax() or not.

It probably ought to be renamed to CPUIDLE_CPU_RELAX which would
better describe its modern purpose.

-- 
RMK's Patch system: https://www.armlinux.org.uk/developer/patches/
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