linux-kernel - Re: [PATCH 1/3] arm: ls1: add CPU hotplug platform support

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Date:	Fri, 26 Sep 2014 14:20:38 +0100
From:	Mark Rutland <mark.rutland@....com>
To:	Russell King - ARM Linux <linux@....linux.org.uk>
Cc:	Chenhui Zhao <chenhui.zhao@...escale.com>,
	"kernel@...gutronix.de" <kernel@...gutronix.de>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"Zhuoyu.Zhang@...escale.com" <Zhuoyu.Zhang@...escale.com>,
	"Jason.Jin@...escale.com" <Jason.Jin@...escale.com>,
	"leoli@...escale.com" <leoli@...escale.com>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH 1/3] arm: ls1: add CPU hotplug platform support

> > This looks to be a carbon copy of the vexpress pseudo-hotplug in
> > arch/arm/mach-vexpress/hotplug.c, which is obviously broken in the way
> > you describe above. Perhaps we should go about ripping that out?
> 
> The Versatile Express does not support suspend so the only problem case
> is kexec.  However, isn't this support needed for big.LITTLE, and as
> the Versatile Express is the platform which these features get developed
> on, having working CPU hotplug seems rather fundamental for ARM kernel
> feature development.
> 
> In that regard, Versatile Express is something of a special case.

It is admittedly helpful during development to perform pseudo-hotplug on
Versatile Express. I have a local patch adding vexpress_cpu_disable so I
can test for bugs that only trigger if CPU0 is hotplugged.

Given that, perhaps we should make it clearer that Versatile Express is
not a reference implementation for CPU hotplug; add some Kconfig (e.g.
VEXPRESS_PSEUDO_HOTPLUG) that depends on !KEXEC && !SUSPEND, and putting
a note in hotplug.c stating it's not suitable as a reference
implementation.

...but perhaps that's overkill.

Mark.
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