linux-kernel - Re: [PATCH v4] ARM: smp: Only expose /sys/.../cpuX/online if hotpluggable

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Date:	Tue, 7 Apr 2015 15:56:35 +0100
From:	Russell King - ARM Linux <linux@....linux.org.uk>
To:	Stephen Boyd <sboyd@...eaurora.org>
Cc:	linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
	Mark Rutland <mark.rutland@....com>,
	Nicolas Pitre <nico@...aro.org>,
	Dave Martin <Dave.Martin@....com>,
	Simon Horman <horms@...ge.net.au>,
	Magnus Damm <magnus.damm@...il.com>, linux-sh@...r.kernel.org,
	Tyler Baker <tyler.baker@...aro.org>,
	Geert Uytterhoeven <geert@...ux-m68k.org>
Subject: Re: [PATCH v4] ARM: smp: Only expose /sys/.../cpuX/online if
 hotpluggable

On Mon, Apr 06, 2015 at 01:24:13PM -0700, Stephen Boyd wrote:
> Writes to /sys/.../cpuX/online fail if we determine the platform
> doesn't support hotplug for that CPU. Furthermore, if the cpu_die
> op isn't specified the system hangs when we try to offline a CPU
> and it comes right back online unexpectedly. Let's figure this
> stuff out before we make the sysfs nodes so that the online file
> doesn't even exist if it isn't (at least sometimes) possible to
> hotplug the CPU.
> 
> Add a new 'cpu_can_disable' op and repoint all 'cpu_disable'
> implementations at it because all implementers use the op to
> indicate if a CPU can be hotplugged or not in a static fashion.
> With PSCI we may need to add a 'cpu_disable' op so that the
> secure OS can be migrated off the CPU we're trying to hotplug.
> In this case, the 'cpu_can_disable' op will indicate that all
> CPUs are hotpluggable by returning true, but the 'cpu_disable' op
> will make a PSCI migration call and occasionally fail, denying
> the hotplug of a CPU. This shouldn't be any worse than x86 where
> we may indicate that all CPUs are hotpluggable but occasionally
> we can't offline a CPU due to check_irq_vectors_for_cpu_disable()
> failing to find a CPU to move vectors to.
> 
> Cc: Mark Rutland <mark.rutland@....com>
> Cc: Nicolas Pitre <nico@...aro.org>
> Cc: Dave Martin <Dave.Martin@....com>
> Cc: Simon Horman <horms@...ge.net.au>
> Cc: Magnus Damm <magnus.damm@...il.com>
> Cc: <linux-sh@...r.kernel.org>
> Cc: Tyler Baker <tyler.baker@...aro.org>
> Cc: Geert Uytterhoeven <geert@...ux-m68k.org>
> Signed-off-by: Stephen Boyd <sboyd@...eaurora.org>

I think this is fine, but I'd like to see some acks for it.  As it's
mostly core ARM stuff, it should be merged through my tree unless there
is a known conflict with arm-soc.  Thanks.

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