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

From: Russell King - ARM Linux
Date: Tue Apr 07 2015 - 10:57:04 EST

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@xxxxxxx>
> Cc: Nicolas Pitre <nico@xxxxxxxxxx>
> Cc: Dave Martin <Dave.Martin@xxxxxxx>
> Cc: Simon Horman <horms@xxxxxxxxxxxx>
> Cc: Magnus Damm <magnus.damm@xxxxxxxxx>
> Cc: <linux-sh@xxxxxxxxxxxxxxx>
> Cc: Tyler Baker <tyler.baker@xxxxxxxxxx>
> Cc: Geert Uytterhoeven <geert@xxxxxxxxxxxxxx>
> Signed-off-by: Stephen Boyd <sboyd@xxxxxxxxxxxxxx>

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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