linux-kernel - Re: [PATCH v3] ARM: smp: Only expose /sys/.../cpuX/online if hotpluggable

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Date:	Thu, 19 Feb 2015 17:14:56 -0500
From:	Simon Horman <horms@...ge.net.au>
To:	Stephen Boyd <sboyd@...eaurora.org>
Cc:	Russell King <linux@....linux.org.uk>,
	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>,
	Magnus Damm <magnus.damm@...il.com>, linux-sh@...r.kernel.org
Subject: Re: [PATCH v3] ARM: smp: Only expose /sys/.../cpuX/online if
 hotpluggable

On Wed, Feb 18, 2015 at 03:27:57PM -0800, Stephen Boyd wrote:
> On 02/18/15 14:27, Simon Horman wrote:
> > On Fri, Feb 13, 2015 at 04:42:54PM -0800, 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 current users use the op to
> >> indicate if a CPU can be hotplugged or not in a static fashion.
> >> With PSCI we may need to introduce 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 1, 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>
> >> Signed-off-by: Stephen Boyd <sboyd@...eaurora.org>
> >> ---
> >>
> >> Changes since v2:
> >>  * Left cpu_disable op in place
> >>  * Split out shmobile function deletion
> >>
> >>  arch/arm/common/mcpm_platsmp.c       | 12 ++++--------
> >>  arch/arm/include/asm/smp.h           | 10 ++++++++++
> >>  arch/arm/kernel/setup.c              |  2 +-
> >>  arch/arm/kernel/smp.c                | 15 ++++++++++++++-
> >>  arch/arm/mach-shmobile/common.h      |  2 +-
> >>  arch/arm/mach-shmobile/platsmp.c     |  4 ++--
> >>  arch/arm/mach-shmobile/smp-r8a7790.c |  2 +-
> >>  arch/arm/mach-shmobile/smp-r8a7791.c |  2 +-
> >>  arch/arm/mach-shmobile/smp-sh73a0.c  |  2 +-
> >>  9 files changed, 35 insertions(+), 16 deletions(-)
> > I think it would make sense to separate the ARM-core changes
> > from the mach-shmobile integration changes.
> 
> Are you saying two (three?) patches to add the op, and then move over
> each struct smp_operations? It's all going through rmk's tree so I'll
> leave that up to him.

I'm also happy to let RMK to decide what he thinks is best.
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