[v5] ARM: smp: Only expose /sys/.../cpuX/online if hotpluggable

Commit Message

Stephen Boyd April 10, 2015, 10:33 p.m. UTC

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 <[e-mail]>
Cc: Nicolas Pitre <[e-mail]>
Cc: Dave Martin <[e-mail]>
Acked-by: Simon Horman <[e-mail]> [shmobile portion]
Tested-by: Simon Horman <[e-mail]>
Cc: Magnus Damm <[e-mail]>
Cc: <[e-mail]>
Cc: Tyler Baker <[e-mail]>
Cc: Geert Uytterhoeven <[e-mail]>
Signed-off-by: Stephen Boyd <[e-mail]>
---

Changes since v4:
 * Fix build error on !SMP (Tyler Baker)
 * Picked up Simon's ack/tested-by

Changes since v3:
 * Return bool instead of int from 'cpu_can_disable' op

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           |  1 +
 arch/arm/include/asm/smp_plat.h      |  9 +++++++++
 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 +-
 10 files changed, 35 insertions(+), 16 deletions(-)

Comments

Tyler Baker April 11, 2015, 5:23 p.m. UTC | #1

On 10 April 2015 at 15:33, Stephen Boyd <[e-mail]> 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 <[e-mail]>
> Cc: Nicolas Pitre <[e-mail]>
> Cc: Dave Martin <[e-mail]>
> Acked-by: Simon Horman <[e-mail]> [shmobile portion]
> Tested-by: Simon Horman <[e-mail]>
> Cc: Magnus Damm <[e-mail]>
> Cc: <[e-mail]>
> Cc: Tyler Baker <[e-mail]>
> Cc: Geert Uytterhoeven <[e-mail]>
> Signed-off-by: Stephen Boyd <[e-mail]>
> ---

Tested-by: Tyler Baker <[e-mail]>

I have tested v5 on top of next-20150410[0] and can confirm the build
regression reported in v4 is resolved[1]. Additionally, 314 boot tests
were performed[2] on arm, arm64 and x86 plaforms, no new regressions
detected.

I have executed the cpu-hotplug kselftest test case using the
multi_v7_defconfig on an array of arm platforms. Platforms which do
not support cpu hotplug show the behavior you would expect. The below
result was executed on a sun9i-a80-optimus.

Running tests in cpu-hotplug
========================================
CPU online/offline summary:
Cpus in online state: 0
Cpus in offline state: 1-7
Limited scope test: one hotplug cpu
(leaves cpu in the original state):
online to offline to online: cpu 0
./cpu-on-off-test.sh: line 186: can't create
/sys/devices/system/cpu/cpu0/online: Permission denied
0: unexpected fail
./cpu-on-off-test.sh: line 186: can't create
/sys/devices/system/cpu/cpu0/online: Permission denied
0: unexpected fail
selftests: cpu-on-off-test.sh [XFAIL]

Comparing this result to a tegra124-jetson-tk1 which supports cpu-hotplug.

Running tests in cpu-hotplug
========================================
CPU online/offline summary:
Cpus in online state: 0-3
Cpus in offline state: 0
Limited scope test: one hotplug cpu
(leaves cpu in the original state):
online to offline to online: cpu 3
[ 14.478074] CPU3: shutdown
selftests: cpu-on-off-test.sh [PASS]

Full list of kselftest cpu-hotplug results:

sun9i-a80-optimus [XFAIL]
sun9i-a80-cubieboard4 [XFAIL]
sun7i-a20-cubietruck [PASS]
tegra124-jetson-tk1 [PASS]
exynos5800-peach-pi [PASS]
exynos5422-odroidxu3 [XFAIL]
exynos5250-arndale [PASS]
exynos5250-snow [PASS]
exynos5420-arndale-octa [PASS]
exynos4412-odroidu3 [PASS]
imx6q-cm-fx6 [PASS]
imx6q-wandboard [PASS]
imx6q-sabrelite [PASS]
qcom-apq8084-ifc6540 [PASS]
qcom-apq8064-ifc6410 [PASS]
hip04-d01 [PASS]
omap4-panda-es [PASS]
am335x-boneblack [XFAIL]
omap3-beagle-xm [XFAIL]
ste-snowball [PASS
zynq-parallella [PASS]
(qemu) vexpress-v2p-ca15-tc1 [XFAIL]
(qemu) vexpress-v2p-ca15_a7 [PASS]
(qemu) vexpress-v2p-ca9 [XFAIL]

>
> Changes since v4:
>  * Fix build error on !SMP (Tyler Baker)
>  * Picked up Simon's ack/tested-by
>
> Changes since v3:
>  * Return bool instead of int from 'cpu_can_disable' op
>
> 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           |  1 +
>  arch/arm/include/asm/smp_plat.h      |  9 +++++++++
>  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 +-
>  10 files changed, 35 insertions(+), 16 deletions(-)
>
> diff --git a/arch/arm/common/mcpm_platsmp.c b/arch/arm/common/mcpm_platsmp.c
> index 92e54d7c6f46..2b25b6038f66 100644
> --- a/arch/arm/common/mcpm_platsmp.c
> +++ b/arch/arm/common/mcpm_platsmp.c
> @@ -65,14 +65,10 @@ static int mcpm_cpu_kill(unsigned int cpu)
>         return !mcpm_wait_for_cpu_powerdown(pcpu, pcluster);
>  }
>
> -static int mcpm_cpu_disable(unsigned int cpu)
> +static bool mcpm_cpu_can_disable(unsigned int cpu)
>  {
> -       /*
> -        * We assume all CPUs may be shut down.
> -        * This would be the hook to use for eventual Secure
> -        * OS migration requests as described in the PSCI spec.
> -        */
> -       return 0;
> +       /* We assume all CPUs may be shut down. */
> +       return true;
>  }
>
>  static void mcpm_cpu_die(unsigned int cpu)
> @@ -92,7 +88,7 @@ static struct smp_operations __initdata mcpm_smp_ops = {
>         .smp_secondary_init     = mcpm_secondary_init,
>  #ifdef CONFIG_HOTPLUG_CPU
>         .cpu_kill               = mcpm_cpu_kill,
> -       .cpu_disable            = mcpm_cpu_disable,
> +       .cpu_can_disable        = mcpm_cpu_can_disable,
>         .cpu_die                = mcpm_cpu_die,
>  #endif
>  };
> diff --git a/arch/arm/include/asm/smp.h b/arch/arm/include/asm/smp.h
> index 18f5a554134f..2563453d7b37 100644
> --- a/arch/arm/include/asm/smp.h
> +++ b/arch/arm/include/asm/smp.h
> @@ -104,6 +104,7 @@ struct smp_operations {
>  #ifdef CONFIG_HOTPLUG_CPU
>         int  (*cpu_kill)(unsigned int cpu);
>         void (*cpu_die)(unsigned int cpu);
> +       bool  (*cpu_can_disable)(unsigned int cpu);
>         int  (*cpu_disable)(unsigned int cpu);
>  #endif
>  #endif
> diff --git a/arch/arm/include/asm/smp_plat.h b/arch/arm/include/asm/smp_plat.h
> index 993e5224d8f7..f9080717fc88 100644
> --- a/arch/arm/include/asm/smp_plat.h
> +++ b/arch/arm/include/asm/smp_plat.h
> @@ -107,4 +107,13 @@ static inline u32 mpidr_hash_size(void)
>  extern int platform_can_secondary_boot(void);
>  extern int platform_can_cpu_hotplug(void);
>
> +#ifdef CONFIG_HOTPLUG_CPU
> +extern int platform_can_hotplug_cpu(unsigned int cpu);
> +#else
> +static inline int platform_can_hotplug_cpu(unsigned int cpu)
> +{
> +       return 0;
> +}
> +#endif
> +
>  #endif
> diff --git a/arch/arm/kernel/setup.c b/arch/arm/kernel/setup.c
> index 6c777e908a24..955d45d0f70c 100644
> --- a/arch/arm/kernel/setup.c
> +++ b/arch/arm/kernel/setup.c
> @@ -992,7 +992,7 @@ static int __init topology_init(void)
>
>         for_each_possible_cpu(cpu) {
>                 struct cpuinfo_arm *cpuinfo = &per_cpu(cpu_data, cpu);
> -               cpuinfo->cpu.hotpluggable = 1;
> +               cpuinfo->cpu.hotpluggable = platform_can_hotplug_cpu(cpu);
>                 register_cpu(&cpuinfo->cpu, cpu);
>         }
>
> diff --git a/arch/arm/kernel/smp.c b/arch/arm/kernel/smp.c
> index cca5b8758185..8c834ecc09db 100644
> --- a/arch/arm/kernel/smp.c
> +++ b/arch/arm/kernel/smp.c
> @@ -173,13 +173,26 @@ static int platform_cpu_disable(unsigned int cpu)
>         if (smp_ops.cpu_disable)
>                 return smp_ops.cpu_disable(cpu);
>
> +       return 0;
> +}
> +
> +int platform_can_hotplug_cpu(unsigned int cpu)
> +{
> +       /* cpu_die must be specified to support hotplug */
> +       if (!smp_ops.cpu_die)
> +               return 0;
> +
> +       if (smp_ops.cpu_can_disable)
> +               return smp_ops.cpu_can_disable(cpu);
> +
>         /*
>          * By default, allow disabling all CPUs except the first one,
>          * since this is special on a lot of platforms, e.g. because
>          * of clock tick interrupts.
>          */
> -       return cpu == 0 ? -EPERM : 0;
> +       return cpu != 0;
>  }
> +
>  /*
>   * __cpu_disable runs on the processor to be shutdown.
>   */
> diff --git a/arch/arm/mach-shmobile/common.h b/arch/arm/mach-shmobile/common.h
> index afc60bad6fd6..f2c4bf437ea7 100644
> --- a/arch/arm/mach-shmobile/common.h
> +++ b/arch/arm/mach-shmobile/common.h
> @@ -13,7 +13,7 @@ extern void shmobile_smp_boot(void);
>  extern void shmobile_smp_sleep(void);
>  extern void shmobile_smp_hook(unsigned int cpu, unsigned long fn,
>                               unsigned long arg);
> -extern int shmobile_smp_cpu_disable(unsigned int cpu);
> +extern bool shmobile_smp_cpu_can_disable(unsigned int cpu);
>  extern void shmobile_invalidate_start(void);
>  extern void shmobile_boot_scu(void);
>  extern void shmobile_smp_scu_prepare_cpus(unsigned int max_cpus);
> diff --git a/arch/arm/mach-shmobile/platsmp.c b/arch/arm/mach-shmobile/platsmp.c
> index 3923e09e966d..b23378f3d7e1 100644
> --- a/arch/arm/mach-shmobile/platsmp.c
> +++ b/arch/arm/mach-shmobile/platsmp.c
> @@ -31,8 +31,8 @@ void shmobile_smp_hook(unsigned int cpu, unsigned long fn, unsigned long arg)
>  }
>
>  #ifdef CONFIG_HOTPLUG_CPU
> -int shmobile_smp_cpu_disable(unsigned int cpu)
> +bool shmobile_smp_cpu_can_disable(unsigned int cpu)
>  {
> -       return 0; /* Hotplug of any CPU is supported */
> +       return true; /* Hotplug of any CPU is supported */
>  }
>  #endif
> diff --git a/arch/arm/mach-shmobile/smp-r8a7790.c b/arch/arm/mach-shmobile/smp-r8a7790.c
> index 930f45cbc08a..947e437cab68 100644
> --- a/arch/arm/mach-shmobile/smp-r8a7790.c
> +++ b/arch/arm/mach-shmobile/smp-r8a7790.c
> @@ -64,7 +64,7 @@ struct smp_operations r8a7790_smp_ops __initdata = {
>         .smp_prepare_cpus       = r8a7790_smp_prepare_cpus,
>         .smp_boot_secondary     = shmobile_smp_apmu_boot_secondary,
>  #ifdef CONFIG_HOTPLUG_CPU
> -       .cpu_disable            = shmobile_smp_cpu_disable,
> +       .cpu_can_disable        = shmobile_smp_cpu_can_disable,
>         .cpu_die                = shmobile_smp_apmu_cpu_die,
>         .cpu_kill               = shmobile_smp_apmu_cpu_kill,
>  #endif
> diff --git a/arch/arm/mach-shmobile/smp-r8a7791.c b/arch/arm/mach-shmobile/smp-r8a7791.c
> index 5e2d1db79afa..b2508c0d276b 100644
> --- a/arch/arm/mach-shmobile/smp-r8a7791.c
> +++ b/arch/arm/mach-shmobile/smp-r8a7791.c
> @@ -58,7 +58,7 @@ struct smp_operations r8a7791_smp_ops __initdata = {
>         .smp_prepare_cpus       = r8a7791_smp_prepare_cpus,
>         .smp_boot_secondary     = r8a7791_smp_boot_secondary,
>  #ifdef CONFIG_HOTPLUG_CPU
> -       .cpu_disable            = shmobile_smp_cpu_disable,
> +       .cpu_can_disable        = shmobile_smp_cpu_can_disable,
>         .cpu_die                = shmobile_smp_apmu_cpu_die,
>         .cpu_kill               = shmobile_smp_apmu_cpu_kill,
>  #endif
> diff --git a/arch/arm/mach-shmobile/smp-sh73a0.c b/arch/arm/mach-shmobile/smp-sh73a0.c
> index 2106d6b76a06..ae7c764fd6b4 100644
> --- a/arch/arm/mach-shmobile/smp-sh73a0.c
> +++ b/arch/arm/mach-shmobile/smp-sh73a0.c
> @@ -68,7 +68,7 @@ struct smp_operations sh73a0_smp_ops __initdata = {
>         .smp_prepare_cpus       = sh73a0_smp_prepare_cpus,
>         .smp_boot_secondary     = sh73a0_boot_secondary,
>  #ifdef CONFIG_HOTPLUG_CPU
> -       .cpu_disable            = shmobile_smp_cpu_disable,
> +       .cpu_can_disable        = shmobile_smp_cpu_can_disable,
>         .cpu_die                = shmobile_smp_scu_cpu_die,
>         .cpu_kill               = shmobile_smp_scu_cpu_kill,
>  #endif
> --
> The Qualcomm Innovation Center, Inc. is a member of the Code Aurora Forum,
> a Linux Foundation Collaborative Project
>

Cheers,

Tyler

[0] https://git.linaro.org/people/tyler.baker/linux.git/shortlog/refs/heads/to-build
[1] http://kernelci.org/build/tbaker/kernel/v4.0-rc7-10845-g43b069424ab4/
[2] http://kernelci.org/boot/all/job/tbaker/kernel/v4.0-rc7-10845-g43b069424ab4/

Russell King - ARM Linux April 13, 2015, 1:42 p.m. UTC | #2

On Fri, Apr 10, 2015 at 03:33:11PM -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 <[e-mail]>
> Cc: Nicolas Pitre <[e-mail]>
> Cc: Dave Martin <[e-mail]>
> Acked-by: Simon Horman <[e-mail]> [shmobile portion]
> Tested-by: Simon Horman <[e-mail]>
> Cc: Magnus Damm <[e-mail]>
> Cc: <[e-mail]>
> Cc: Tyler Baker <[e-mail]>
> Cc: Geert Uytterhoeven <[e-mail]>
> Signed-off-by: Stephen Boyd <[e-mail]>

Let's see some more acks for this...

Stephen Boyd June 9, 2015, 7:08 p.m. UTC | #3

On 04/13/2015 06:42 AM, Russell King - ARM Linux wrote:
> On Fri, Apr 10, 2015 at 03:33:11PM -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 <[e-mail]>
>> Cc: Nicolas Pitre <[e-mail]>
>> Cc: Dave Martin <[e-mail]>
>> Acked-by: Simon Horman <[e-mail]> [shmobile portion]
>> Tested-by: Simon Horman <[e-mail]>
>> Cc: Magnus Damm <[e-mail]>
>> Cc: <[e-mail]>
>> Cc: Tyler Baker <[e-mail]>
>> Cc: Geert Uytterhoeven <[e-mail]>
>> Signed-off-by: Stephen Boyd <[e-mail]>
> Let's see some more acks for this...
>

Nobody else has acked this so far. Shall I put it in the patch tracker
now? Or is there someone more specific we need an ack from?

Russell King - ARM Linux June 9, 2015, 9:02 p.m. UTC | #4

On Tue, Jun 09, 2015 at 12:08:28PM -0700, Stephen Boyd wrote:
> On 04/13/2015 06:42 AM, Russell King - ARM Linux wrote:
> > On Fri, Apr 10, 2015 at 03:33:11PM -0700, Stephen Boyd wrote:
> >> Cc: Mark Rutland <[e-mail]>
> >> Cc: Nicolas Pitre <[e-mail]>
> >> Cc: Dave Martin <[e-mail]>
> >> Acked-by: Simon Horman <[e-mail]> [shmobile portion]
> >> Tested-by: Simon Horman <[e-mail]>
> >> Cc: Magnus Damm <[e-mail]>
> >> Cc: <[e-mail]>
> >> Cc: Tyler Baker <[e-mail]>
> >> Cc: Geert Uytterhoeven <[e-mail]>
> >> Signed-off-by: Stephen Boyd <[e-mail]>
> > Let's see some more acks for this...
> >
> 
> Nobody else has acked this so far. Shall I put it in the patch tracker
> now? Or is there someone more specific we need an ack from?

I guess if people haven't responded by now, they're never going to respond,
and I guess we take their non-response as tacit approval for the change.

Maybe we ought to have tacit-acked-by: which we can add when someones
been Cc'd and hasn't responded. :)

Catalin Marinas June 10, 2015, 9:58 a.m. UTC | #5

On Tue, Jun 09, 2015 at 10:02:00PM +0100, Russell King - ARM Linux wrote:
> On Tue, Jun 09, 2015 at 12:08:28PM -0700, Stephen Boyd wrote:
> > On 04/13/2015 06:42 AM, Russell King - ARM Linux wrote:
> > > On Fri, Apr 10, 2015 at 03:33:11PM -0700, Stephen Boyd wrote:
> > >> Cc: Mark Rutland <[e-mail]>
> > >> Cc: Nicolas Pitre <[e-mail]>
> > >> Cc: Dave Martin <[e-mail]>
> > >> Acked-by: Simon Horman <[e-mail]> [shmobile portion]
> > >> Tested-by: Simon Horman <[e-mail]>
> > >> Cc: Magnus Damm <[e-mail]>
> > >> Cc: <[e-mail]>
> > >> Cc: Tyler Baker <[e-mail]>
> > >> Cc: Geert Uytterhoeven <[e-mail]>
> > >> Signed-off-by: Stephen Boyd <[e-mail]>
> > > Let's see some more acks for this...
> > >
> > 
> > Nobody else has acked this so far. Shall I put it in the patch tracker
> > now? Or is there someone more specific we need an ack from?
> 
> I guess if people haven't responded by now, they're never going to respond,
> and I guess we take their non-response as tacit approval for the change.
> 
> Maybe we ought to have tacit-acked-by: which we can add when someones
> been Cc'd and hasn't responded. :)

I think we also need a "To:" tag for people who don't notice when they
are cc'ed ;).

Patch

diff --git a/arch/arm/common/mcpm_platsmp.c b/arch/arm/common/mcpm_platsmp.c
index 92e54d7c6f46..2b25b6038f66 100644
--- a/arch/arm/common/mcpm_platsmp.c
+++ b/arch/arm/common/mcpm_platsmp.c
@@ -65,14 +65,10 @@  static int mcpm_cpu_kill(unsigned int cpu)
 	return !mcpm_wait_for_cpu_powerdown(pcpu, pcluster);
 }
 
-static int mcpm_cpu_disable(unsigned int cpu)
+static bool mcpm_cpu_can_disable(unsigned int cpu)
 {
-	/*
-	 * We assume all CPUs may be shut down.
-	 * This would be the hook to use for eventual Secure
-	 * OS migration requests as described in the PSCI spec.
-	 */
-	return 0;
+	/* We assume all CPUs may be shut down. */
+	return true;
 }
 
 static void mcpm_cpu_die(unsigned int cpu)
@@ -92,7 +88,7 @@  static struct smp_operations __initdata mcpm_smp_ops = {
 	.smp_secondary_init	= mcpm_secondary_init,
 #ifdef CONFIG_HOTPLUG_CPU
 	.cpu_kill		= mcpm_cpu_kill,
-	.cpu_disable		= mcpm_cpu_disable,
+	.cpu_can_disable	= mcpm_cpu_can_disable,
 	.cpu_die		= mcpm_cpu_die,
 #endif
 };
diff --git a/arch/arm/include/asm/smp.h b/arch/arm/include/asm/smp.h
index 18f5a554134f..2563453d7b37 100644
--- a/arch/arm/include/asm/smp.h
+++ b/arch/arm/include/asm/smp.h
@@ -104,6 +104,7 @@  struct smp_operations {
 #ifdef CONFIG_HOTPLUG_CPU
 	int  (*cpu_kill)(unsigned int cpu);
 	void (*cpu_die)(unsigned int cpu);
+	bool  (*cpu_can_disable)(unsigned int cpu);
 	int  (*cpu_disable)(unsigned int cpu);
 #endif
 #endif
diff --git a/arch/arm/include/asm/smp_plat.h b/arch/arm/include/asm/smp_plat.h
index 993e5224d8f7..f9080717fc88 100644
--- a/arch/arm/include/asm/smp_plat.h
+++ b/arch/arm/include/asm/smp_plat.h
@@ -107,4 +107,13 @@  static inline u32 mpidr_hash_size(void)
 extern int platform_can_secondary_boot(void);
 extern int platform_can_cpu_hotplug(void);
 
+#ifdef CONFIG_HOTPLUG_CPU
+extern int platform_can_hotplug_cpu(unsigned int cpu);
+#else
+static inline int platform_can_hotplug_cpu(unsigned int cpu)
+{
+	return 0;
+}
+#endif
+
 #endif
diff --git a/arch/arm/kernel/setup.c b/arch/arm/kernel/setup.c
index 6c777e908a24..955d45d0f70c 100644
--- a/arch/arm/kernel/setup.c
+++ b/arch/arm/kernel/setup.c
@@ -992,7 +992,7 @@  static int __init topology_init(void)
 
 	for_each_possible_cpu(cpu) {
 		struct cpuinfo_arm *cpuinfo = &per_cpu(cpu_data, cpu);
-		cpuinfo->cpu.hotpluggable = 1;
+		cpuinfo->cpu.hotpluggable = platform_can_hotplug_cpu(cpu);
 		register_cpu(&cpuinfo->cpu, cpu);
 	}
 
diff --git a/arch/arm/kernel/smp.c b/arch/arm/kernel/smp.c
index cca5b8758185..8c834ecc09db 100644
--- a/arch/arm/kernel/smp.c
+++ b/arch/arm/kernel/smp.c
@@ -173,13 +173,26 @@  static int platform_cpu_disable(unsigned int cpu)
 	if (smp_ops.cpu_disable)
 		return smp_ops.cpu_disable(cpu);
 
+	return 0;
+}
+
+int platform_can_hotplug_cpu(unsigned int cpu)
+{
+	/* cpu_die must be specified to support hotplug */
+	if (!smp_ops.cpu_die)
+		return 0;
+
+	if (smp_ops.cpu_can_disable)
+		return smp_ops.cpu_can_disable(cpu);
+
 	/*
 	 * By default, allow disabling all CPUs except the first one,
 	 * since this is special on a lot of platforms, e.g. because
 	 * of clock tick interrupts.
 	 */
-	return cpu == 0 ? -EPERM : 0;
+	return cpu != 0;
 }
+
 /*
  * __cpu_disable runs on the processor to be shutdown.
  */
diff --git a/arch/arm/mach-shmobile/common.h b/arch/arm/mach-shmobile/common.h
index afc60bad6fd6..f2c4bf437ea7 100644
--- a/arch/arm/mach-shmobile/common.h
+++ b/arch/arm/mach-shmobile/common.h
@@ -13,7 +13,7 @@  extern void shmobile_smp_boot(void);
 extern void shmobile_smp_sleep(void);
 extern void shmobile_smp_hook(unsigned int cpu, unsigned long fn,
 			      unsigned long arg);
-extern int shmobile_smp_cpu_disable(unsigned int cpu);
+extern bool shmobile_smp_cpu_can_disable(unsigned int cpu);
 extern void shmobile_invalidate_start(void);
 extern void shmobile_boot_scu(void);
 extern void shmobile_smp_scu_prepare_cpus(unsigned int max_cpus);
diff --git a/arch/arm/mach-shmobile/platsmp.c b/arch/arm/mach-shmobile/platsmp.c
index 3923e09e966d..b23378f3d7e1 100644
--- a/arch/arm/mach-shmobile/platsmp.c
+++ b/arch/arm/mach-shmobile/platsmp.c
@@ -31,8 +31,8 @@  void shmobile_smp_hook(unsigned int cpu, unsigned long fn, unsigned long arg)
 }
 
 #ifdef CONFIG_HOTPLUG_CPU
-int shmobile_smp_cpu_disable(unsigned int cpu)
+bool shmobile_smp_cpu_can_disable(unsigned int cpu)
 {
-	return 0; /* Hotplug of any CPU is supported */
+	return true; /* Hotplug of any CPU is supported */
 }
 #endif
diff --git a/arch/arm/mach-shmobile/smp-r8a7790.c b/arch/arm/mach-shmobile/smp-r8a7790.c
index 930f45cbc08a..947e437cab68 100644
--- a/arch/arm/mach-shmobile/smp-r8a7790.c
+++ b/arch/arm/mach-shmobile/smp-r8a7790.c
@@ -64,7 +64,7 @@  struct smp_operations r8a7790_smp_ops __initdata = {
 	.smp_prepare_cpus	= r8a7790_smp_prepare_cpus,
 	.smp_boot_secondary	= shmobile_smp_apmu_boot_secondary,
 #ifdef CONFIG_HOTPLUG_CPU
-	.cpu_disable		= shmobile_smp_cpu_disable,
+	.cpu_can_disable	= shmobile_smp_cpu_can_disable,
 	.cpu_die		= shmobile_smp_apmu_cpu_die,
 	.cpu_kill		= shmobile_smp_apmu_cpu_kill,
 #endif
diff --git a/arch/arm/mach-shmobile/smp-r8a7791.c b/arch/arm/mach-shmobile/smp-r8a7791.c
index 5e2d1db79afa..b2508c0d276b 100644
--- a/arch/arm/mach-shmobile/smp-r8a7791.c
+++ b/arch/arm/mach-shmobile/smp-r8a7791.c
@@ -58,7 +58,7 @@  struct smp_operations r8a7791_smp_ops __initdata = {
 	.smp_prepare_cpus	= r8a7791_smp_prepare_cpus,
 	.smp_boot_secondary	= r8a7791_smp_boot_secondary,
 #ifdef CONFIG_HOTPLUG_CPU
-	.cpu_disable		= shmobile_smp_cpu_disable,
+	.cpu_can_disable	= shmobile_smp_cpu_can_disable,
 	.cpu_die		= shmobile_smp_apmu_cpu_die,
 	.cpu_kill		= shmobile_smp_apmu_cpu_kill,
 #endif
diff --git a/arch/arm/mach-shmobile/smp-sh73a0.c b/arch/arm/mach-shmobile/smp-sh73a0.c
index 2106d6b76a06..ae7c764fd6b4 100644
--- a/arch/arm/mach-shmobile/smp-sh73a0.c
+++ b/arch/arm/mach-shmobile/smp-sh73a0.c
@@ -68,7 +68,7 @@  struct smp_operations sh73a0_smp_ops __initdata = {
 	.smp_prepare_cpus	= sh73a0_smp_prepare_cpus,
 	.smp_boot_secondary	= sh73a0_boot_secondary,
 #ifdef CONFIG_HOTPLUG_CPU
-	.cpu_disable		= shmobile_smp_cpu_disable,
+	.cpu_can_disable	= shmobile_smp_cpu_can_disable,
 	.cpu_die		= shmobile_smp_scu_cpu_die,
 	.cpu_kill		= shmobile_smp_scu_cpu_kill,
 #endif