[PATCH] ARM: Use WFI when possible when halting a core

[Chen-Yu Tsai]

On Thu, Jul 27, 2017 at 8:17 PM, Russell King - ARM Linux
<linux at armlinux.org.uk> wrote:
> On Thu, Jul 27, 2017 at 08:08:03PM +0800, Chen-Yu Tsai wrote:
>> On ARM, the kernel busy loops after cleaning up when a core is to be
>> halted. This may have the undesired effect of increasing the core
>> temperature, at a time when no power or thermal management is
>> available, such as a kernel panic or shutdown to halt.
>>
>> x86 uses the more efficient HLT (halt) instruction. The equivalent
>> instruction on ARM is WFI (wait for interrupt).
>>
>> This patch makes the busy loops in the ARM halt/reboot/panic paths
>> use WFI when available (as defined in arch/arm/include/asm/barrier.h).
>>
>> A touch test indicates that this lowers the surface temperature of the
>> Allwinner H3 SoC, with all 4 cores brought up with SMP, from painfully
>> hot to just warm to the touch after shutdown to halt.
>>
>> Signed-off-by: Chen-Yu Tsai <wens at csie.org>
>> ---
>>
>> I asked about this some time ago, and the answer was no one had done
>> it. So here it is. It is somewhat similar to the patch "arm64:
>> Improve parking of stopped CPUs", for arm64 obviously.
>
> There are various errata around the "wfi" instruction on various CPUs
> which means that this simple implementation isn't going to work
> reliably.
>
> This is where cpu_do_idle() comes in - please use that instead.

Thanks for the tip. Looks like this one is implemented for every
platform. No need for #ifdefs anymore.

ChenYu