Re: [PATCH v2] arch: arm64: add early_param idle=<wfi|yield|nop>

[Anshuman Khandual <anshuman.khandual@arm.com>]

On 13/07/26 3:27 PM, Sudeep Holla wrote:
> On Sat, Jul 11, 2026 at 09:35:25AM +0200, Yureka Lilian wrote:
>> Overriding the idle mechanism might be useful for debugging and performance
>> testing. Add a cmdline parameter for it, similar to the existing idle=
>> parameter already present for the x86 and ppc architectures.
>>
>> It is also useful on platforms where the WFI instruction misbehaves,
>> such as Apple Silicon SoCs. Generally, a misbehaving instruction should
>> be treated as an erratum and patched using the alternatives framework.
>> However, in the Apple Silicon case we need more flexibility because it is
>> difficult to detect whether the erratum applies. For example, Linux VMs
>> inside macOS have the same MIDR and may even seem like they're running
>> in EL2 in the case of NV, but should continue using WFI (it's trapped and
>> handled correctly by the hypervisor there). Thus, we prefer to
>> let the m1n1 bootloader add the idle=nop parameter[1].
>>
>> Link[1]: https://lore.kernel.org/all/99b69262-e54b-424e-baa2-96ef7013b87a@kernel.org/
>> Suggested-by: Will Deacon <will@kernel.org>
>> Signed-off-by: Yureka Lilian <yureka@cyberchaos.dev>
>> ---
>> Changes in v2:
>> - Applied suggestions by Anshuman Khandual (Thanks!)
>> - Link to v1: https://patch.msgid.link/20260705-arm64-idle-param-v1-1-7454249f473f@cyberchaos.dev
>> ---
>>  Documentation/admin-guide/kernel-parameters.txt | 23 +++++++++++++++++++
>>  arch/arm64/kernel/idle.c                        | 30 +++++++++++++++++++++++--
>>  arch/arm64/kernel/idle.h                        | 13 +++++++++++
>>  arch/arm64/lib/delay.c                          |  5 ++++-
>>  4 files changed, 68 insertions(+), 3 deletions(-)
>>
>> diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
>> index b2d7d3540ded..d7f5471edf8f 100644
>> --- a/Documentation/admin-guide/kernel-parameters.txt
>> +++ b/Documentation/admin-guide/kernel-parameters.txt
>> @@ -2239,6 +2239,29 @@ Kernel parameters
>>  
>>  			idle=nomwait: Disable mwait for CPU C-states
>>  
>> +			[ARM64,EARLY]
>> +			Format: idle=wfi, idle=yield, idle=nop
>> +
>> +			idle=wfi: Use the WFI (Wait For Interrupt) hint
>> +			instruction in the idle loop. This is the default and
>> +			allows the CPU to enter a low-power state until an
>> +			interrupt arrives.
> 
> Just curious as when and why one would need to use idle=wfi if that is the
> default behaviour. I am missing the need to have it.

I guess once there is a list to chose options from
in the command line, should not the default option 
be listed there as well ?