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

[Will Deacon <will@kernel.org>]

On Mon, Jul 13, 2026 at 10:57:38AM +0100, 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.

It's probably useful to have so that you can override an idle= option
present earlier in the cmdline. e.g.

	idle=nop idle=wfi

will give you the wfi behaviour, which is handy if the cmdline is
stitched topgether from different sources.

Will