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

[Sudeep Holla]

On Mon, Jul 13, 2026 at 12:59:31PM +0100, Will Deacon wrote:
> 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@xxxxxxxxxx/
> > > Suggested-by: Will Deacon <will@xxxxxxxxxx>
> > > Signed-off-by: Yureka Lilian <yureka@xxxxxxxxxxxxxx>
> > > ---
> > > Changes in v2:
> > > - Applied suggestions by Anshuman Khandual (Thanks!)
> > > - Link to v1: https://patch.msgid.link/20260705-arm64-idle-param-v1-1-7454249f473f@xxxxxxxxxxxxxx
> > > ---
> > >  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.
> 

Thanks, that makes sense.

-- 
Regards,
Sudeep