Re: [PATCH v2] arch: arm64: add early_param idle=<wfi|yield|nop>
From: Yureka Lilian
Date: Fri Jul 31 2026 - 15:33:17 EST
On 7/31/26 18:16, Will Deacon wrote:
Hi Yureka,Ultimately I'm happy we found a solution which covers the Apple Silicon use case while also not making too much of a mess elsewhere, and I appreciate your feedback a lot!
On Mon, Jul 27, 2026 at 10:49:57AM +0200, Yureka Lilian wrote:
On 7/22/26 23:43, Will Deacon wrote:I think nohlt will break the PSCI cpuidle proposal here:
On Sat, Jul 11, 2026 at 09:35:25AM +0200, Yureka Lilian wrote:In practise, I could only find the following uses of WFI / WFIT / wfi() /
diff --git a/arch/arm64/lib/delay.c b/arch/arm64/lib/delay.cRather than scatter the idle implementation check across all users of
index e278e060e78a..2452990ed37a 100644
--- a/arch/arm64/lib/delay.c
+++ b/arch/arm64/lib/delay.c
@@ -15,6 +15,8 @@
#include <clocksource/arm_arch_timer.h>
+#include "../kernel/idle.h"
+
#define USECS_TO_CYCLES(time_usecs) \
xloops_to_cycles((time_usecs) * 0x10C7UL)
@@ -49,7 +51,8 @@ void __delay(unsigned long cycles)
* Start with WFIT. If an interrupt makes us resume
* early, use a WFET loop to complete the delay.
*/
- wfit(end);
+ if (likely(idle == ARM64_IDLE_WFI))
+ wfit(end);
WFI*, why not move this into the macro itself? That way, the callers can
all stay like they are but the macro behaves as specified.
wfit() in arm64 code: 1) the default idle loop and delay() function;
arguably the only "real" users 2) parking cores after different kinds of
unexpected situations / crashes.
Just to confirm, you are suggesting to add the conditionals to
arch/arm64/include/asm/barrier.h, and have wfi() and wfit() macros not
actually do WFI / WFIT depending on the value of the idle param?
I'm torn about this: We should maybe first discuss what effect idle= should
have: Should it prevent WFI running anywhere in the kernel when idle=nop, OR
is its intended use case to change the default arm64 implementations for
idle and delay, while still allowing other parts of the kernel to use WFI
(for example, for custom cpuidle implementation)? I think the existing nohlt
parameter, which disables all idle states, is more fitting for the first
goal, even though it currently does not prevent the wfit in the delay
function.
https://lore.kernel.org/all/20260708-efi-psci-v1-0-9efb3abf0e4c@xxxxxxxxxx/
For preventing WFI anywhere, I think alternatives patching based on theI keep changing my mind about this patch :/
earlyparam would be the most reliable way to achieve that, and second most
reliable way is putting the conditional in the wfi() / wfit() macros.
But my assumption is that what we want to achieve is actually closer to
changing only the default idle implementation, while allowing an idle state
registered at later point to still do its thing (including using WFI for
this purpose). And for this (and to be more flexible with other idle modes
e.g. yield), what I proposed in this patch makes more sense, and I'm
relatively confident it will fulfill the Apple Silicon use case without
adding many more scattered checks other than these two.
Does this make sense?
What do you think about a funny sort of hybrid approach where:
* We have idle=, but it really only affects the idle loop
* We have a cpu_errata entry to detect (based on the MIDR) this CPU
and then avoid patching in the WFIT instructions (i.e. don't detect
the ARM64_HAS_WFXT capability).
I think that would solve the case for you, as well as giving others finer
grained control over the idle implementation and adding the infrastructure
we need to handle a CPU with broken wfit.
The idle param portion, if implemented like you say only affecting the idle loop, should be rather uncontroversial.
Getting the other part of it (the WFIT in the delay function) right is much more difficult, but it also has less of an performance / efficiency impact if it's a bit on the conservative side and we can find a good solution independently of the idle param.
Will
Thanks,
- Yureka