[PATCH] arm64: suspend: Remove forcing error from suspend finisher

[Maulik Shah]

Successful cpu_suspend() may not always want to return to cpu_resume() to
save the work and latency involved.

consider a scenario,

when single physical CPU (pCPU) is used on different virtual machines (VMs)
as virtual CPUs (vCPUs). VM-x's vCPU can request a powerdown state after
saving the context by invoking __cpu_suspend_enter() whereas VM-y's vCPU is
requesting a shallower than powerdown state. The hypervisor aggregates to a
non powerdown state for pCPU. A wakeup event for VM-x's vCPU may want to
resume the execution at the same place instead of jumping to cpu_resume()
as the HW never reached till powerdown state which would have lost the
context.

While the vCPU of VM-x had latency impact of saving the context in suspend
entry path but having the return to same place saves the latency to restore
the context in resume path.

consider another scenario,

Newer CPUs include a feature called “powerdown abandon”. The feature is
based on the observation that events like GIC wakeups have a high
likelihood of happening while the CPU is in the middle of its powerdown
sequence (at wfi). Older CPUs will powerdown and immediately power back
up when this happens. The newer CPUs will “give up” mid way through if
no context has been lost yet. This is possible as the powerdown operation
is lengthy and a large part of it does not lose context [1].

As the wakeup arrived after SW powerdown is done but before HW is fully
powered down. From SW view this is still a successful entry to suspend
and since the HW did not loose the context there is no reason to return at
entry address cpu_resume() to restore the context.

Remove forcing the failure at kernel if the execution does not resume at
cpu_resume() as kernel has no reason to treat such returns as failures
when the firmware has already filled in return as success.

[1] https://trustedfirmware-a.readthedocs.io/en/v2.14.0/design/firmware-design.html#cpu-specific-operations-framework

Signed-off-by: Maulik Shah <maulik.shah@xxxxxxxxxxxxxxxx>
---
 arch/arm64/kernel/suspend.c | 15 +++++++--------
 1 file changed, 7 insertions(+), 8 deletions(-)

diff --git a/arch/arm64/kernel/suspend.c b/arch/arm64/kernel/suspend.c
index eaaff94329cddb8d1fb8d1523395453f3501c9a5..b54e578f0f8b03c1dba38157c6012bb064adaa12 100644
--- a/arch/arm64/kernel/suspend.c
+++ b/arch/arm64/kernel/suspend.c
@@ -144,15 +144,14 @@ int cpu_suspend(unsigned long arg, int (*fn)(unsigned long))
 		ret = fn(arg);
 
 		/*
-		 * Never gets here, unless the suspend finisher fails.
-		 * Successful cpu_suspend() should return from cpu_resume(),
-		 * returning through this code path is considered an error
-		 * If the return value is set to 0 force ret = -EOPNOTSUPP
-		 * to make sure a proper error condition is propagated
+		 * Successful HW power down should return at cpu_resume()
+		 * however successful SW power down may still want to
+		 * return here to save the work and latency involved in
+		 * restoring the context when the HW never lost it.
+		 *
+		 * If the return value is set to 0 do not force failure
+		 * from here.
 		 */
-		if (!ret)
-			ret = -EOPNOTSUPP;
-
 		ct_cpuidle_exit();
 	} else {
 		ct_cpuidle_exit();

---
base-commit: 343f51842f4ed7143872f3aa116a214a5619a4b9
change-id: 20260316-suspend_ret-d5e202d34cc9

Best regards,
-- 
Maulik Shah <maulik.shah@xxxxxxxxxxxxxxxx>