[Question] About handling PMU context loss in the deepest idle state where the core is powered down

Xiongfeng Wang wangxiongfeng2 at huawei.com
Thu Jan 9 19:28:26 PST 2020

On 2020/1/10 0:46, Lorenzo Pieralisi wrote:
> On Thu, Jan 09, 2020 at 03:03:19PM +0000, Will Deacon wrote:
>> [+Lorenzo]
>> On Thu, Jan 09, 2020 at 10:43:40AM +0800, Xiongfeng Wang wrote:
>>> Sorry to bother you. It's just that we have come across some problems
>>> about PMU recently.
>> No bother, and thanks for including the mailing list.
>>> We are working on deep power state on CPU cores. In the deepest idle
>>> state, the core will be powered down. In our implementation, the PMU
>>> and the core are in the same power domain, so the PMU will also be
>>> powered down. But I didn't find where we saved the PMU context in
>>> kernel before entering the deepest idle state.
>>> Before we enter the system sleep state, we update the kernel PMU
>>> counter and stop the PMU in 'cpu_pm_pmu_notify()'. But we didn't do
>>> that before we enter idle state.
> ACPI or DT firmware ? I suspect that's ACPI, with LPI idle state
> flags set to 0x0 (3.1.3 - save and restore flags):
> http://infocenter.arm.com/help/topic/com.arm.doc.den0048a/DEN0048A_ARM_FFH_Specification.pdf
> If that's the case a firmware update is needed (ie currently the kernel
> expects the PMU state to be retained).
> arch/arm64/kernel/cpuidle.c

That totally solved my problem.
I set the LPI idle state flag in firmware, and the 'cpu_pm_pmu_notify()' can be called
before I enter the context-lost idle state. Thanks a lot !

> In DT in the PSCI CPUidle driver we run the notifiers irrespective
> of the idle state depth so I don't think this behaviour can happen
> in a DT bootstrapped system.
> I am just guessing - please let me know if my assumption is correct.

Yes, it's correct. We are using ACPI.

>>> I only find some system registers saving in 'psci_cpu_suspend_enter()->cpu_susend()->cpu_do_suspend()'
>> I'm not sure what you mean by "system sleep state"
> I think they mean suspend-to-RAM - in suspend-to-RAM the notifiers
> are run through syscore operations which are decoupled from CPUidle.

Yes, I mean suspend-to-RAM.


> Regardless, CPUidle should call the notifiers if instructed by firmware
> correctly.
> Thanks,
> Lorenzo
> .

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