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

From: Xiongfeng Wang <wangxiongfeng2@huawei.com>
To: Will Deacon <will@kernel.org>
Cc: mark.rutland@arm.com, wangzhe59@huawei.com,
	lorenzo.pieralisi@arm.com,
	Catalin Marinas <catalin.marinas@arm.com>,
	huawei.libin@huawei.com, guohanjun@huawei.com,
	linux-arm-kernel@lists.infradead.org
Subject: Re: [Question] About handling PMU context loss in the deepest idle state where the core is powered down
Date: Fri, 10 Jan 2020 11:45:57 +0800	[thread overview]
Message-ID: <31ec228b-a51b-637d-1c1c-543c81653a96@huawei.com> (raw)
In-Reply-To: <20200109150318.GF12236@willie-the-truck>

On 2020/1/9 23:03, 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.
>> 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", but if you're putting
> the CPU into a state where register contents is lost, then I think you need
> those PM notifiers to run. My understanding was that CPUidle took care of
> this. Why is that not sufficient for you?

Sorry, I didn't notice the PM notifiers are called in CPUidle when 'arch_flags' is set.
I thought it's only called in 'cpu_pm_syscore_ops', I mean Susepnd-to-RAM.

Thanks,
Xiongfeng

> 
>> Do you have some suggestion about how we can handle this problem ?
>> Should we save and restore the PMU context before and after the deepest idle state . I don't know
>> if we can take care of the PMU  counters and interrupts properly by saving and restoring
>> the PMU registers.
>> Or we should update the kernel counter and stop the PMU, like what we did in system sleep situation.
>> Or we should let the firmware to handle the saving and restoring problem.
> 
> If the idle state is somehow autonomous (i.e. the kernel is not aware of
> it), then I suppose firmware has to save/restore the register state that
> is not otherwise preserved. However, I'd like to understand if this is
> really what's happening and why CPUidle isn't just doing the right thing.
> 
> Thanks,
> 
> Will
> 
> .
> 

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