Re: [PATCH v2 2/2] ARM64 / cpuidle: Use new cpuidle macro for entering retention state

[Sudeep Holla <sudeep.holla@arm.com>]

On 14/11/17 16:15, Prakash, Prashanth wrote:
> 
> 
> On 11/13/2017 5:33 AM, Sudeep Holla wrote:
>>
>> On 09/11/17 00:38, Prashanth Prakash wrote:
>>> CPU_PM_CPU_IDLE_ENTER_RETENTION skips calling cpu_pm_enter() and
>>> cpu_pm_exit(). By not calling cpu_pm functions in idle entry/exit
>>> paths we can reduce the latency involved in entering and exiting
>>> the low power idle state.
>>>
>>> On ARM64 based Qualcomm server platform we measured below overhead
>>> for calling cpu_pm_enter and cpu_pm_exit for retention states.
>>>
>>> workload: stress --hdd #CPUs --hdd-bytes 32M  -t 30
>>> 	Average overhead of cpu_pm_enter - 1.2us
>>> 	Average overhead of cpu_pm_exit  - 3.1us
>>>
>>> Signed-off-by: Prashanth Prakash <pprakash@codeaurora.org>
>>> ---
>>>  arch/arm64/kernel/cpuidle.c | 8 +++++++-
>>>  1 file changed, 7 insertions(+), 1 deletion(-)
>>>
>>> diff --git a/arch/arm64/kernel/cpuidle.c b/arch/arm64/kernel/cpuidle.c
>>> index fd69108..f2d1381 100644
>>> --- a/arch/arm64/kernel/cpuidle.c
>>> +++ b/arch/arm64/kernel/cpuidle.c
>>> @@ -47,6 +47,8 @@ int arm_cpuidle_suspend(int index)
>>>  
>>>  #include <acpi/processor.h>
>>>  
>>> +#define ARM64_LPI_IS_RETENTION_STATE(arch_flags) (!(arch_flags))
>>> +
>> This is fine, but just to be safer, is it better to check for all
>> flags to be set as we can't/don't support any partial retention modes.
> I am not sure I completely understand.
> 
> If any bit is set in arch_flags, then we lose the context corresponding to that bit, so
> a full retention state will have no bit set, so that's what we are checking for.
> Or am i missing something?
> 

Ah you are right, I somehow misinterpreted the flags exactly to be
opposite(i.e. state retained) rather than state lost, sorry for the noise.

-- 
Regards,
Sudeep