[PATCH v2 0/2] Avoid calling cpu_pm functions for retention idle states

[Prashanth Prakash]

CPU_PM_CPU_IDLE_ENTER() treats all idle states whose idx != 0 as a
state that loses some context, but we can have deeper idle states that
doesn't lose any software context. If a CPU is entering such a low power
idle state where it retains the context, then there is no need to call
cpu_pm_enter()/cpu_pm_exit().

Add a new macro(CPU_PM_CPU_IDLE_ENTER_RETENTION) to be used by cpuidle
drivers when they are entering retention state. By not calling cpu_pm_enter
and cpu_pm_exit we reduce the latency involved in entering and exiting
the retention states.

On ARM64 based Qualcomm server platform we measured below overhead for
for calling cpu_pm_enter and cpu_pm_exit for retention states.

workload: stress --hdd #CPUs --hdd-bytes 32M  -t 30
Overhead of cpu_pm_enter - 1.2us(Average), 6.5us(Max)
Overhead of cpu_pm_exit  - 3.1us(Average), 11.1us(Max)

Listed below are 5 functions that were notified on ENTER/EXIT on the
test platform:
      gic_cpu_pm_notifier
      arch_timer_cpu_pm_notify
      cpu_pm_pmu_notify
      hyp_init_cpu_pm_notifier
      fpsimd_cpu_pm_notifier

Prashanth Prakash (2):
  cpuidle: Add new macro to enter a retention idle state
  ARM64 / cpuidle: Use new cpuidle macro for entering retention state

Changes in v2:
 - Reordered cpuidle.h macros for better readablity (Rafael)

 arch/arm64/kernel/cpuidle.c |  8 +++++++-
 include/linux/cpuidle.h     | 40 ++++++++++++++++++++++++----------------
 2 files changed, 31 insertions(+), 17 deletions(-)

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