ARM64 / cpuidle: Use new cpuidle macro for entering retention state

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
Acked-by: Sudeep Holla <sudeep.holla@arm.com>
Signed-off-by: Prashanth Prakash <pprakash@codeaurora.org>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>

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))
+
 int acpi_processor_ffh_lpi_probe(unsigned int cpu)
 {
 	return arm_cpuidle_init(cpu);
@@ -54,6 +56,10 @@ int acpi_processor_ffh_lpi_probe(unsigned int cpu)
 
 int acpi_processor_ffh_lpi_enter(struct acpi_lpi_state *lpi)
 {
+	if (ARM64_LPI_IS_RETENTION_STATE(lpi->arch_flags))
+		return CPU_PM_CPU_IDLE_ENTER_RETENTION(arm_cpuidle_suspend,
+						lpi->index);
+	else
 		return CPU_PM_CPU_IDLE_ENTER(arm_cpuidle_suspend, lpi->index);
 }
 #endif