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To enumerate platform Low Power Idle states, Intel platforms are using
Low Power Idle Table (LPIT). More details about this table can be
downloaded from:
http://www.uefi.org/sites/default/files/resources/Intel_ACPI_Low_Power_S0_Idle.pdf
Residencies for each low power state can be read via FFH
(Function fixed hardware) or a memory mapped interface.
On platforms supporting S0ix sleep states, there can be two types of
residencies:
- CPU PKG C10 (Read via FFH interface)
- Platform Controller Hub (PCH) SLP_S0 (Read via memory mapped interface)
The following attributes are added dynamically to the cpuidle
sysfs attribute group:
/sys/devices/system/cpu/cpuidle/low_power_idle_cpu_residency_us
/sys/devices/system/cpu/cpuidle/low_power_idle_system_residency_us
The "low_power_idle_cpu_residency_us" attribute shows time spent
by the CPU package in PKG C10
The "low_power_idle_system_residency_us" attribute shows SLP_S0
residency, or system time spent with the SLP_S0# signal asserted.
This is the lowest possible system power state, achieved only when CPU is in
PKG C10 and all functional blocks in PCH are in a low power state.