How to activate PCIe ASPM

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1. Article purpose[edit source]

The purpose of this article is to explain how to activate the PCIe ASPM.

ASPM (Active-State Power Management) is a mechanism to progressively reduce power consumption thanks to the L0s/L1 substates.

L0s (standby) only turnoff the controller transmit lane, but the link remains active
L1 the link becomes inactive, and a link recovery is needed to reactivate the link

Linux allow 3 configuration mode for ASPM

default: Use default settings specified by the firmware on the system.
powersave: sets ASPM to save power, regardless of the cost due to link recovery latency
performance: sets ASPM to avoid PCIE link entering ASPM during idle times

Warning

Default is CONFIG_PCIEDEFAULT, equivalent to performance on STM32MP25

2. How to activate the power saving mode[edit source]

To change the PCIe ASPM to power save mode, first check that the PCIe endpoint Capability supports ASPM:

root@stm32mp2:~# lspci -vv -s 03:00.0 | grep ASPM
LnkCap: Port #0, Speed 5GT/s, Width x1, ASPM L0s L1, Exit Latency L0s <4us, L1 unlimited
ClockPM+ Surprise- LLActRep- BwNot- ASPMOptComp-
LnkCtl: ASPM Disabled; RCB 64 bytes, Disabled- CommClk+

It's possible to dynamically change the policy either from the /sys/fs

root@stm32mp2:~# echo powersave > /sys/module/pcie_aspm/parameters/policy

or using the boot flag:

pcie_aspm.policy=powersave

root@stm32mp2:~# lspci -vv -s 03:00.0 | grep ASPM

               LnkCap: Port #0, Speed 5GT/s, Width x1, ASPM L0s L1, Exit Latency L0s <4us, L1 unlimited
                       ClockPM+ Surprise- LLActRep- BwNot- ASPMOptComp-
               LnkCtl: ASPM L0s L1 Enabled; RCB 64 bytes, Disabled- CommClk+

Default config can be changed with

CONFIG_PCIEASPM_POWERSAVE=y