Setting LED trigger to 'cpu' doesn't work with 64-bit kernel

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Assessment

Difficulty
4/5
Estimated time
3-5 days
Newbie friendliness
35/100
Issue type
Bug
Clarity
Mostly clear
Activity status
Stale
Tech stack
c, linux

Research direction

Start by reproducing the issue with echo cpu | sudo tee /sys/class/leds/led0/trigger on the reported Raspberry Pi 4 or CM4 setup using a 64-bit kernel, then compare it with the 32-bit behavior. Inspect the kernel's LED trigger handling associated with /sys/class/leds/ledX/trigger; done means the cpu and cpuX triggers make the LED reflect CPU activity on 64-bit kernels without errors.

Written by the indexing model from the issue text.

Description

Describe the bug
Setting cpu or cpuX as the LED trigger in /sys/class/leds/ledX/trigger doesn't work with 64-bit kernel (using arm_64bit=1 in config.txt), but does work with 32-bit kernel. cat /sys/class/leds/ledX/trigger shows cpu and cpuX as available options.

To reproduce
echo cpu | sudo tee /sys/class/leds/led0/trigger

Expected behaviour
LED should flicker with CPU usage

Actual behaviour
LED stays off and no errors

System
Pi Model:
Raspberry Pi 4 and CM4

cat /etc/rpi-issue
Raspberry Pi reference 2021-05-07
Generated using pi-gen, https://github.com/RPi-Distro/pi-gen, dcfd74d7d1fa293065ac6d565711e9ff891fe2b8, stage2

vcgencmd version
May 27 2021 14:02:31
Copyright (c) 2012 Broadcom
version 7d9a298cda813f747b51fe17e1e417e7bf5ca94d (clean) (release) (start_cd)

uname -a
Linux router.localnet 5.10.17-v8+ #1421 SMP PREEMPT Thu May 27 14:01:37 BST 2021 aarch64 GNU/Linux

Dominant language
C
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Merged PRs (30d)
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