raspberrypi
pi0 arm core voltage isnt being set right with freq scaling
Describe the bug the core voltage the firmware picks for a given frequency depends on what range of frequencies you can select
with the default config, manually cycling thru the 4 available frequencies gives the following:
arm_clock pllb divisor core_v
700mhz 2.8ghz /4 1.2
800mhz 3.2ghz /4 1.35
900mhz 1.8ghz /2 1.35
1ghz 2ghz /2 1.35
as you might expect, the lowest clock has a lower voltage, however!
arm_freq_min=100
arm_freq=100
this changes which frequencies are available, and things now start to get weird:
arm_clock pllb divisor core_v
100mhz 2.4ghz /24 1.2
200mhz 2.4ghz /12 1.35
300mhz 2.4ghz /8 1.35
400mhz 3.2ghz /8 1.35
500mhz 2ghz /4 1.35
600mhz 2.4ghz /4 1.35
700mhz 2.8ghz /4 1.35
previously, 700mhz ran at 1.2v, but now the whole 200mhz-700mhz range runs at 1.35v instead, and the new slowest speed is the only one at 1.2v it looks like the voltage selection code isnt aware of the actual frequency, and is just blindly giving the slowest speed 1.2v, and all others 1.35v
To reproduce
root@pi400:~# echo userspace > /sys/devices/system/cpu/cpufreq/policy0/scaling_governor
root@pi400:~# echo 700000 > /sys/devices/system/cpu/cpufreq/policy0/scaling_setspeed
root@pi400:~# vcgencmd measure_clock arm ; vcgencmd measure_clock pllb ; vcgencmd measure_volts core
Expected behaviour 700mhz should have the same voltage, reguardless of what other clocks i have available
Actual behaviour Add a clear and concise description of what actually happened.
System Copy and paste the results of the raspinfo command in to this section. Alternatively, copy and paste a pastebin link, or add answers to the following questions:
- Which model of Raspberry PiZeroW:
Revision : 9000c1 - Which OS and version (
cat /etc/rpi-issue)?Generated using pi-gen, https://github.com/RPi-Distro/pi-gen, cce27bd6f44a3b2e83855645986b3e21f771e852, stage4 - Which firmware version (
vcgencmd version)?Nov 30 2020 22:13:46 version ab1181cc0cb6df52bfae3b1d3fef0ce7c325166c (clean) (release) (start) - Which kernel version (
uname -a)?Linux pi400 5.4.79+ #1373 Mon Nov 23 13:18:15 GMT 2020 armv6l GNU/Linux
This behaviour seems to be somehow expected, though not exactly documented linke that. over_voltage_min (1.2A by default) is only applied when the ARM runs at its lowest frequency, regardless which one this is based on the arm_freq_min/arm_freq and over_voltage (1.35A by default) is applied in any other case.
I have a long waiting pull request open to have this behaviour documented, including the aim to bring the question to the developers attention whether this is expected and reasonable that way of if it might deserve a change: https://github.com/raspberrypi/documentation/pull/1698
A lowered arm_freq_min alone, without a lowered arm_freq, has no effect as well in your case, right?
That is a dedicated issue which I'd like to see addressed to allow saving power and heat dissipation: https://github.com/raspberrypi/firmware/issues/1431#issuecomment-732214981
@MichaIng the original goal of my underclocking, was to run the arm at ~25mhz, which i did sucessfully do, but this bug is something i found along the way, and i can see it harming power usage
given that 700mhz runs at 1.2v with all defaults then the entire 100mhz-700mhz range should also run at 1.2v just fine, and that would then reduce the waste heat
Yes as said, it behaves as expected, which does not mean that it is somehow smart. You need to lower arm_freq_min together with over_voltage_min and arm_freq together with over_voltage. Voltage is not automatically lowered when you lower the frequencies, but it detects and sticks with the hardcoded defaults only based on the fact whether the ARM runs at it's lowest scheduling frequency currently or not.
https://forums.raspberrypi.com/viewtopic.php?p=2172164#p2172164
and i have observed the same bug on the pi5 as well from this measurement, it looks like there is just a min/max voltage/clock it then generates 100mhz steps in the clock, and just cuts the voltage range up into even chunks and slaps it on the clocks
so when the min clock changes, but min-voltage doesnt, it winds up using different voltages for everything