Feature request: add option to disable GPU frequency scaling

[edo1]

I need stable core clock frequency because SPI bus is used.
I use my PRi in headless mode, so GPU performance doesn't matter to me. And I prefer to keep GPU power consumption as low as possible.
I want to use max (non-overcocked) CPU frequency. At least with performance governor RPi boots faster. And idle temperature difference between 600MHz and 1400MHz is only 0.5-1℃ on 3B+.

There is my current config, it works on RPi 3B+:

gpu_freq=250
force_turbo=1

I dislike explicit frequency setting, I'm unsure if it is compatible with RPi 4 and future models.

What about new option? Something like gpu_turbo=0 would be fine.

[pelwell]

On a Pi with onboard Bluetooth, enable_uart=1 is another way to stop the core frequency from changing.

[edo1]

I tried it, it doesn't work to me

# for src in arm core h264 isp v3d uart pwm emmc pixel vec hdmi dpi ; do   echo -e "$src:\t$(vcgencmd measure_clock $src)" ; done
arm:    frequency(45)=600000000
core:   frequency(1)=250000000
h264:   frequency(28)=0
isp:    frequency(42)=250000000
v3d:    frequency(43)=250000000
uart:   frequency(22)=47999000
pwm:    frequency(25)=0
emmc:   frequency(47)=200000000
pixel:  frequency(29)=148500000
vec:    frequency(10)=0
hdmi:   frequency(9)=163682000
dpi:    frequency(4)=0
# echo performance > /sys/devices/system/cpu/cpufreq/policy0/scaling_governor 
# for src in arm core h264 isp v3d uart pwm emmc pixel vec hdmi dpi ; do   echo -e "$src:\t$(vcgencmd measure_clock $src)" ; done
arm:    frequency(45)=1400000000
core:   frequency(1)=400000000
h264:   frequency(28)=300000000
isp:    frequency(42)=300000000
v3d:    frequency(43)=300000000
uart:   frequency(22)=48000000
pwm:    frequency(25)=0
emmc:   frequency(47)=200000000
pixel:  frequency(29)=148500000
vec:    frequency(10)=0
hdmi:   frequency(9)=163683000
dpi:    frequency(4)=0

And I'm unsure if enable_uart=1 is compatible with dtoverlay=disable-bt (I need ttyAMA0 because ttyS0 lacks MARK/SPACE parity support).

[pelwell]

You are correct - the two are incompatible. disable-bt effectively turns a BT Pi into a non-BT Pi, at which point the clock-fixing is disabled.

Although I'm not yet convinced about the choice of name, I think this would be a useful addition - particularly now that Pi 4 uses different clock speeds.

[edo1]

English is not my native language, I completely rely on your choice of name.

[pelwell]

@popcornmix To be maximally useful we need two options - one to fix the core clock at the highest supported frequency, and one to fix it at the lowest. How about fixed_core=1 for the lower bound and fixed_core=2 for the higher, the default being 0?

[pelwell]

In relation to raspberrypi/linux#3381, there should also be a way of setting the value from Device Tree.

[amahoneyLIT]

just want to confirm - is there currently a way to disable GPU scaling on the RPi 4? I am also looking for optimal settings for SPI usage, also disabled bluetooth and using uarts

referring to https://www.raspberrypi.org/documentation/configuration/config-txt/overclocking.md

gpu_freq # sets core_freq, h264_freq, isp_freq, and v3d_freq together. 
         # defaults to 500, 600 is the only other accepted value. 

gpu_freq_min # defaults to 500

core_freq      # For Pi 4B the default value is 500, 600 is the only other accepted value
core_freq_min  # The default value is 250. On Pi 4B the default is 275 when hdmi_enable_4kp60 is set

So correct me if I'm wrong, we'll currently have to do core_freq=500 and core_freq_min=500, and this issue is requesting being able to do core_freq=250 and core_freq_min=250, which is currently not allowed according to the values of 500 or 600 being required for core_freq even though the documentation suggests core_freq_min defaults to 250 and is allowed

Appreciate any input, thanks!

[pelwell]

For an unchanging core clock, the absolute max/min values chosen for the core frequency don't matter - only that they are the same. Choose the absolute values appropriately for the platform and the workload - 500 on the Pi 4 is likely to be fine:

core_freq=500
core_freq_min=500

[amahoneyLIT]

I see. I tried this

force_turbo=1
core_freq=250
core_freq_min=250

and confirmed that vcgencmd measure_clock core showed 250MHz under load/idle conditions

I'm confused why the documentation states for core_freq that

For Pi 4B the default value is 500, 600 is the only other accepted value

https://www.raspberrypi.org/documentation/configuration/config-txt/overclocking.md

[amahoneyLIT]

@edo1 think that

force_turbo=1
gpu_freq=250
gpu_freq_min=250

does the trick to keep arm clock at 1500MHz and everything else at lowest 250MHz on the RPi 4

[JamesH65]

I see. I tried this

force_turbo=1
core_freq=250
core_freq_min=250

and confirmed that vcgencmd measure_clock core showed 250MHz under load/idle conditions

I'm confused why the documentation states for core_freq that

For Pi 4B the default value is 500, 600 is the only other accepted value

https://www.raspberrypi.org/documentation/configuration/config-txt/overclocking.md

That page is currently under review.