RPI4 clock speeds and serial port

[tech-lists]

hi,

On Thu, Mar 18, 2021 at 10:00:53AM -0700, bob prohaska wrote:

>/boot/msdos/config.txt contains
>
>root at nemesis:~ # more /boot/msdos/config.txt
>[all]
>arm_64bit=1
>dtparam=audio=on,i2c_arm=on,spi=on
>dtoverlay=mmc
>dtoverlay=disable-bt
>device_tree_address=0x4000
>kernel=u-boot.bin
>
>[pi4]
>#hdmi_safe=1
>armstub=armstub8-gic.bin
>
>Can one change the cpu speed without disturbing the serial console
>by using something like
>
>arm_freq=1750
>
>in config.txt, provided adequate cooling provisions are made?

I have this config.txt on both a rpi4 stable/13 and main/14. I have a
flirc case on stable/13 and one of those cases that looks like armour
made from solid metal on main/rpi4.
In both cases, the approved 5.1V @3.1W power supply is used.
The highest temp I've seen under load is 72 degC and that's at 25 degC 
ambient. On both machines my config.txt looks like this:

arm_control=0x200
dtparam=audio=on,i2c_arm=on,spi=on
dtoverlay=mmc
dtoverlay=pwm
dtoverlay=disable-bt
device_tree_address=0x4000
kernel=u-boot.bin
over_voltage=6
arm_freq=2000
sdram_freq_min=3200

In order to clock it, I need to enable powerd on startup in
/etc/rc.conf:

powerd_enable="YES"

Because I also want that higher clock to run as much as possible, I tell
powerd to have an absurdly low trigger to get the clocked speed:

powerd_flags="-r 1"

and here's the clocked speed:

% sysctl -a | fgrep cpu.0
dev.cpu.0.temperature: 49.6C
dev.cpu.0.freq_levels: 2000/-1 600/-1
dev.cpu.0.freq: 2000
dev.cpu.0.%parent: cpulist0
dev.cpu.0.%pnpinfo: name=cpu at 0 compat=arm,cortex-a72
dev.cpu.0.%location: 
dev.cpu.0.%driver: cpu
dev.cpu.0.%desc: Open Firmware CPU

> I'd rather not complicate use of the serial console at this point

not sure why the serial console would be a factor. I have a
console-wired rpi4 running main/14 and I never considered it. I console
into this machine like this:

cu -l /dev/cuaU0 -s 115200

-- 
J.
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