RPI4 clock speeds and serial port

[bob prohaska]

On Thu, Mar 18, 2021 at 05:28:27PM +0000, tech-lists wrote:
> 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

72 C is rather toasty 8-)

> 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
>

Maybe on FreeBSD it doesn't matter. Pi-related sites report that the
mini-UART baudrate is tied to the clock of the GPU, if that changes
the baudrate changes too. It wasn't clear to me if the speed-changes to
config.txt affected the GPU clock. Apparently not, at least in FreeBSD.

Thank you!

bob prohaska
 
> -- 
> J.