RPI4 clock speeds and serial port

[Mark Millard]

On 2021-Mar-18, at 11:07, bob prohaska <fbsd at www.zefox.net> wrote:

> 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

Note that use of disable-bt (which is
something standard to my context as well).
This switches would serial hardware is
used, such that the hardware that is not
cpu/gpu clock rate dependent is used.

I forgot to mention this context in my
prior reply.

>>> 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.

You are correct that the mini-UART has the dependencies.
But use of:

dtoverlay=disable-bt

avoids use of the mini-UART. The same is true
of instead using:

dtoverlay=miniuart-bt

but, then, Bluetooth suffers the variability.
(I just do not try to use Bluetooth.)


===
Mark Millard
marklmi at yahoo.com
( dsl-only.net went
away in early 2018-Mar)