RPI4 clock speeds and serial port

[Mark Millard]

On 2021-Mar-18, at 14:21, Mark Millard <marklmi at yahoo.com> wrote:

> On 2021-Mar-18, at 13:38, Diane Bruce <d at db.net> wrote:
> 
>> On Thu, Mar 18, 2021 at 11:07:36AM -0700, bob prohaska 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:
>>>> 
>> ...
>>> bob prohaska
>>> 
>> 
>> I've been meaning to get someone to look this over but basically this
>> rejigs the mini-uart baud rate depending on the CPU clock..
>> 
>> 
>> https://reviews.freebsd.org/D24339
> 
> I'm confused. The RPi* firmware dynamically
> assigns which UART used for the serial console
> based on other settings (including a default
> context).
> 
> For the default, with Bluetooth in use the
> miniuart is used for the serial console and
> the PL011 for Bluetooth. (Only the miniuart
> has the speed variability issue.)
> 
> But with either ( in config.txt ):
> 
> dtoverlay=disable-bt
> or:
> dtoverlay=miniuart-bt
> 
> the PL011 is used for the serial console.
> That last uses the miniuart for Bluetooth
> as well, letting it suffer the variability.
> 
> Is the proposed change compatible with
> this? Is it compatible with Bluetooth
> being what is using the miniuart?
> 
> (Note: I happen to use dtoverlay=disable-bt .)
> 

Now I'm even more confused:

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

reports for the RPi4:

QUOTE
Changing core_freq in config.txt is not supported on the Pi 4
any change from the default will almost certainly cause a
failure to boot.
END QUOTE

Or, more completely, it reports ties with hdmi_enable_4kp60
and enable_tvout for core_freq:

QUOTE
The core_freq of the Raspberry Pi 4 can change from the default
if either hdmi_enable_4kp60 or enable_tvout are used, due to
relationship between internal clocks and the particular
requirements of the requested display modes.

Display option	Frequency
Default	500
enable_tvout	360
hdmi_enable_4kp60	550

Changing core_freq in config.txt is not supported on the Pi 4,
any change from the default will almost certainly cause a
failure to boot.

It is recommended when overclocking to use the individual
frequency settings (isp_freq, v3d_freq etc) rather than gpu_freq,
as since it attempts to set core_freq (which cannot be changed on]
the Pi 4), it is not likely to have the desired effect.
END QUOTE

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

also documents the Pi3/Pi3A+/Pi3B+ as using a 400 MHz core_freq
(Pi0/W as well). Only the Pi1/Pi2 list 250 MHz. My guess is that
"Pi2" means RPi2 V1.1 and below, not the RPi2 V1.2 as well.

It is true that the "core_freq_min" figures are almost
all 250 MHz: RPi4 lists 250 MHz / 275 MHz with no clear
indication of the context difference for 275 MHz that I
can find.

Have you covered what all the:

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

page reports for things that you are planning
to touch?

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