raspberry pi zero W serial port overlay fun

[Patrick Welche <prlw1%talktalk.net@localhost>]

[I posted this to port-arm around 4th July, but hasn't made it. Reposting
 here in case useful...]


tl;dr On a raspberry pi zero W, updating the firmware allows the disable-bt
      overlay to function resulting in a stable serial console.

Experimental method(!)

- grab https://nycdn.netbsd.org/pub/NetBSD-daily/HEAD/202207031950Z/evbarm-earmv6hf/binary/gzimg/rpi.img.gz
- gunzip / dd to card / remove "console=fb" from first line of cmdline.txt
- connect raspberry pi zero W 1.1 to serial port via pins 4,6,8,10 which boots
- watch output with tip & baud rate 115200
- all is well until /etc/rc.local is run, changing the cpu frequency,
  which messes up the baud rate because the raspberry pi zero w, by
  default, reserves its real UART for bluetooth, and attaches a "mini"
  UART to the console, and this "mini" UART doesn't appear to have its
  own clock:
...
[   1.0000000] simplebus0 at armfdt0: Raspberry Pi Zero W Rev 1.1
...
[   1.0000000] plcom0 at simplebus1: ARM PL011 UART
...
[   1.0000000] com0 at simplebus1: BCM AUX UART, 1-byte FIFO
[   1.0000000] com0: console
...
Starting local daemons:.
JRQ
   ܊VP�҇KZ��     �       ���Q


At this point, we can either:
1) delete /etc/rc.local
2) try to make use of the "real" UART by disabling bluetooth

1) works, 2) would be nice.

2):
- create overlays directory in SD card's /boot partition:
    mount /dev/sd0e /mnt
    cd /mnt
    mkdir overlays
    cp /tmp/firmware/boot/overlays/disable-bt.dtbo overlays

- add "dtoverlay=disable-bt" to config.txt

- boot, but no change:

[   1.0000000] com0 at simplebus1: BCM AUX UART, 1-byte FIFO
[   1.0000000] com0: console



- Grab precompiled raspberry pi firmware:
    cd /tmp
    git clone --depth=1 https://github.com/raspberrypi/firmware.git

    Compare disable-bt.dtb0 to rpi.img dtb using file:
    /tmp/firmware/boot/overlays/disable-bt.dtbo: Device Tree Blob version 17, size=1073, boot CPU=0, string block size=145, DT structure block size=872
    /mnt/dtb/bcm2835-rpi-zero-w.dtb: Device Tree Blob version 17, size=19566, boot CPU=0, string block size=1770, DT structure block size=16700

    Same version, so looking hopeful.

- replace firmware:

    mount /dev/sd0e /mnt
    cd /mnt
    for sfx in elf bin dat; do
      rm *.${sfx}
      cp /tmp/firmware/boot/*.${sfx} .
    done;
    rm dtb/*
    cp /tmp/firmware/boot/bcm2708-rpi-zero-w.dtb dtb

  NB in rpi.img.gz was called: bcm2835-rpi-zero-w.dtb, bcm2708, yet
  dtc -I dtb -O dts  ./boot/overlays/disable-bt.dtbo
  shows
/dts-v1/;

/ {
        compatible = "brcm,bcm2835";
...


Now, boot, and success!

[   1.0000000] plcom0 at simplebus1: ARM PL011 UART
[   1.0000000] plcom0: txfifo disabled
[   1.0000000] plcom0: console

rpi# sysctl machdep.cpu.frequency
machdep.cpu.frequency.target = 1000
machdep.cpu.frequency.current = 1000
machdep.cpu.frequency.min = 700
machdep.cpu.frequency.max = 1000
machdep.cpu.frequency.available = 700 1000

and that is read over the working serial console.

BTW my impression is there is no bluetooth support, so we are not
missing anything by disabling it? (as opposed to miniuart-bt).


Cheers,

Patrick