[Bug 1873520] Re: fan-shim interrupts u-boot

[rewound]

syscfg.txt is intended to contain system-made configuration changes and
user configuration changes should be placed in usercfg.txt. Consequently
I edited, in the boot partition of the RPi4's SD card mounted in a
laptop, usercfg.txt and inserted enable_uart=0 there. Xubuntu 20.04 LTS
now boots correctly with Fan Shim mounted.

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https://bugs.launchpad.net/bugs/1873520

Title:
  fan-shim interrupts u-boot

Status in u-boot package in Ubuntu:
  Confirmed

Bug description:
  The popular pimoroni fan-shim uses the serial lines on the GPIO header
  to control an RGBLED. The default configuration of Ubuntu includes a
  serial console, which can interrupt the u-boot process with an
  appropriately timed key-press. When the fan-shim is installed, it
  effectively spams the serial console continuously with the result that
  the boot process is interrupted and lots of repeated characters appear
  on the screen.

  The workaround, for now, is to add the following line to the
  syscfg.txt file, disabling the serial port:

    enable_uart=0

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