[Bug 290021] uart(4) causes system freeze with EFI bootloader on Atom C3558

https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=290021

            Bug ID: 290021
           Summary: uart(4) causes system freeze with EFI bootloader on
                    Atom C3558
           Product: Base System
           Version: 14.3-RELEASE
          Hardware: amd64
                OS: Any
            Status: New
          Severity: Affects Some People
          Priority: ---
         Component: kern
          Assignee: bugs@FreeBSD.org
          Reporter: koston@iki.fi

Created attachment 264318
  --> https://bugs.freebsd.org/bugzilla/attachment.cgi?id=264318&action=edit
/var/run/dmesg.boot with 'boot -v'

Hardware: Qotom Q20321G9 (Intel Atom C3558 based mini-PC)

Goal was to gain a fully functional serial console for remote administration.
Hardware has a single RJ45 (RS232) socket, connected to device:

<Intel Denverton UART> port 0xe040-0xe047 mem 0xdf617000-0xdf6170ff irq 16 at
device 26.0 on pci0

Kernel console is successfully configured with:
'hw.uart.console=io:0xe040,br:115200'

When system is booted via EFI using 'console=efi', kernel console works, but
userland does not. Attempting to use the connected /dev/uartX device usually
causes the system to freeze completely.

In detail, when a non-system console is configured in /etc/ttys, getty process
manages to squeeze out text ")FreeBSD/amd64" but nothing else. Afterwards,
system freezes when attempting to:

- Restart getty after modifying /etc/ttys: 'kill -HUP 1'
- Read capabilities of the UART device: 'pciconf -lc 0:0:26:0'
- Reboot: 'shutdown -r now' (after "Writing early boot entropy file: ." in
video console)

Configuring a system console causes the host to freeze already during bootup,
after pushing out text "rc: WARNING". When kernel console is not configured and
a non-system console is, nothing is printed (but otherwise problem remains).

Regardless of any console being configured or not, if a serial client writes to
the port, system will freeze when attempting to run: 'pciconf -lc 0:0:26:0'.

Tested through FreeBSD 13.5-REL to 15.0-ALPHA4, issue persists. Problem does
not exist when system is booted using legacy bootloader.

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