[SOLVED] Serial on host stops working during boot

[dhellstern]

My motherboard has a COM header, so I recently got the equipment I needed to hook it up and manage via serial. However, I'm unable to use serial after the system is booted. The port is at "ttyS0". I set it up in GRUB (note that my setup is serial-only until SSH comes up, the only GPU in the system is not actually used by GRUB or Linux, only for passthrough):

GRUB_TERMINAL=serial
GRUB_SERIAL_COMMAND="serial --unit=0 --speed=115200"

And then I set it up for Linux, by adding "console=ttyS0,115200" to the kernel command line. Now, when I boot the host, I first see the GRUB menu, and can interact with it fully, and then I see the normal loading initramfs messages, followed by most of the normal Linux boot process. However, once it gets to the "Login Service", there is no longer any output and I cannot interact with the terminal:

...
         Starting Proxmox VE Login Banner...
         Starting Enable support fo…l executable binary formats...
[  OK  ] Reached target ZFS volumes are ready.
[  OK  ] Started Preprocess NFS configuration.
[  OK  ] Started Set console font and keymap.
[  OK  ] Started Create Volatile Files and Directories.
[  OK  ] Started Load AppArmor profiles.
[  OK  ] Started Proxmox VE firewall logger.
         Mounting Arbitrary Executable File Formats File System...
         Starting Raise network interfaces...
[   17.623601] Generic FE-GE Realtek PHY r8169-800:00: attached PHY driver [Generic FE-GE Realtek PHY] (mii_bus:phy_addr=r8169-800:00, irq=IGNORE)
         Starting Network Time Synchronization...
         Starting Update UTMP about System Boot/Shutdown...
         Starting RPC bind portmap service...
[  OK  ] Reached target NFS client services.
[  OK  ] Started Proxmox VE Login Banner.
[  OK  ] Started Commit Proxmox VE network changes.
[  OK     17.739718] r8169 0000:08:00.0 enp8s0: Link is Down
0m] Mounted Arbitrary Executable File Formats File System.
[  OK  ] Started    17.760897] 8021q: 802.1Q VLAN Support v1.8
1;39mRPC bind portmap service.
[  OK  ] Started Network Time Synchronization.
[  OK  ] Started Enable support for…nal executable binary formats.
[  OK  ] Reached target System Time Synchronized.
[  OK  ] Reached target RPC Port Mapper.
[  OK  ] Started Update UTMP about System Boot/Shutdown.
[  OK  ] Reached target System Initialization.
[  OK  ] Listening on D-Bus System Message Bus Socket.
[  OK  ] Reached target Sockets.
[  OK  ] Reached target Basic System.
         Starting Proxmox VE LXC Syscall Daemon...
[  OK  ] Started D-Bus System Message Bus.
         Starting PVE Qemu Event Daemon...
[  OK  ] Started ZFS Event Daemon (zed).
         Starting System Logging Service...
[  OK  ] Started Daily apt download activities.
[  OK  ] Started Daily apt upgrade and clean activities.
         Starting LSB: The monitor UPS software....
[  OK  ] Started Daily rotation of log files.
[  OK  ] Started Daily man-db regeneration.
         Starting Login Service...
[  OK  ] Started Proxmox VE [   18.118065] softdog: initialized. soft_noboot=0 soft_margin=60 sec soft_panic=0 (nowayout=0)
watchdog multiplexer.
         Starting Kernel Samepage Merging (KSM) Tuning Daemon...
[  OK  ] Started Daily Cleanup of Temporary Directories.
[  OK  ] Started Daily PVE download activities.
         Starting Self Monitoring a…g Technology (SMART) Daemon...
         Starting ZFS file system shares...
[  OK  ] Started Proxmox VE replication runner.
[  OK  ] Reached target Timers.
[  OK  ] Started FUSE filesy[   18.258047] new mount options do not match the existing superblock, will be ignored
stem for LXC.
         Starting Initialize hardware monitoring sensors...
[  OK  ] Started Proxmox VE LXC Syscall Daemon.
[  OK  ] Started System Logging Service.
[  OK  ] Started PVE Qemu Event Daemon.
[  OK  ] Started Kernel Samepage Merging (KSM) Tuning Daemon.
[  OK  ] Started ZFS file system shares.
[  OK  ] Reached target ZFS startup target.
[  OK  ] Started Initialize hardware monitoring sensors.
[  OK  ] Started Login Service.

If I SSH in, I can see that the "serial-getty@ttyS0" service is running, and its journal shows zero errors or warnings. Similarly, dmesg and the syslog have no errors about this. Even if I restart the service, the serial terminal still doesn't work. If I run "stty -a -F /dev/ttyS0", I can see that for some reason, the baud rate is set to 2400, even though it was correct during boot. I can manually set it back to 115200 baud, but if I do that and then type anything on the client side of the terminal, it immediately reverts to 2400 after showing that character. However, if I manually reset it and then restart "serial-getty@ttyS0", it maintains 115200 baud and remains interactive.

I've been unable to find a solution anywhere on the internet, perhaps my Google-fu is not strong enough but it's difficult to find an issue where it works partially, unlike what most people experience, where it doesn't work at all.

SOLVED thanks to the Unix StackExchange:
https://unix.stackexchange.com/ques...-host-stops-working-during-boot/650197#650197