[Bug 290295] aws ec2 freebsd 14 arm serial console gone

From: <bugzilla-noreply_at_freebsd.org>
Date: Thu, 16 Oct 2025 14:02:31 UTC 
https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=290295

            Bug ID: 290295
           Summary: aws ec2 freebsd 14 arm serial console gone
           Product: Base System
           Version: 14.3-STABLE
          Hardware: arm64
                OS: Any
            Status: New
          Severity: Affects Many People
          Priority: ---
         Component: arm
          Assignee: freebsd-arm@FreeBSD.org
          Reporter: clive@CirX.ORG
                CC: Andrew@FreeBSD.org, cperciva@FreeBSD.org,
                    imp@FreeBSD.org, jhibbits@FreeBSD.org,
                    lwhsu@FreeBSD.org

I'm not sure what changed in aws around Sep or Oct 2025, I noticed that serial
console is gone on arm64 instances. As to amd64 instances, all works. I ran
more tests on various snapshots and found that in FreeBSD 15, the issue is
resolved after 5.0-PRERELEASE-arm64-20250822. Then I tested several kernel
builds by reverting commits one by one, and luckily found the one-line fix 

https://cgit.freebsd.org/src/commit/?h=releng/15.0&id=a685a263b803012d9546abaca4dcf52f80c6b8d6

    Although it's a one line patch, but for me it's not easy to apply it on
stable/14. Either find the equivalent bits in stable/14 and fix it, or apply
all the fundamental changes in stable/15 (uart, acpica, maybe pci as well) to
fix it.

    Since 14.4 release is still months away, lwhsu@ recommended opening a
ticket here. I’m happy to test any potential fix.

    Additional tests I did before submitting the ticket:

Up to table/14-n272680-258cde4b2e48
https://cgit.freebsd.org/src/commit/?h=stable/14&id=258cde4b2e4832bfaa49c800754aec8800a388c7
The serial console is still missing.

Up to stable/15-n280713-24f4ec797e9b
https://cgit.freebsd.org/src/commit/?h=stable/15&id=24f4ec797e9b19abd4488554c78e7ecc27313428
The serial console is still detected and and usable.

    Both are GENERIC kernel. Tested on instance types: t4g.small and
m8g.2xlarge. 

    Sorry for the long CC list. It's based on the commit log, reviews, and of
course our aws hero.

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