build_library/oem/ami: Drop bare earlycon on arm64

[jordeu]

Flatcar on arm64 EC2 boots with a bare earlycon, which takes the UART address from the ACPI SPCR table. On Graviton that address is wrong, and when an NVMe device lands there (t4g with an extra EBS volume) the kernel hangs during early boot. This drops earlycon and keeps console=ttyS0, so the serial console still works; only pre-console early output is lost.

Details

• earlycon was added in d30359a together with the ttyAMA0 -> ttyS0 console fix. The console fix stays.
• SPCR on Graviton reports 0x80000000 while the real UART is elsewhere (e.g. 0x8000c000). With an NVMe BAR mapped at 0x80000000, the early 8250 console polls it forever.
• Scaleway and Azure set earlycon with an explicit driver/address and are not affected.
• nvme_core.io_timeout is unchanged.
• Stable 4757.2.1 ships the same line and is affected too; up to maintainers whether to backport.

How to use / testing

Boot an arm64 AMI built from this branch on a t4g instance with an additional EBS volume attached; it should reach userspace and the serial console should still show kernel output. tormath1 offered to build a test image for this.

The change was prepared with the help of an LLM.

Fixes flatcar/Flatcar#2446

CI 🟢 : https://jenkins.flatcar.org/job/container/job/packages_all_arches/825/cldsv/

[Copilot]

🟢 Approval recommended

The focused configuration change matches the documented failure and preserves the regular serial console.

0 open findings

What changed in this PR

Removes unsafe automatic UART discovery from AWS arm64 boot configuration while retaining normal serial output.

Changes:

• Drops bare earlycon from EC2 arm64 kernel arguments.
• Documents the resulting Graviton boot-hang fix.

File	Description
build_library/​oem/​ami/​grub.cfg	Removes bare earlycon on arm64.
changelog/​bugfixes/​2026-10-09-ec2-arm64-earlycon.md	Records the AWS arm64 boot fix.

🧠 Review effort: Balanced

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[tormath1]

@jordeu thanks for the PR - here's an AMI image for ARM64 if you want to give it a try: https://bincache.flatcar-linux.net/images/arm64/9999.9.100+aws-earlycon/

[jordeu]

Thanks for building it! I've tested it on t4g.large in eu-west-2, with Ignition config {"ignition":{"version":"3.3.0"}}:

Image	Extra 8 GiB gp3	Result
stable 4757.2.1 (ami-0203c6338a91abaf0)	yes	hangs at EFI stub: Exiting boot services...
9999.9.100+aws-earlycon	yes	boots, reaches multi-user, status checks ok
9999.9.100+aws-earlycon	no	boots, reaches multi-user, status checks ok

With the test image, /proc/cmdline has console=tty1 console=ttyS0,115200n8 and no earlycon, and the second NVMe controller (00:1f.0) is detected normally. The serial console still shows the full kernel log.

[chewi]

Backported to 4820 (Alpha), 4790 (Beta), 4757 (Stable).