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uefi: fall back to the Graviton UART when the SPCR points into memory - #64
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Since 2026-10-14 the Graviton3 (c7g) firmware in eu-north-1 describes its console as a 16550 at 0x80000000 with byte access. That address is the end of the low RAM block in the UEFI memory map, not a device; the UART is still the 16550 at 0x90a0000 with dword registers it always was. The kernel took the table at its word and hung in the serial driver's first poll before it could print anything, so every c7g boot showed only "miniosv UEFI stub starting". Graviton4 (c8g) firmware is unaffected. The stub now checks the SPCR address against the memory map when the ACPI OEM ID is AMAZON: an address inside, or exactly at the end of, a non-MMIO entry is memory, and the stub substitutes the known Graviton layout and says so. Other firmware is trusted as before. A diagnostic line prints the SPCR fields and the OEM ID, since this is the only trace a Graviton boot leaves when the console pick is wrong. Verified on c7g.large: the kernel boots in 345 ms and the PMU benches run. Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com> Claude-Session: https://claude.ai/code/session_016x6mtxvvKCfAojAVKt4ZYe
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Since a couple of days,
c7ginstances fail to show any console output. This is the fix claude came up with:Graviton3 (c7g) firmware in eu-north-1 describes its console as a 16550 at 0x80000000 with byte access. That address is the end of the low RAM block in the UEFI memory map, not a device; the UART is still the 16550 at 0x90a0000 with dword registers it always was. The kernel took the table at its word and hung in the serial driver's first poll before it could print anything, so every c7g boot showed only "miniosv UEFI stub starting". Graviton4 (c8g) firmware is unaffected.
The stub now checks the SPCR address against the memory map when the ACPI OEM ID is AMAZON: an address inside, or exactly at the end of, a non-MMIO entry is memory, and the stub substitutes the known Graviton layout and says so. Other firmware is trusted as before. A diagnostic line prints the SPCR fields and the OEM ID, since this is the only trace a Graviton boot leaves when the console pick is wrong.
Verified on c7g.large: the kernel boots in 345 ms and the PMU benches run.