ADR-007 — PL011 UART RX as virt input

• Status: Accepted
• Date: 2026-09-09

Context

FR-08 already covers hardware interrupts (timer via the virt GIC, M5 / ADR-006) and named “later input” as the rest of that ID. Roadmap M6 is that input path. The x86-era reading of FR-08 was a keyboard. Frozen IDs stay; the arm64 text is revised here.

Options on QEMU virt:

1. PL011 UART RX at 0x0900_0000 (already the console).
2. virtio-input / virtio-keyboard (virtio-mmio, virtqueues, DMA). No paging or heap yet (M7/M8). Too much device stack for one milestone.
3. PL011 UARTCR.LBE loopback as a self-test. QEMU 8.2 (Ubuntu 24.04 / this repo’s GHA and cloud probe) does not implement LBE — hw/char/pl011.c still says the loopback bit is unimplemented. LBE landed in later QEMU. A loopback-only probe would be a false pass on newer hosts and a false fail on 8.2.

Host stdin on -serial stdio does reach pl011_receive → the RX FIFO. UARTLCR_H.FEN toggles reset that FIFO, so a byte sent before uart::init is lost. Inject after Hello World! (init already ran).

Decision

1. M6 input is PL011 RX poll, not virtio-keyboard and not a UART RX IRQ. FR-08’s “via the virt GIC” remains the M5 timer. Input does not take a new FR ID.
2. Prove a received byte with a host inject: scripts/qemu-serial-inject.py writes 0x41 ('A') to QEMU stdin after it sees Hello World!. The hello kernel polls UARTFR.RXFE / UARTDR (DAIF.I still masked after the timer window) and prints input: rx 0x41.
3. Fail closed. Missing marker or input: rx missed fails scripts/qemu-smoke.sh. #[test_case] only asserts the FIFO is empty when cargo test does not inject — the character proof is the serial smoke.
4. Do not claim Raspberry Pi UART, virtio-input, GICv3 UART SPI, or QEMU LBE.

Consequences

• scripts/qemu-smoke.sh needs python3 to drive the inject (Dockerfile installs it).
• cargo test / scripts/qemu-aarch64.sh stay inject-free so M2–M5 cases do not wait on stdin.
• A later virtio-keyboard or UART-RX-via-GIC path needs a new ADR; it is not this milestone.