feedback_aarch64_irq_path_pitfall.md

name	feedback-aarch64-irq-path-pitfall
description	When AArch64 input/timing "doesn't work," check the IRQ vector and IRQ-time function calls before suspecting the device. Two C-ABI traps lurk.
metadata	node_type type originSessionId memory feedback 5e4de8fe-6dd6-4370-af76-b4662b3ce1d6

When AArch64 input or timing "doesn't work" — tk_dly_tsk hangs, shell input silently disappears, semaphores never wake — look at the IRQ delivery path first, not the device.

Two C-ABI traps to check whenever an IRQ handler in cpu_support.S calls into C:

1. Save x30 before any bl / blr. A function whose only "epilogue" is ret will silently loop into itself after the BL clobbers the link register. The first IRQ silently never completes; the kernel spins forever in the tail of the handler. Symptom from the outside: exactly one tick, then nothing.

2. Don't keep the IAR value in a caller-saved register across a blr to a C handler. w1-w18 are call-clobbered by the AArch64 ABI, so any C function will overwrite them. Writing a stale w1 to GICC_EOIR sends garbage to the GIC; the IRQ stays "active," and the GIC then filters out subsequent equal- or lower-priority interrupts. Symptom from the outside: the first tick works, every subsequent tick is dropped. Stash IAR on the stack across the call, or use a callee-saved register you've explicitly preserved.

Why: Both bugs hide in the AArch64 port and survived Phase 1 + Phase 2b because the only consumer of timer ticks at the time was tk_dly_tsk(1) in sio_read_line, and nobody had typed into the shell yet (the net/ai commands were never actually exercised interactively until Phase 2c). Each bug alone would look like "shell input doesn't work," masking the real failure mode.

How to apply: Anytime you see tk_dly_tsk / tk_wai_sem block longer than the timeout, before you debug the device driver, add a one-character print inside the timer IRQ handler (or a static volatile unsigned irq_count increment). If exactly one print appears, you have one of these two bugs.

Related: [[moment-2026-05-20-phase2c-irq-fix]] documents the diagnosis session.