EXTI: clear pending bits before dispatching handlers

[xhlsa]

Problem

EXTI_IRQHandler() clears each line's pending bit after its handler returns. If a new edge arrives on that line while the handler is running, it sets the pending bit again, and the write that follows wipes it. The interrupt is lost.

This is harmless for a gyro data-ready line, where the next sample brings another edge. It is fatal for a device that raises exactly one edge per step and then waits. A radio's BUSY/DIO line is the example: the driver waits for an interrupt that already arrived and was discarded.

Fix

Clear the snapshot of active lines once, before dispatching (as Betaflight's EXTI_IRQHandler does). An edge that arrives during dispatch stays pending, so the IRQ fires again on return.

Found / tested

• Found porting Betaflight's SPI ExpressLRS (SX1280) receiver. Its interrupt chain stalled within seconds of linking.
• A host-side unit test models EXTI pending/enable/software-trigger and runs the unmodified SX1280 driver with randomized BUSY timing. It stalls with the old clear order and runs clean with the new one. The test lives on the ELRS branch; happy to adapt it here if useful.
• Flown on an AT32F435 board. Build-tested on STM32F4 and AT32F43x targets.

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PR Summary by Qodo

Prevent lost EXTI edges during handler dispatch

🐞 Bug fix 🕐 10-20 Minutes

AI Description

• Clears snapshotted EXTI pending lines before invoking registered handlers.
• Preserves same-line edges arriving during dispatch for subsequent IRQ delivery.

Diagram

sequenceDiagram
    participant EXTI as EXTI Hardware
    participant IRQ as IRQ Dispatcher
    participant Handler as Line Handler
    EXTI->>IRQ: Raise pending IRQ
    IRQ->>EXTI: Snapshot enabled pending
    IRQ->>EXTI: Clear snapshot
    IRQ->>Handler: Dispatch active line
    EXTI-->>EXTI: New edge stays pending
    Handler-->>IRQ: Return
    EXTI->>IRQ: Re-enter IRQ

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High-Level Assessment

Clearing the complete active snapshot before dispatch is the preferred approach for write-one-to-clear EXTI registers. Unlike post-handler or per-line clearing, it preserves every edge arriving after the snapshot—including edges on lines waiting for dispatch—while requiring only one register write.

Files changed (1) +5 / -1

Bug fix (1) +5 / -1

exti.cClear active EXTI pending bits before callback dispatch +5/-1

Clear active EXTI pending bits before callback dispatch

• Moves the write-one-to-clear operation ahead of the handler loop and clears the captured active-line snapshot in one write. Edges arriving while callbacks execute now remain pending for subsequent interrupt delivery instead of being erased after callback return.

src/main/drivers/exti.c

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Code Review by Qodo

🐞 Bugs (0) 📘 Rule violations (0) 📎 Requirement gaps (0)

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