steami: Stabilize config zone firmware (I2C IRQ race condition).

[nedseb]

Context

The config zone commands (READ_CONFIG 0x31, WRITE_CONFIG 0x30, CLEAR_CONFIG 0x32) using the STM32F103 internal flash work correctly after a clean build, but the initial investigation revealed a race condition in the I2C interrupt handler.

Root cause identified

Two combined factors:

1. Flash timing incompatible with 30ms hook — W25Q64JV sector erase takes 400ms typical (up to 2s max per datasheet p.76). During this time, the flash rejects all SPI commands except Read Status.

2. I2C IRQ race condition — The I2C callback (HAL_I2C_SlaveRxCpltCallback) runs in interrupt context with priority 0 (highest) and directly modifies shared state (current_task, task_rx, task_rx_len, tx_i2c_data) without critical sections or memory barriers.

Corrections needed

Firmware side

1. Add __disable_irq() / __enable_irq() around current_task modifications in on_I2C_receive_command() (IRQ context) and process_task() (thread context)
2. Make config operations non-blocking (state machine like WRITE_DATA): TASK_CLEAR_CONFIG → send erase → TASK_WAIT_FLASH_BUSY → poll w25q64_is_busy()
3. Use TASK_WAIT_FLASH_BUSY after WRITE_CONFIG and CLEAR_CONFIG

MicroPython side

1. Increase polling delay to 50ms instead of 5ms to reduce I2C pressure
2. Add sleep after write/erase operations before starting status polling

Current state

The internal flash config zone (PR #4, merged) works correctly. This issue tracks the stabilization of the external W25Q64 flash operations for potential future use.

Related

• PR steami: Add config zone in F103 internal flash gap #4 (merged) — Internal flash config zone implementation
• Issue steami_config: Add persistent configuration module. micropython-steami-lib#166 — steami_config module