ISR-Safe API 설계 — FromISR 패턴·Higher Priority Wake·Deferred Work

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/* Task context */
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xQueueSend(q, &item, xTicksToWait);
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xSemaphoreTake(sem, xTicksToWait);
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/* ISR context */
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xQueueSendFromISR(q, &item, &xHigherPriorityTaskWoken);
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xSemaphoreGiveFromISR(sem, &xHigherPriorityTaskWoken);

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void uart_rx_isr(void) {
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    uint8_t byte = UART->RDR;
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    BaseType_t xHigherPriorityTaskWoken = pdFALSE;
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    xQueueSendFromISR(rx_queue, &byte, &xHigherPriorityTaskWoken);
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    /* ISR 끝에서 — 더 높은 priority task wake됐다면 yield */
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    portYIELD_FROM_ISR(xHigherPriorityTaskWoken);
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}

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uint32_t saved = portSET_INTERRUPT_MASK_FROM_ISR();
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{
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    /* ISR-safe critical — IRQ priority temp boost */
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}
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portCLEAR_INTERRUPT_MASK_FROM_ISR(saved);

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static inline uint32_t portSET_INTERRUPT_MASK_FROM_ISR(void) {
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    uint32_t saved_basepri;
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    __asm volatile (
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        "mrs %0, basepri              \n"
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        "mov r0, %1                   \n"
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        "msr basepri, r0              \n"
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        : "=r"(saved_basepri)
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        : "i"(configMAX_SYSCALL_INTERRUPT_PRIORITY)
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        : "r0"
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    );
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    return saved_basepri;
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}

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void some_isr(void) {
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    xSemaphoreTake(sem, 100);   // ✗ 컴파일 에러
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}

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/* Deferred task — high priority */
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void deferred_handler_task(void *p) {
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    for (;;) {
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        xSemaphoreTake(uart_rx_sem, portMAX_DELAY);
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        /* ISR 대신 여기서 무거운 처리 */
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        process_uart_packet();
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    }
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}
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/* ISR — 짧게 */
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void uart_irq(void) {
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    BaseType_t pxHP = pdFALSE;
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    xSemaphoreGiveFromISR(uart_rx_sem, &pxHP);
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    portYIELD_FROM_ISR(pxHP);
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}

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/* FreeRTOS internal — config로 활성화 */
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void prvTimerTask(void *p) {
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    for (;;) {
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        /* Wait for command */
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        xQueueReceive(xTimerQueue, &cmd, portMAX_DELAY);
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        switch (cmd.type) {
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            case PEND_FUNC_CALL:
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                cmd.func(cmd.arg1, cmd.arg2);   // ISR에서 요청한 함수 실행
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                break;
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            case TIMER_EXPIRED:
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                cmd.timer->callback();
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                break;
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        }
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    }
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}

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void heavy_isr(void) {
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    BaseType_t pxHP = pdFALSE;
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    xTimerPendFunctionCallFromISR(do_heavy_work, arg1, arg2, &pxHP);
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    portYIELD_FROM_ISR(pxHP);
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}

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#define portYIELD_FROM_ISR(x) do { \
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    if (x != pdFALSE) { \
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        portYIELD();   /* SCB->ICSR = PENDSVSET */ \
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    } \
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} while (0)

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portEND_SWITCHING_ISR(xHigherPriorityTaskWoken);

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volatile uint32_t shared_counter;
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void task(void *p) {
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    portENTER_CRITICAL();
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    uint32_t v = shared_counter;
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    portEXIT_CRITICAL();
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    /* use v */
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}
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void isr(void) {
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    shared_counter++;   // 32-bit atomic on M-class
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}

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void isr(void) {
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    xQueueSend(q, ...);   // ✗ task API
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}

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void isr(void) {
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    BaseType_t pxHP = pdFALSE;
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    xSemaphoreGiveFromISR(sem, &pxHP);
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    /* portYIELD_FROM_ISR 안 호출 */
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}

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NVIC_SetPriority(UART1_IRQn, 0);   // Highest — FreeRTOS API 금지 영역