FreeRTOS解析

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FreeRTOS源码解析

基本配置

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#define portSTACK_GROWTH			( -1 )

任务管理

任务控制块TCB

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/*
 * Task control block.  A task control block (TCB) is allocated for each task,
 * and stores task state information, including a pointer to the task's context
 * (the task's run time environment, including register values)
 */
typedef struct tskTaskControlBlock       /* The old naming convention is used to prevent breaking kernel aware debuggers. */
{
    volatile StackType_t * pxTopOfStack; /**< Points to the location of the last item placed on the tasks stack.  THIS MUST BE THE FIRST MEMBER OF THE TCB STRUCT. */

    ListItem_t xStateListItem;                  /**< The list that the state list item of a task is reference from denotes the state of that task (Ready, Blocked, Suspended ). */
    ListItem_t xEventListItem;                  /**< Used to reference a task from an event list. */
    UBaseType_t uxPriority;                     /**< The priority of the task.  0 is the lowest priority. */
    StackType_t * pxStack;                      /**< Points to the start of the stack. */
    char pcTaskName[ configMAX_TASK_NAME_LEN ]; /**< Descriptive name given to the task when created.  Facilitates debugging only. */ /*lint !e971 Unqualified char types are allowed for strings and single characters only. */

    UBaseType_t uxBasePriority; /**< The priority last assigned to the task - used by the priority inheritance mechanism. */
    UBaseType_t uxMutexesHeld;

    volatile uint32_t ulNotifiedValue[ configTASK_NOTIFICATION_ARRAY_ENTRIES ];
    volatile uint8_t ucNotifyState[ configTASK_NOTIFICATION_ARRAY_ENTRIES ];

    uint8_t ucDelayAborted;
} tskTCB;

/* The old tskTCB name is maintained above then typedefed to the new TCB_t name
 * below to enable the use of older kernel aware debuggers. */
typedef tskTCB TCB_t;

动态创建任务

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// tasks.c
BaseType_t xTaskCreate( TaskFunction_t pxTaskCode,
                            const char * const pcName, /*lint !e971 Unqualified char types are allowed for strings and single characters only. */
                            const configSTACK_DEPTH_TYPE usStackDepth,
                            void * const pvParameters,
                            UBaseType_t uxPriority,
                            TaskHandle_t * const pxCreatedTask )

portSTACK_GROWTH < 0，栈向下增长；为避免栈增长到TCB中，需要先申请栈再申请TCB结构。

pxNewTCB->pxStack = pxStack;将栈赋值到TCB中。

prvInitialiseNewTask初始化TCB结构。

prvAddNewTaskToReadyList将任务添加到就绪列表中。

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static void prvAddNewTaskToReadyList( TCB_t * pxNewTCB )

taskENTER_CRITICAL进入临界区，关闭中断。

prvAddTaskToReadyList将任务按优先级加入就绪队列中。

taskEXIT_CRITICAL退出临界区，开中断。

开始任务调度

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// tasks.c
void vTaskStartScheduler( void )

xTaskCreate创建idle任务。

xTimerCreateTimerTask创建计时器任务。

portDISABLE_INTERRUPTS禁用中断。

xPortStartScheduler调用开启硬件调度接口，一般是启动tick计时器。

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// port.c
BaseType_t xPortStartScheduler( void )

portDISABLE_INTERRUPTS禁用中断。

configSETUP_TICK_INTERRUPT配置tick计时器。

vPortRestoreTaskContext开始执行第一个任务。

汇编程序

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// portASM.S
/******************************************************************************
 * vPortRestoreTaskContext is used to start the scheduler.
 *****************************************************************************/
.align 8
.type vPortRestoreTaskContext, %function
vPortRestoreTaskContext:
.set freertos_vector_base,	_freertos_vector_table

	/* Install the FreeRTOS interrupt handlers. */
	LDR		X1, =freertos_vector_base
#if defined( GUEST )
	MSR		VBAR_EL1, X1
#else
	MSR		VBAR_EL3, X1
#endif
	DSB		SY
	ISB		SY

	/* Start the first task. */
	portRESTORE_CONTEXT

任务切换

MPS2_QEMU会调用汇编处理程序xPortPendSVHandler

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void xPortPendSVHandler( void )
{
    /* This is a naked function. */

    __asm volatile
    (
        "	mrs r0, psp							\n"
        "	isb									\n"
        "										\n"
        "	ldr	r3, pxCurrentTCBConst			\n"/* Get the location of the current TCB. */
        "	ldr	r2, [r3]						\n"
        "										\n"
        "	stmdb r0!, {r4-r11}					\n"/* Save the remaining registers. */
        "	str r0, [r2]						\n"/* Save the new top of stack into the first member of the TCB. */
        "										\n"
        "	stmdb sp!, {r3, r14}				\n"
        "	mov r0, %0							\n"
        "	msr basepri, r0						\n"
        "	bl vTaskSwitchContext				\n"
        "	mov r0, #0							\n"
        "	msr basepri, r0						\n"
        "	ldmia sp!, {r3, r14}				\n"
        "										\n"/* Restore the context, including the critical nesting count. */
        "	ldr r1, [r3]						\n"
        "	ldr r0, [r1]						\n"/* The first item in pxCurrentTCB is the task top of stack. */
        "	ldmia r0!, {r4-r11}					\n"/* Pop the registers. */
        "	msr psp, r0							\n"
        "	isb									\n"
        "	bx r14								\n"
        "										\n"
        "	.align 4							\n"
        "pxCurrentTCBConst: .word pxCurrentTCB	\n"
        ::"i" ( configMAX_SYSCALL_INTERRUPT_PRIORITY )
    );
}

OpenAI辅助理解

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void PendSV_Handler(void) {
    /* 保存当前任务的上下文 */
    __asm volatile (
        "MRS     R0, PSP              \n" /* 获取进程堆栈指针 */
        "STMDB   R0!, {R4-R11}        \n" /* 保存R4-R11寄存器到堆栈 */
        "LDR     R1, =pxCurrentTCB    \n" /* 加载当前TCB指针地址 */
        "LDR     R2, [R1]             \n" /* 加载当前TCB指针 */
        "STR     R0, [R2]             \n" /* 保存堆栈指针到当前TCB */
    );

    /* 切换到下一个任务 */
    vTaskSwitchContext();

    /* 恢复下一个任务的上下文 */
    __asm volatile (
        "LDR     R1, =pxCurrentTCB    \n" /* 加载当前TCB指针地址 */
        "LDR     R2, [R1]             \n" /* 加载当前TCB指针 */
        "LDR     R0, [R2]             \n" /* 加载堆栈指针 */
        "LDMIA   R0!, {R4-R11}        \n" /* 恢复R4-R11寄存器 */
        "MSR     PSP, R0              \n" /* 恢复进程堆栈指针 */
        "BX      LR                   \n" /* 返回任务 */
    );
}

然后调用切换上下文函数

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// task.c
void vTaskSwitchContext( void )

taskSELECT_HIGHEST_PRIORITY_TASK选择最高优先级任务

时钟管理

时钟节拍

MPS2_QEMU会调用处理程序xPortSysTickHandler

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// port.c
void xPortSysTickHandler( void )

portDISABLE_INTERRUPTS关闭中断。

xTaskIncrementTick尝试增加tick值，成功后触发PendSV中断，强行进行一次调度。

portENABLE_INTERRUPTS打开中断。

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// tasks.c
BaseType_t xTaskIncrementTick( void )

taskSWITCH_DELAYED_LISTS系统tick值反转时，将延时任务队列与反转反转队列互换。

判断是否有超时的任务，并将其移入就绪队列中。

任务延时

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void vTaskDelay( const TickType_t xTicksToDelay )

vTaskSuspendAll关闭调度器。

prvAddCurrentTaskToDelayedList将当前任务加到延时列表中。

xTaskResumeAll恢复调度器。

portYIELD_WITHIN_API当恢复调度时未进行再次调度，则强制进行调度并让任务进入睡眠状态。

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static void prvAddCurrentTaskToDelayedList( TickType_t xTicksToWait,
                                            const BaseType_t xCanBlockIndefinitely )

uxListRemove将当前任务移除队列中。

xTimeToWake = xConstTickCount + xTicksToWait计算下次唤醒的时刻。需要考虑时间反转的场景。