嵌入式學習（3）-FreeRTOS的建立任務過程

建立任務過程了解

• xTaskCreate（）分析
• prvInitialiseNewTask（）函數分析
• vListInitialiseItem（）函數分析
• prvAddNewTaskToReadyList()函數分析
• prvAddTaskToReadyList()函數分析
• pxPortInitialiseStack（）函數分析

xTaskCreate（）分析

BaseType_t xTaskCreate(	TaskFunction_t pxTaskCode,
							const char * const pcName,
							const uint16_t usStackDepth,
							void * const pvParameters,
							UBaseType_t uxPriority,
							TaskHandle_t * const pxCreatedTask ) 
	{
	TCB_t *pxNewTCB;
	BaseType_t xReturn;
		#if( portSTACK_GROWTH > 0 )
		{
			pxNewTCB = ( TCB_t * ) pvPortMalloc( sizeof( TCB_t ) );
			if( pxNewTCB != NULL )
			{
				pxNewTCB->pxStack = ( StackType_t * ) pvPortMalloc( ( ( ( size_t ) usStackDepth ) * sizeof( StackType_t ) ) );
				if( pxNewTCB->pxStack == NULL )
				{
					vPortFree( pxNewTCB );
					pxNewTCB = NULL;
				}
			}
		}
		#else 
		{
		StackType_t *pxStack;
			pxStack = ( StackType_t * ) pvPortMalloc( ( ( ( size_t ) usStackDepth ) * sizeof( StackType_t ) ) ); 
			if( pxStack != NULL )
			{
				/* Allocate space for the TCB. */
				pxNewTCB = ( TCB_t * ) pvPortMalloc( sizeof( TCB_t ) ); /*lint !e961 MISRA exception as the casts are only redundant for some paths. */

				if( pxNewTCB != NULL )
				{
					/* Store the stack location in the TCB. */
					pxNewTCB->pxStack = pxStack;
				}
				else
				{
					/* The stack cannot be used as the TCB was not created.  Free
					it again. */
					vPortFree( pxStack );
				}
			}
			else
			{
				pxNewTCB = NULL;
			}
		}
		#endif /* portSTACK_GROWTH */

		if( pxNewTCB != NULL )
		{
			#if( tskSTATIC_AND_DYNAMIC_ALLOCATION_POSSIBLE != 0 )
			{
				/* Tasks can be created statically or dynamically, so note this
				task was created dynamically in case it is later deleted. */
				pxNewTCB->ucStaticallyAllocated = tskDYNAMICALLY_ALLOCATED_STACK_AND_TCB;
			}
			#endif /* configSUPPORT_STATIC_ALLOCATION */

			prvInitialiseNewTask( pxTaskCode, pcName, ( uint32_t ) usStackDepth, pvParameters, uxPriority, pxCreatedTask, pxNewTCB, NULL );
			prvAddNewTaskToReadyList( pxNewTCB );
			xReturn = pdPASS;
		}
		else
		{
			xReturn = errCOULD_NOT_ALLOCATE_REQUIRED_MEMORY;
		}

		return xReturn;
	}
           

首先判斷堆棧增長方向，通過堆棧增長方向來設定堆棧。

然後給任務快申請記憶體，申請成功就初始化堆棧字段pxStack 。

再标記一下這是使用動态建立任務函數。

然後初始化任務。

将新建立的任務加入到就緒清單中。

prvInitialiseNewTask（）函數分析

static void prvInitialiseNewTask( 	TaskFunction_t pxTaskCode,
									const char * const pcName,
									const uint32_t ulStackDepth,
									void * const pvParameters,
									UBaseType_t uxPriority,
									TaskHandle_t * const pxCreatedTask,
									TCB_t *pxNewTCB,
									const MemoryRegion_t * const xRegions ) /*lint !e971 Unqualified char types are allowed for strings and single characters only. */
{
StackType_t *pxTopOfStack;
UBaseType_t x;

	#if( portUSING_MPU_WRAPPERS == 1 )
		/* Should the task be created in privileged mode? */
		BaseType_t xRunPrivileged;
		if( ( uxPriority & portPRIVILEGE_BIT ) != 0U )
		{
			xRunPrivileged = pdTRUE;
		}
		else
		{
			xRunPrivileged = pdFALSE;
		}
		uxPriority &= ~portPRIVILEGE_BIT;
	#endif /* portUSING_MPU_WRAPPERS == 1 */

	/* Avoid dependency on memset() if it is not required. */
	#if( ( configCHECK_FOR_STACK_OVERFLOW > 1 ) || ( configUSE_TRACE_FACILITY == 1 ) || ( INCLUDE_uxTaskGetStackHighWaterMark == 1 ) )
	{
		/* Fill the stack with a known value to assist debugging. */
		( void ) memset( pxNewTCB->pxStack, ( int ) tskSTACK_FILL_BYTE, ( size_t ) ulStackDepth * sizeof( StackType_t ) );
	}
	#endif /* ( ( configCHECK_FOR_STACK_OVERFLOW > 1 ) || ( ( configUSE_TRACE_FACILITY == 1 ) || ( INCLUDE_uxTaskGetStackHighWaterMark == 1 ) ) ) */

	/* Calculate the top of stack address.  This depends on whether the stack
	grows from high memory to low (as per the 80x86) or vice versa.
	portSTACK_GROWTH is used to make the result positive or negative as required
	by the port. */
	#if( portSTACK_GROWTH < 0 )
	{
		pxTopOfStack = pxNewTCB->pxStack + ( ulStackDepth - ( uint32_t ) 1 );
		pxTopOfStack = ( StackType_t * ) ( ( ( portPOINTER_SIZE_TYPE ) pxTopOfStack ) & ( ~( ( portPOINTER_SIZE_TYPE ) portBYTE_ALIGNMENT_MASK ) ) ); /*lint !e923 MISRA exception.  Avoiding casts between pointers and integers is not practical.  Size differences accounted for using portPOINTER_SIZE_TYPE type. */

		/* Check the alignment of the calculated top of stack is correct. */
		configASSERT( ( ( ( portPOINTER_SIZE_TYPE ) pxTopOfStack & ( portPOINTER_SIZE_TYPE ) portBYTE_ALIGNMENT_MASK ) == 0UL ) );
	}
	#else /* portSTACK_GROWTH */
	{
		pxTopOfStack = pxNewTCB->pxStack;

		/* Check the alignment of the stack buffer is correct. */
		configASSERT( ( ( ( portPOINTER_SIZE_TYPE ) pxNewTCB->pxStack & ( portPOINTER_SIZE_TYPE ) portBYTE_ALIGNMENT_MASK ) == 0UL ) );

		/* The other extreme of the stack space is required if stack checking is
		performed. */
		pxNewTCB->pxEndOfStack = pxNewTCB->pxStack + ( ulStackDepth - ( uint32_t ) 1 );
	}
	#endif /* portSTACK_GROWTH */

	/* Store the task name in the TCB. */
	for( x = ( UBaseType_t ) 0; x < ( UBaseType_t ) configMAX_TASK_NAME_LEN; x++ )
	{
		pxNewTCB->pcTaskName[ x ] = pcName[ x ];

		/* Don't copy all configMAX_TASK_NAME_LEN if the string is shorter than
		configMAX_TASK_NAME_LEN characters just in case the memory after the
		string is not accessible (extremely unlikely). */
		if( pcName[ x ] == 0x00 )
		{
			break;
		}
		else
		{
			mtCOVERAGE_TEST_MARKER();
		}
	}

	/* Ensure the name string is terminated in the case that the string length
	was greater or equal to configMAX_TASK_NAME_LEN. */
	pxNewTCB->pcTaskName[ configMAX_TASK_NAME_LEN - 1 ] = '\0';

	/* This is used as an array index so must ensure it's not too large.  First
	remove the privilege bit if one is present. */
	if( uxPriority >= ( UBaseType_t ) configMAX_PRIORITIES )
	{
		uxPriority = ( UBaseType_t ) configMAX_PRIORITIES - ( UBaseType_t ) 1U;
	}
	else
	{
		mtCOVERAGE_TEST_MARKER();
	}

	pxNewTCB->uxPriority = uxPriority;
	#if ( configUSE_MUTEXES == 1 )
	{
		pxNewTCB->uxBasePriority = uxPriority;
		pxNewTCB->uxMutexesHeld = 0;
	}
	#endif /* configUSE_MUTEXES */

	vListInitialiseItem( &( pxNewTCB->xStateListItem ) );
	vListInitialiseItem( &( pxNewTCB->xEventListItem ) );

	/* Set the pxNewTCB as a link back from the ListItem_t.  This is so we can get
	back to	the containing TCB from a generic item in a list. */
	listSET_LIST_ITEM_OWNER( &( pxNewTCB->xStateListItem ), pxNewTCB );

	/* Event lists are always in priority order. */
	listSET_LIST_ITEM_VALUE( &( pxNewTCB->xEventListItem ), ( TickType_t ) configMAX_PRIORITIES - ( TickType_t ) uxPriority ); /*lint !e961 MISRA exception as the casts are only redundant for some ports. */
	listSET_LIST_ITEM_OWNER( &( pxNewTCB->xEventListItem ), pxNewTCB );

	#if ( portCRITICAL_NESTING_IN_TCB == 1 )
	{
		pxNewTCB->uxCriticalNesting = ( UBaseType_t ) 0U;
	}
	#endif /* portCRITICAL_NESTING_IN_TCB */

	#if ( configUSE_APPLICATION_TASK_TAG == 1 )
	{
		pxNewTCB->pxTaskTag = NULL;
	}
	#endif /* configUSE_APPLICATION_TASK_TAG */

	#if ( configGENERATE_RUN_TIME_STATS == 1 )
	{
		pxNewTCB->ulRunTimeCounter = 0UL;
	}
	#endif /* configGENERATE_RUN_TIME_STATS */

	#if ( portUSING_MPU_WRAPPERS == 1 )
	{
		vPortStoreTaskMPUSettings( &( pxNewTCB->xMPUSettings ), xRegions, pxNewTCB->pxStack, ulStackDepth );
	}
	#else
	{
		/* Avoid compiler warning about unreferenced parameter. */
		( void ) xRegions;
	}
	#endif

	#if( configNUM_THREAD_LOCAL_STORAGE_POINTERS != 0 )
	{
		for( x = 0; x < ( UBaseType_t ) configNUM_THREAD_LOCAL_STORAGE_POINTERS; x++ )
		{
			pxNewTCB->pvThreadLocalStoragePointers[ x ] = NULL;
		}
	}
	#endif

	#if ( configUSE_TASK_NOTIFICATIONS == 1 )
	{
		pxNewTCB->ulNotifiedValue = 0;
		pxNewTCB->ucNotifyState = taskNOT_WAITING_NOTIFICATION;
	}
	#endif

	#if ( configUSE_NEWLIB_REENTRANT == 1 )
	{
		/* Initialise this task's Newlib reent structure. */
		_REENT_INIT_PTR( ( &( pxNewTCB->xNewLib_reent ) ) );
	}
	#endif

	#if( INCLUDE_xTaskAbortDelay == 1 )
	{
		pxNewTCB->ucDelayAborted = pdFALSE;
	}
	#endif

	/* Initialize the TCB stack to look as if the task was already running,
	but had been interrupted by the scheduler.  The return address is set
	to the start of the task function. Once the stack has been initialised
	the	top of stack variable is updated. */
	#if( portUSING_MPU_WRAPPERS == 1 )
	{
		pxNewTCB->pxTopOfStack = pxPortInitialiseStack( pxTopOfStack, pxTaskCode, pvParameters, xRunPrivileged );
	}
	#else /* portUSING_MPU_WRAPPERS */
	{
		pxNewTCB->pxTopOfStack = pxPortInitialiseStack( pxTopOfStack, pxTaskCode, pvParameters );
	}
	#endif /* portUSING_MPU_WRAPPERS */

	if( ( void * ) pxCreatedTask != NULL )
	{
		/* Pass the handle out in an anonymous way.  The handle can be used to
		change the created task's priority, delete the created task, etc.*/
		*pxCreatedTask = ( TaskHandle_t ) pxNewTCB;
	}
	else
	{
		mtCOVERAGE_TEST_MARKER();
	}
}
           

這段代碼比較長，一點一點分析，首先

#if( ( configCHECK_FOR_STACK_OVERFLOW > 1 ) || ( configUSE_TRACE_FACILITY == 1 ) || ( INCLUDE_uxTaskGetStackHighWaterMark == 1 ) )
	{
		/* Fill the stack with a known value to assist debugging. */
		( void ) memset( pxNewTCB->pxStack, ( int ) tskSTACK_FILL_BYTE, ( size_t ) ulStackDepth * sizeof( StackType_t ) );
	}
           

這裡是判斷是否使能了堆棧溢出檢測功能和追蹤功能，如果使能了，就用0XA5U來填充堆棧。tskSTACK_FILL_BYTE的值為0xA5U.

然後根據堆棧增長方向，計算棧頂的位址。

然後将任務的名字的字元串儲存起來，添加結束符’\0’。

判斷目前任務的優先級是否超過了FreeRTOS系統的最大優先級。超過了就将優先級設定為最大優先級減一。

初始化任務控制塊的優先級字段uxPriority。

如果使能了互斥信号量，就初始化相應的字段。

使用vListInitialiseItem（）函數初始化清單項。

設定狀态清單項和事件清單項為目前任務的控制塊。

後面有一些條件編譯，根據自己使用情況來開啟，這裡是在開啟後進行了一些初始化字段。

後面初始化了任務堆棧，使用pxPortInitialiseStack（）。

最後生成任務句柄，可以看出，任務句柄就是任務控制塊。

vListInitialiseItem（）函數分析

void vListInitialiseItem( ListItem_t * const pxItem )
{
	/* Make sure the list item is not recorded as being on a list. */
	pxItem->pvContainer = NULL;

	/* Write known values into the list item if
	configUSE_LIST_DATA_INTEGRITY_CHECK_BYTES is set to 1. */
	listSET_FIRST_LIST_ITEM_INTEGRITY_CHECK_VALUE( pxItem );
	listSET_SECOND_LIST_ITEM_INTEGRITY_CHECK_VALUE( pxItem );
}
           

這裡的函數都沒有實作，說明這裡隻是将清單進行了初始化，對pvContainer 給了0。

prvAddNewTaskToReadyList()函數分析

static void prvAddNewTaskToReadyList( TCB_t *pxNewTCB )
{
	/* Ensure interrupts don't access the task lists while the lists are being
	updated. */
	taskENTER_CRITICAL();
	{
		uxCurrentNumberOfTasks++;
		if( pxCurrentTCB == NULL )
		{
			/* There are no other tasks, or all the other tasks are in
			the suspended state - make this the current task. */
			pxCurrentTCB = pxNewTCB;

			if( uxCurrentNumberOfTasks == ( UBaseType_t ) 1 )
			{
				/* This is the first task to be created so do the preliminary
				initialisation required.  We will not recover if this call
				fails, but we will report the failure. */
				prvInitialiseTaskLists();
			}
			else
			{
				mtCOVERAGE_TEST_MARKER();
			}
		}
		else
		{
			/* If the scheduler is not already running, make this task the
			current task if it is the highest priority task to be created
			so far. */
			if( xSchedulerRunning == pdFALSE )
			{
				if( pxCurrentTCB->uxPriority <= pxNewTCB->uxPriority )
				{
					pxCurrentTCB = pxNewTCB;
				}
				else
				{
					mtCOVERAGE_TEST_MARKER();
				}
			}
			else
			{
				mtCOVERAGE_TEST_MARKER();
			}
		}

		uxTaskNumber++;

		#if ( configUSE_TRACE_FACILITY == 1 )
		{
			/* Add a counter into the TCB for tracing only. */
			pxNewTCB->uxTCBNumber = uxTaskNumber;
		}
		#endif /* configUSE_TRACE_FACILITY */
		traceTASK_CREATE( pxNewTCB );

		prvAddTaskToReadyList( pxNewTCB );

		portSETUP_TCB( pxNewTCB );
	}
	taskEXIT_CRITICAL();

	if( xSchedulerRunning != pdFALSE )
	{
		/* If the created task is of a higher priority than the current task
		then it should run now. */
		if( pxCurrentTCB->uxPriority < pxNewTCB->uxPriority )
		{
			taskYIELD_IF_USING_PREEMPTION();
		}
		else
		{
			mtCOVERAGE_TEST_MARKER();
		}
	}
	else
	{
		mtCOVERAGE_TEST_MARKER();
	}
}
           

先進入臨界區。uxCurrentNumberOfTasks（）函數用來記錄任務數量。

如果任務控制塊為空，說明目前沒有任務，或者其他任務都處于挂起态，那就将此做為目前任務。初始化任務清單。

如果目前有任務，那就判斷目前是否開啟任務排程。如果目前排程任務還未運作，那就判斷優先級，将pxCurrentTCB設定為高優先級任務的任務控制塊。

任務數量加一。

traceTASK_CREATE( pxNewTCB );函數未實作

添加任務到就緒清單中，

退出臨界區。

如果目前任務排程器已經開啟。而且新任務的優先級最高，那就進行一次任務切換。

prvAddTaskToReadyList()函數分析

#define prvAddTaskToReadyList( pxTCB )																\
	traceMOVED_TASK_TO_READY_STATE( pxTCB );														\
	taskRECORD_READY_PRIORITY( ( pxTCB )->uxPriority );												\
	vListInsertEnd( &( pxReadyTasksLists[ ( pxTCB )->uxPriority ] ), &( ( pxTCB )->xStateListItem ) ); \
	tracePOST_MOVED_TASK_TO_READY_STATE( pxTCB )
           

添加任務到就緒清單函數是一個宏，其中traceMOVED_TASK_TO_READY_STATE( pxTCB ); 未實作

taskRECORD_READY_PRIORITY函數用于硬體設定優先級。

在就将任務添加到就緒清單結尾。

pxPortInitialiseStack（）函數分析

StackType_t *pxPortInitialiseStack( StackType_t *pxTopOfStack, TaskFunction_t pxCode, void *pvParameters )
{
	/* Simulate the stack frame as it would be created by a context switch
	interrupt. */
	pxTopOfStack--; /* Offset added to account for the way the MCU uses the stack on entry/exit of interrupts. */
	*pxTopOfStack = portINITIAL_XPSR;	/* xPSR */
	pxTopOfStack--;
	*pxTopOfStack = ( ( StackType_t ) pxCode ) & portSTART_ADDRESS_MASK;	/* PC */
	pxTopOfStack--;
	*pxTopOfStack = ( StackType_t ) prvTaskExitError;	/* LR */

	pxTopOfStack -= 5;	/* R12, R3, R2 and R1. */
	*pxTopOfStack = ( StackType_t ) pvParameters;	/* R0 */
	pxTopOfStack -= 8;	/* R11, R10, R9, R8, R7, R6, R5 and R4. */

	return pxTopOfStack;
}
           

在任務初始化函數裡會對任務的堆棧初始化，首先将寄存器xPSRd的值設定為portINITIAL_XPSR，然後将PC的值設定為pxCode ，然後将LR的值設定為prvTaskExitError。跳過R12,R3,R2,R1.将R0設定為pvParameters。跳過八個寄存器。這裡的寄存器順序是晶片内設定好的。最後的堆棧如圖所示。