【5】STM32·FreeRTOS·临界段保护与调度器挂起

一、临界段代码保护简介

二、临界段代码保护函数介绍

2.1、调用示例

2.2、内部实现

三、任务调度器的挂起和恢复

3.1、调用示例

3.2、内部实现

一、临界段代码保护简介

什么是临界段：临界段代码也叫做临界区，是指那些必须完整运行，不能被打断的代码段

适用场合如：

1、外设	需严格按照时序初始化的外设：IIC、SPI等等
2、系统	系统自身需求
3、用户	用户需求

中断和任务调度可以打断当前程序的运行

二、临界段代码保护函数介绍

FreeRTOS 在进入临界段代码的时候需要关闭中断，当处理完临界段以后再开中断

函数	描述
taskENTER_CRITICAL()	任务级进入临界段
taskEXIT_CRITICAL()	任务级退出临界段
taskENTER_CRITICAL_FROM_ISR()	中断级进入临界段
taskEXIT_CRITICAL_FROM_ISR()	中断级退出临界段

2.1、调用示例

任务级临界区调用格式示例：

taskENTER_CRITICAL();
{...    /* 临界区 */
}
taskEXIT_CRITICAL();

中断级临界区调用格式示例：

uint32_t save_status;
save_status = taskENTER_CRITICAL_FROM_ISR();
{...    /* 临界区 */
}
taskEXIT_CRITICAL_FROM_ISR(save_status);

特点

1、成对使用

2、支持嵌套

3、尽量保持临界段耗时短

2.2、内部实现

任务级进入临界段：taskENTER_CRITICAL()

#define taskENTER_CRITICAL()               portENTER_CRITICAL()
#define portENTER_CRITICAL()               vPortEnterCritical()void vPortEnterCritical( void )
{portDISABLE_INTERRUPTS();uxCriticalNesting++;/* This is not the interrupt safe version of the enter critical function so* assert() if it is being called from an interrupt context.  Only API* functions that end in "FromISR" can be used in an interrupt.  Only assert if* the critical nesting count is 1 to protect against recursive calls if the* assert function also uses a critical section. */if( uxCriticalNesting == 1 ){configASSERT( ( portNVIC_INT_CTRL_REG & portVECTACTIVE_MASK ) == 0 );}
}#define portDISABLE_INTERRUPTS()           vPortRaiseBASEPRI()static portFORCE_INLINE void vPortRaiseBASEPRI( void ){uint32_t ulNewBASEPRI = configMAX_SYSCALL_INTERRUPT_PRIORITY;__asm{/* Set BASEPRI to the max syscall priority to effect a critical* section. */
/* *INDENT-OFF* */msr basepri, ulNewBASEPRIdsbisb
/* *INDENT-ON* */}}

任务级退出临界段：taskEXIT_CRITICAL()

#define taskEXIT_CRITICAL()                portEXIT_CRITICAL()
#define portEXIT_CRITICAL()                vPortExitCritical()void vPortExitCritical( void )
{configASSERT( uxCriticalNesting );uxCriticalNesting--;if( uxCriticalNesting == 0 ){portENABLE_INTERRUPTS();}
}#define portENABLE_INTERRUPTS()            vPortSetBASEPRI( 0 )static portFORCE_INLINE void vPortSetBASEPRI( uint32_t ulBASEPRI ){__asm{/* Barrier instructions are not used as this function is only used to* lower the BASEPRI value. */
/* *INDENT-OFF* */msr basepri, ulBASEPRI
/* *INDENT-ON* */}}

中断级进入临界段：taskENTER_CRITICAL_FROM_ISR()

#define taskENTER_CRITICAL_FROM_ISR()      portSET_INTERRUPT_MASK_FROM_ISR()
#define portSET_INTERRUPT_MASK_FROM_ISR()  ulPortRaiseBASEPRI()static portFORCE_INLINE uint32_t ulPortRaiseBASEPRI( void ){uint32_t ulReturn, ulNewBASEPRI = configMAX_SYSCALL_INTERRUPT_PRIORITY;__asm{/* Set BASEPRI to the max syscall priority to effect a critical* section. */
/* *INDENT-OFF* */mrs ulReturn, baseprimsr basepri, ulNewBASEPRIdsbisb
/* *INDENT-ON* */}return ulReturn;}

中断级退出临界段：taskEXIT_CRITICAL_FROM_ISR()

#define taskEXIT_CRITICAL_FROM_ISR( x )           portCLEAR_INTERRUPT_MASK_FROM_ISR( x )
#define portCLEAR_INTERRUPT_MASK_FROM_ISR( x )    vPortSetBASEPRI( x )static portFORCE_INLINE void vPortSetBASEPRI( uint32_t ulBASEPRI ){__asm{/* Barrier instructions are not used as this function is only used to* lower the BASEPRI value. */
/* *INDENT-OFF* */msr basepri, ulBASEPRI
/* *INDENT-ON* */}}

三、任务调度器的挂起和恢复

挂起任务调度器，调用此函数不需要关闭中断

函数	描述
vTaskSuspendAll()	挂起任务调度器
xTaskResumeAll()	恢复任务调度器

3.1、调用示例

vTaskSuspendAll();
{···    /* 内容 */
}
xTaskResumeAll();

1、与临界区不一样的是，挂起任务调度器，未关闭中断

2、它仅仅是防止了任务之间的资源争夺，中断照样可以直接响应

3、挂起调度器的方式，适用于临界区位于任务与任务之间

4、既不用去延时中断，又可以做到临界区的安全

3.2、内部实现

挂起任务调度器：vTaskSuspendAll()

void vTaskSuspendAll( void )
{/* A critical section is not required as the variable is of type* BaseType_t.  Please read Richard Barry's reply in the following link to a* post in the FreeRTOS support forum before reporting this as a bug! -* https://goo.gl/wu4acr *//* portSOFTWARE_BARRIER() is only implemented for emulated/simulated ports that* do not otherwise exhibit real time behaviour. */portSOFTWARE_BARRIER();/* The scheduler is suspended if uxSchedulerSuspended is non-zero.  An increment* is used to allow calls to vTaskSuspendAll() to nest. */++uxSchedulerSuspended;/* Enforces ordering for ports and optimised compilers that may otherwise place* the above increment elsewhere. */portMEMORY_BARRIER();
}

当变量 uxSchedulerSuspended 的值不为 0，将会导致 Systick 无法触发 PendSV 中断，即挂起任务调度器

恢复任务调度器：xTaskResumeAll()

BaseType_t xTaskResumeAll( void )
{TCB_t * pxTCB = NULL;BaseType_t xAlreadyYielded = pdFALSE;/* If uxSchedulerSuspended is zero then this function does not match a* previous call to vTaskSuspendAll(). */configASSERT( uxSchedulerSuspended );/* It is possible that an ISR caused a task to be removed from an event* list while the scheduler was suspended.  If this was the case then the* removed task will have been added to the xPendingReadyList.  Once the* scheduler has been resumed it is safe to move all the pending ready* tasks from this list into their appropriate ready list. */taskENTER_CRITICAL();{--uxSchedulerSuspended;if( uxSchedulerSuspended == ( UBaseType_t ) pdFALSE ){if( uxCurrentNumberOfTasks > ( UBaseType_t ) 0U ){/* Move any readied tasks from the pending list into the* appropriate ready list. */while( listLIST_IS_EMPTY( &xPendingReadyList ) == pdFALSE ){pxTCB = listGET_OWNER_OF_HEAD_ENTRY( ( &xPendingReadyList ) ); /*lint !e9079 void * is used as this macro is used with timers and co-routines too.  Alignment is known to be fine as the type of the pointer stored and retrieved is the same. */listREMOVE_ITEM( &( pxTCB->xEventListItem ) );portMEMORY_BARRIER();listREMOVE_ITEM( &( pxTCB->xStateListItem ) );prvAddTaskToReadyList( pxTCB );/* If the moved task has a priority higher than or equal to* the current task then a yield must be performed. */if( pxTCB->uxPriority >= pxCurrentTCB->uxPriority ){xYieldPending = pdTRUE;}else{mtCOVERAGE_TEST_MARKER();}}if( pxTCB != NULL ){/* A task was unblocked while the scheduler was suspended,* which may have prevented the next unblock time from being* re-calculated, in which case re-calculate it now.  Mainly* important for low power tickless implementations, where* this can prevent an unnecessary exit from low power* state. */prvResetNextTaskUnblockTime();}/* If any ticks occurred while the scheduler was suspended then* they should be processed now.  This ensures the tick count does* not  slip, and that any delayed tasks are resumed at the correct* time. */{TickType_t xPendedCounts = xPendedTicks; /* Non-volatile copy. */if( xPendedCounts > ( TickType_t ) 0U ){do{if( xTaskIncrementTick() != pdFALSE ){xYieldPending = pdTRUE;}else{mtCOVERAGE_TEST_MARKER();}--xPendedCounts;} while( xPendedCounts > ( TickType_t ) 0U );xPendedTicks = 0;}else{mtCOVERAGE_TEST_MARKER();}}if( xYieldPending != pdFALSE ){#if ( configUSE_PREEMPTION != 0 ){xAlreadyYielded = pdTRUE;}#endiftaskYIELD_IF_USING_PREEMPTION();}else{mtCOVERAGE_TEST_MARKER();}}}else{mtCOVERAGE_TEST_MARKER();}}taskEXIT_CRITICAL();return xAlreadyYielded;
}

当变量 uxSchedulerSuspended 的值等于 0，则允许调度

1、当任务数量大于 0 时，恢复调度器才有意义，如果没有一个已创建的任务就无意义

2、移除等待就绪列表中的列表项，恢复至就绪列表，直到 xPendingReadyList 列表为空

3、如果恢复的任务优先级比当前正在执行任务优先级更高，则将 xYieldPending 赋值为 pdTRUE，表示需要进行一次任务切换

4、在调度器被挂起的期间内，是否有丢失未处理的滴答数。xPendedTicks 是丢失的滴答数，有则调用 xTaskIncrementTick() 补齐丢失的滴答数

5、判断是否允许任务切换

6、返回任务是否已经切换，已经切换返回 pdTRUE，反之返回 pdFALSE