FreeRTOS的任务调度

1.启动任务调度器 vTaskStartScheduler

c 复制代码
void vTaskStartScheduler( void )
{
BaseType_t xReturn;

	/* Add the idle task at the lowest priority. */
	#if ( INCLUDE_xTaskGetIdleTaskHandle == 1 )
	{
		/* Create the idle task, storing its handle in xIdleTaskHandle so it can
		be returned by the xTaskGetIdleTaskHandle() function. */
		xReturn = xTaskCreate( prvIdleTask, "IDLE", tskIDLE_STACK_SIZE, ( void * ) NULL, ( tskIDLE_PRIORITY | portPRIVILEGE_BIT ), &xIdleTaskHandle ); /*lint !e961 MISRA exception, justified as it is not a redundant explicit cast to all supported compilers. */
	}
	#else
	{
		/* Create the idle task without storing its handle. */
		 /* 动态创建空闲任务 */
		xReturn = xTaskCreate( prvIdleTask, "IDLE", tskIDLE_STACK_SIZE, ( void * ) NULL, ( tskIDLE_PRIORITY | portPRIVILEGE_BIT ), NULL );  /*lint !e961 MISRA exception, justified as it is not a redundant explicit cast to all supported compilers. */
	}
	#endif /* INCLUDE_xTaskGetIdleTaskHandle */

	#if ( configUSE_TIMERS == 1 )
	{
		if( xReturn == pdPASS )
		{
			xReturn = xTimerCreateTimerTask();
		}
		else
		{
			mtCOVERAGE_TEST_MARKER();
		}
	}
	#endif /* configUSE_TIMERS */

	if( xReturn == pdPASS )
	{
		/* Interrupts are turned off here, to ensure a tick does not occur
		before or during the call to xPortStartScheduler().  The stacks of
		the created tasks contain a status word with interrupts switched on
		so interrupts will automatically get re-enabled when the first task
		starts to run. */
		portDISABLE_INTERRUPTS(); /* 关闭中断 */

		#if ( configUSE_NEWLIB_REENTRANT == 1 )
		{
			/* Switch Newlib's _impure_ptr variable to point to the _reent
			structure specific to the task that will run first. */
			_impure_ptr = &( pxCurrentTCB->xNewLib_reent );
		}
		#endif /* configUSE_NEWLIB_REENTRANT */

		xNextTaskUnblockTime = portMAX_DELAY;  /* 时间最大 */
		xSchedulerRunning = pdTRUE;      /* 调度运行标志 */
		xTickCount = ( TickType_t ) 0U;  /* tick 初始化为 0 */

		/* If configGENERATE_RUN_TIME_STATS is defined then the following
		macro must be defined to configure the timer/counter used to generate
		the run time counter time base. */
		portCONFIGURE_TIMER_FOR_RUN_TIME_STATS();

		/* Setting up the timer tick is hardware specific and thus in the
		portable interface. */
		 /* 启动调度器 */
		if( xPortStartScheduler() != pdFALSE )
		{
			/* Should not reach here as if the scheduler is running the
			function will not return. */
		}
		else
		{
			/* Should only reach here if a task calls xTaskEndScheduler(). */
		}
	}
	else
	{
		/* This line will only be reached if the kernel could not be started,
		because there was not enough FreeRTOS heap to create the idle task
		or the timer task. */
		configASSERT( xReturn );
	}
}

2. xPortStartScheduler

c 复制代码
BaseType_t xPortStartScheduler( void )
{
	/* configMAX_SYSCALL_INTERRUPT_PRIORITY must not be set to 0.
	See http://www.FreeRTOS.org/RTOS-Cortex-M3-M4.html */
	configASSERT( configMAX_SYSCALL_INTERRUPT_PRIORITY );

	/* This port can be used on all revisions of the Cortex-M7 core other than
	the r0p1 parts.  r0p1 parts should use the port from the
	/source/portable/GCC/ARM_CM7/r0p1 directory. */
	configASSERT( portCPUID != portCORTEX_M7_r0p1_ID );
	configASSERT( portCPUID != portCORTEX_M7_r0p0_ID );

	#if( configASSERT_DEFINED == 1 )
	{
		volatile uint32_t ulOriginalPriority;
		volatile uint8_t * const pucFirstUserPriorityRegister = ( uint8_t * ) ( portNVIC_IP_REGISTERS_OFFSET_16 + portFIRST_USER_INTERRUPT_NUMBER );
		volatile uint8_t ucMaxPriorityValue;

		/* Determine the maximum priority from which ISR safe FreeRTOS API
		functions can be called.  ISR safe functions are those that end in
		"FromISR".  FreeRTOS maintains separate thread and ISR API functions to
		ensure interrupt entry is as fast and simple as possible.

		Save the interrupt priority value that is about to be clobbered. */
		ulOriginalPriority = *pucFirstUserPriorityRegister;

		/* Determine the number of priority bits available.  First write to all
		possible bits. */
		*pucFirstUserPriorityRegister = portMAX_8_BIT_VALUE;

		/* Read the value back to see how many bits stuck. */
		ucMaxPriorityValue = *pucFirstUserPriorityRegister;

		/* The kernel interrupt priority should be set to the lowest
		priority. */
		configASSERT( ucMaxPriorityValue == ( configKERNEL_INTERRUPT_PRIORITY & ucMaxPriorityValue ) );

		/* Use the same mask on the maximum system call priority. */
		ucMaxSysCallPriority = configMAX_SYSCALL_INTERRUPT_PRIORITY & ucMaxPriorityValue;

		/* Calculate the maximum acceptable priority group value for the number
		of bits read back. */
		ulMaxPRIGROUPValue = portMAX_PRIGROUP_BITS;
		while( ( ucMaxPriorityValue & portTOP_BIT_OF_BYTE ) == portTOP_BIT_OF_BYTE )
		{
			ulMaxPRIGROUPValue--;
			ucMaxPriorityValue <<= ( uint8_t ) 0x01;
		}

		/* Shift the priority group value back to its position within the AIRCR
		register. */
		
		ulMaxPRIGROUPValue <<= portPRIGROUP_SHIFT;
		ulMaxPRIGROUPValue &= portPRIORITY_GROUP_MASK;

		/* Restore the clobbered interrupt priority register to its original
		value. */
		*pucFirstUserPriorityRegister = ulOriginalPriority;
	}
	#endif /* conifgASSERT_DEFINED */

	/* Make PendSV and SysTick the lowest priority interrupts. */
	/* 为了保证系统的实时性,配置systick和pendsv为最低的优先级 */
	portNVIC_SYSPRI2_REG |= portNVIC_PENDSV_PRI;
	portNVIC_SYSPRI2_REG |= portNVIC_SYSTICK_PRI;

	/* Start the timer that generates the tick ISR.  Interrupts are disabled
	here already. */
	/* 配置滴答定时器systick的定时周期,并开启systick中断 */
	vPortSetupTimerInterrupt();

	/* Initialise the critical nesting count ready for the first task. */
	uxCriticalNesting = 0;

	/* Ensure the VFP is enabled - it should be anyway. */
	prvEnableVFP();

	/* Lazy save always. */
	*( portFPCCR ) |= portASPEN_AND_LSPEN_BITS;

	/* Start the first task. */
	 /* 启动第一个任务 */
	prvStartFirstTask();

	/* Should not get here! */
	return 0;
}

3.prvStartFirstTask

c 复制代码
__asm void prvStartFirstTask( void )
{
	PRESERVE8

	/* Use the NVIC offset register to locate the stack. */
	ldr r0, =0xE000ED08
	ldr r0, [r0]
	ldr r0, [r0]
	/* Set the msp back to the start of the stack. */
	msr msp, r0
	/* Globally enable interrupts. */
	cpsie i
	cpsie f
	dsb
	isb
	/* Call SVC to start the first task. */

	 /* 调用SVC */
	svc 0
	nop
	nop
}

4.vPortSVCHandler

c 复制代码
__asm void vPortSVCHandler( void )
{
	PRESERVE8

	/* Get the location of the current TCB. */
	/* 获取当前任务控制块 */
	ldr	r3, =pxCurrentTCB
	ldr r1, [r3]
	ldr r0, [r1]
	/* Pop the core registers. */
	ldmia r0!, {r4-r11, r14}
	msr psp, r0
	isb
	mov r0, #0
	msr	basepri, r0
	bx r14
}

在prvStartFirstTask()中通过调用SVC指令触发了SVC中断,而第一个任务的启动就是在SVC中断服务函数中完成的

5. xPortPendSVHandler

c 复制代码
__asm void xPortPendSVHandler( void )
{
	extern uxCriticalNesting;
	extern pxCurrentTCB;
	extern vTaskSwitchContext;

	PRESERVE8

	mrs r0, psp
	isb
	/* Get the location of the current TCB. */
	ldr	r3, =pxCurrentTCB
	ldr	r2, [r3]

	/* Is the task using the FPU context?  If so, push high vfp registers. */
	tst r14, #0x10
	it eq
	vstmdbeq r0!, {s16-s31}

	/* Save the core registers. */
	stmdb r0!, {r4-r11, r14}

	/* Save the new top of stack into the first member of the TCB. */
	str r0, [r2]

	stmdb sp!, {r3}
	mov r0, #configMAX_SYSCALL_INTERRUPT_PRIORITY
	msr basepri, r0
	dsb
	isb
	bl vTaskSwitchContext
	mov r0, #0
	msr basepri, r0
	ldmia sp!, {r3}

	/* The first item in pxCurrentTCB is the task top of stack. */
	ldr r1, [r3]
	ldr r0, [r1]

	/* Pop the core registers. */
	ldmia r0!, {r4-r11, r14}

	/* Is the task using the FPU context?  If so, pop the high vfp registers
	too. */
	tst r14, #0x10
	it eq
	vldmiaeq r0!, {s16-s31}

	msr psp, r0
	isb
	#ifdef WORKAROUND_PMU_CM001 /* XMC4000 specific errata */
		#if WORKAROUND_PMU_CM001 == 1
			push { r14 }
			pop { pc }
			nop
		#endif
	#endif

	bx r14
}
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