FreeRTOS的任务调度
1.启动任务调度器 vTaskStartScheduler
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
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
__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
__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
__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
}