xTaskNotifyGive issue - basic usage

Kernel
1

nndpkz wrote on Thursday, June 07, 2018:

Hi guys,

I am trying to implement example from 9.3 chapter from Mastering The FreeRTOS Real-Time Kernel book. I am using ZYNQ-7000. Now, since I didn’t check how to simulate software interrupts on Zynq, I just decided to simply call a function from which I will call xTaskNotifyGive. This is just an illustration example. This is what I have:

int main( void )
{
	time_t xTimeNow;

	xil_printf( "Hello world!\n\r" );

	time( &xTimeNow );
	prvSRand( (uint32_t)xTimeNow );

	vPortInstallFreeRTOSVectorTable( );

	Xil_DCacheEnable( );

	xTaskCreate( vPlaybackTask, "Playback", 1024, NULL, configMAX_PRIORITIES - 4, NULL );
	xTaskCreate( vPeriodicTask, "Periodic", 1024, NULL, configMAX_PRIORITIES - 6, NULL );

	/* Start the RTOS scheduler. */
	xil_printf( "vTaskStartScheduler\n" );
	vTaskStartScheduler( );

	return 0;
}

static
void vExampleInterruptServiceRoutine( void )
{
	xTaskNotifyGive( vPlaybackTask );
}

static
void vPeriodicTask( void *pvParameters )
{
	while( 1 )
	{
		vTaskDelay( xInterruptFrequency );
		xil_printf("Periodic task - About to generate / simulate an interrupt!\n");
		vExampleInterruptServiceRoutine();
		xil_printf("Periodic task - Interrupt generated!\n");
	}
}

static
void vPlaybackTask( void *pvParameters )
{
	const TickType_t xMaxExpectedBlockTime = xInterruptFrequency + pdMS_TO_TICKS( 10 );
	uint32_t ulEventsToProcess = 0;

	while( 1 )
	{
		ulEventsToProcess  = ulTaskNotifyTake( pdTRUE, xMaxExpectedBlockTime  );
		if( ulEventsToProcess != 0 )
		{
			while( ulEventsToProcess > 0 )
			{
				xil_printf( "Playback task -> processing -> run DMA.\n" );
				ulEventsToProcess--;
			}
		}
	}
}

however, ulTaskNotifyTake never returns a value different from 0. I have also checked that xTaskNotifyGive() is actually called.

Maybe I am missing something obvious here?

Best regards,
Nenad

P.S. The same example from Chapter 6.4 which uses binary semaphore instead of Task Notify is working as expected.

2

rtel wrote on Thursday, June 07, 2018:

I think this recent thread:
https://sourceforge.net/p/freertos/discussion/382005/thread/543fb83e/
showed xil_printf() may not be thread safe and may cause problems (which
is why we normally serialse logging by only calling printf() form a
single task) so in this case it might be that xil_printf() is taking
longer to execute than you are allowing with the allowable margin you
have set using pdMS_TO_TICKS( 10 ). Try increasing to pdMS_TO_TICKS(
1000 ) as a sanity check.

3

nndpkz wrote on Thursday, June 07, 2018:

Hi Richard,

yes, I remember the xil_printf() issue, however, since this example worked with binary semaphore, I was thinking that something else is problem.
I forgot to mention that I already tried to set to portMAX_DELAY, but ulTaskNotifyTake() never returned. I will examine xil_printf() again.

Thanks.

4

rtel wrote on Thursday, June 07, 2018:

Ah - I see the problem.

The parameter to xTaskNotifyGive() is supposed to be the handle of the
task the notification is sent to. The parameter you are passing is a
pointer to the function that implements the task, not the task’s handle.

The last parameter in your call to:

xTaskCreate( vPlaybackTask, “Playback”, 1024, NULL, configMAX_PRIORITIES

  • 4, NULL );

is null so you never save the task’s handle. Do something like this:

// File scope
static TaskHandle_t PlaybackHandle;

// Update call to xTaskCreate to save the handle.
xTaskCreate( vPlaybackTask, “Playback”, 1024, NULL, configMAX_PRIORITIES

  • 4, &PlaybackHandle );

// Update call to xTaskNotifyGive() to use the handle.
xTaskNotifyGive( PlaybackHandle );

5

nndpkz wrote on Friday, June 08, 2018:

Yes, that was a problem. I knew I was missing something obvious. But, I also thought some assert would fail if something like this happens.

All in all, thank you very much for your help!