source: rtems-docs/porting/idle_thread.rst @ 489740f

Last change on this file since 489740f was 489740f, checked in by Chris Johns <chrisj@…>, on May 20, 2016 at 2:47:09 AM

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IDLE Thread

Does Idle Thread Have a Floating Point Context?

The setting of the macro CPU_IDLE_TASK_IS_FP is based on the answer to the question: Should the IDLE task have a floating point context? If the answer to this question is TRUE, then the IDLE task has a floating point context associated. This is equivalent to creating a task in the Classic API (using rtems_task_create) as a RTEMS_FLOATING_POINT task. If CPU_IDLE_TASK_IS_FP is set to TRUE, then a floating point context switch occurs when the IDLE task is switched in and out. This adds to the execution overhead of the system but is necessary on some ports.

If FALSE, then the IDLE task does not have a floating point context.

NOTE: Setting CPU_IDLE_TASK_IS_FP to TRUE negatively impacts the time required to preempt the IDLE task from an interrupt because the floating point context must be saved as part of the preemption.

The following illustrates how to set this macro: .. code:: c


CPU Dependent Idle Thread Body


The CPU_PROVIDES_IDLE_THREAD_BODY macro setting is based upon the answer to the question: Does this port provide a CPU dependent IDLE task implementation? If the answer to this question is yes, then the CPU_PROVIDES_IDLE_THREAD_BODY macro should be set to TRUE, and the routine _CPU_Thread_Idle_body must be provided. This routine overrides the default IDLE thread body of _Thread_Idle_body. If the CPU_PROVIDES_IDLE_THREAD_BODY macro is set to FALSE, then the generic _Thread_Idle_body is the default IDLE thread body for this port. Regardless of whether or not a CPU dependent IDLE thread implementation is provided, the BSP can still override it.

This is intended to allow for supporting processors which have a low power or idle mode. When the IDLE thread is executed, then the CPU can be powered down when the processor is idle.

The order of precedence for selecting the IDLE thread body is:

# BSP provided

# CPU dependent (if provided)

# generic (if no BSP and no CPU dependent)

The following illustrates setting the CPU_PROVIDES_IDLE_THREAD_BODY macro: .. code:: c


Implementation details of a CPU model specific IDLE thread body are in the next section.

Idle Thread Body

The _CPU_Thread_Idle_body routine only needs to be provided if the porter wishes to include a CPU dependent IDLE thread body. If the port includes a CPU dependent implementation of the IDLE thread body, then the CPU_PROVIDES_IDLE_THREAD_BODY macro should be defined to TRUE. This routine is prototyped as follows: .. code:: c

void *_CPU_Thread_Idle_body( uintptr_t );

As mentioned above, RTEMS does not require that a CPU dependent IDLE thread body be provided as part of the port. If CPU_PROVIDES_IDLE_THREAD_BODY is defined to FALSE, then the CPU independent algorithm is used. This algorithm consists of a "branch to self" which is implemented in a routine as follows. .. code:: c

void *_Thread_Idle_body( uintptr_t ignored ) { while( 1 ) ; }

If the CPU dependent IDLE thread body is implementation centers upon using a "halt", "idle", or "shutdown" instruction, then don't forget to put it in an infinite loop as the CPU will have to reexecute this instruction each time the IDLE thread is dispatched. .. code:: c

void *_CPU_Thread_Idle_body( uintptr_t ignored ) { for( ; ; ) /* insert your "halt" instruction here */ ; }

Be warned. Some processors with onboard DMA have been known to stop the DMA if the CPU were put in IDLE mode. This might also be a problem with other on-chip peripherals. So use this hook with caution.

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