From 33cd2d43a18851aafdcb6bbdeebab679d8ab1c86 Mon Sep 17 00:00:00 2001 From: Joel Sherrill Date: Wed, 6 Oct 1999 19:36:28 +0000 Subject: New manual. First version to CVS. Just starting to see if it builds. --- doc/porting/idlethread.t | 108 +++++++++++++++++++++++++++++++++++++++++++++++ 1 file changed, 108 insertions(+) create mode 100644 doc/porting/idlethread.t (limited to 'doc/porting/idlethread.t') diff --git a/doc/porting/idlethread.t b/doc/porting/idlethread.t new file mode 100644 index 0000000000..98632147fa --- /dev/null +++ b/doc/porting/idlethread.t @@ -0,0 +1,108 @@ +@chapter IDLE Thread + +@section 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: + +@example +#define CPU_IDLE_TASK_IS_FP FALSE +@end example + +@section CPU Dependent Idle Thread Body + +@subsection CPU_PROVIDES_IDLE_THREAD_BODY Macro Setting + +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: + +@enumerate +@item BSP provided + +@item CPU dependent (if provided) + +@item generic (if no BSP and no CPU dependent) + +@end itemize + +The following illustrates setting the CPU_PROVIDES_IDLE_THREAD_BODY macro: + +@example +#define CPU_PROVIDES_IDLE_THREAD_BODY TRUE +@end example + +Implementation details of a CPU model specific IDLE thread body are in the +next section. + +@subsection 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: + +@example +void _CPU_Thread_Idle_body( void ); +@end example + +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 the XXX routine as follows. + +@example +XXX check name and exact implementation +void _Thread_Idle_body( void ) +@{ + while( 1 ) ; +@} +@end example + +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. + +@example +void _CPU_Thread_Idle_body( void ) +@{ + + for( ; ; ) + /* insert your "halt" instruction here */ ; +@} +@end example + +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. + -- cgit v1.2.3