RTEMS: Processor Dependent Context Management

| RTEMS 5.4-rc2

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Processor Dependent Context Management
Internal » SuperCore » CPU Architecture Support » Example

Destroys the context of the thread.

Destroys the context of the thread.

Initializes and validates the CPU context with values derived from the pattern parameter.

Clobbers all volatile registers with values derived from the pattern parameter.

From the highest level viewpoint, there are 2 types of context to save.

1. Interrupt registers to save
2. Task level registers to save

Since RTEMS handles integer and floating point contexts separately, this means we have the following 3 context items:

1. task level context stuff:: Context_Control
2. floating point task stuff:: Context_Control_fp
3. special interrupt level context :: CPU_Interrupt_frame

On some processors, it is cost-effective to save only the callee preserved registers during a task context switch. This means that the ISR code needs to save those registers which do not persist across function calls. It is not mandatory to make this distinctions between the caller/callee saves registers for the purpose of minimizing context saved during task switch and on interrupts. If the cost of saving extra registers is minimal, simplicity is the choice. Save the same context on interrupt entry as for tasks in this case.

Additionally, if gdb is to be made aware of RTEMS tasks for this CPU, then care should be used in designing the context area.

On some CPUs with hardware floating point support, the Context_Control_fp structure will not be used or it simply consist of an array of a fixed number of bytes. This is done when the floating point context is dumped by a "FP save context" type instruction and the format is not really defined by the CPU. In this case, there is no need to figure out the exact format – only the size. Of course, although this is enough information for RTEMS, it is probably not enough for a debugger such as gdb. But that is another problem.

Port Specific Information:

XXX document implementation including references if appropriate

The size of the floating point context area. On some CPUs this will not be a "sizeof" because the format of the floating point area is not defined – only the size is. This is usually on CPUs with a "floating point save context" instruction.

Port Specific Information:

XXX document implementation including references if appropriate

Should be large enough to run all RTEMS tests. This ensures that a "reasonable" small application should not have any problems.

Port Specific Information:

XXX document implementation including references if appropriate

It must be implemented as a macro and an implementation is optional. The default implementation does nothing.

Parameters

  [in] | _the_thread | The corresponding thread.
  [in] | _the_context | The context to destroy.

Port Specific Information:

XXX document implementation including references if appropriate

Initialize the context to a state suitable for starting a task after a context restore operation. Generally, this involves:

- setting a starting address
- preparing the stack
- preparing the stack and frame pointers
- setting the proper interrupt level in the context
- initializing the floating point context

This routine generally does not set any unnecessary register in the context. The state of the "general data" registers is undefined at task start time.

The ISR dispatch disable field of the context must be cleared to zero if it is used by the CPU port. Otherwise, a thread restart results in unpredictable behaviour.

Parameters

  [in] | _the_context | is the context structure to be initialized
  [in] | _stack_base | is the lowest physical address of this task's stack
  [in] | _size | is the size of this task's stack
  [in] | _isr | is the interrupt disable level
  [in] | _entry_point | is the thread's entry point. This is always _Thread_Handler
  [in] | _is_fp | is TRUE if the thread is to be a floating point thread. This is typically only used on CPUs where the FPU may be easily disabled by software such as on the SPARC where the PSR contains an enable FPU bit.
  [in] | _tls_area | The thread-local storage (TLS) area.

Port Specific Information:

XXX document implementation including references if appropriate

This routine switches from the run context to the heir context.

Parameters

  [in] | run | points to the context of the currently executing task
  [in] | heir | points to the context of the heir task

Port Specific Information:

XXX document implementation including references if appropriate

This routine is generally used only to restart self in an efficient manner. It may simply be a label in _CPU_Context_switch.

Parameters

  [in] | new_context | points to the context to be restored.

NOTE: May be unnecessary to reload some registers.

Port Specific Information:

XXX document implementation including references if appropriate

This routine saves the floating point context passed to it.

Parameters

  [in] | fp_context_ptr | is a pointer to a pointer to a floating point context area

Returns
  on output *fp_context_ptr will contain the address that should be used with _CPU_Context_restore_fp to restore this context.

Port Specific Information:

XXX document implementation including references if appropriate

This routine restores the floating point context passed to it.

Parameters

  [in] | fp_context_ptr | is a pointer to a pointer to a floating point context area to restore

Returns
  on output *fp_context_ptr will contain the address that should be used with _CPU_Context_save_fp to save this context.

Port Specific Information:

XXX document implementation including references if appropriate

This function is used only in test sptests/spcontext01.

Parameters

  [in] | pattern | Pattern used to generate distinct register values.

See also
  _CPU_Context_validate().

This function will not return if the CPU context remains consistent. In case this function returns the CPU port is broken.

This function is used only in test sptests/spcontext01.

Parameters

  [in] | pattern | Pattern used to generate distinct register values.

See also
  _CPU_Context_volatile_clobber().