[PATCH 2/4] score: SMP initialization and shutdown changes

[Chris Johns]

On 20/02/2014 12:42 am, Sebastian Huber wrote:
> +/**
> + * @brief State of a processor.
> + *
> + * @dot
> + * digraph states {
> + *   bi [label="PER_CPU_STATE_BEFORE_INITIALIZATION"];
> + *   rsm [label="PER_CPU_STATE_READY_TO_START_MULTITASKING"];
> + *   sm [label="PER_CPU_STATE_START_MULTITASKING"];
> + *   ds [label="PER_CPU_STATE_DO_SHUTDOWN"];
> + *   u [label="PER_CPU_STATE_UP"];
> + *   s [label="PER_CPU_STATE_SHUTDOWN"];
> + *   bi -> rsm [label="secondary processor\ncompleted initialization"];
> + *   bi -> u [label="main processor\nstarts multitasking"];
> + *   rsm -> sm [label="main processor\ncompleted initialization"];
> + *   rsm -> ds [label="a fatal error occurred"];
> + *   ds -> s [label="do shutdown\nstate observed"];
> + *   sm -> u [label="secondary processor\nstarts multitasking"];
> + *   u -> s [label="shutdown initiated"];
> + * }
> + * @enddot

I do not see sm to ds if the main core goes to s after going from bi to u ?

I also see u to s and not to ds; why as the state is called "do 
shutdown" ? Should this state be PER_CPU_STATE_SHUTTING_DOWN ?

I do not see why we have main and secondary processors ? This is 
symmetric multiprocessing which means each core is the same therefore 
capable of completing any required task. I understand there are paths 
which need to complete so if we have states for these phases as gates 
then any processor that arrives should be able to enter the gate 
(spinning lock for those that need to wait) and complete the work. This 
means a degraded state can exist and things at least start. The 
application would need to detect and manage the degraded state and so 
RTEMS should not be concerned with this condition other than doing its 
best to run where possible.

I assume if we have n cores where n is 1..cpus available we enter the 
static constructors once and 'main' [1] once and this independent of the 
number of defined and/or operating cores. If an application's static 
constructor starts further threads it needs to manage the concurrency 
issues and main is only entered once.

[1] I am not concerned with the Classic API init task tables with more 
than one init task and what happens with static constructors as this is 
not defined by any standard. I will never directly use init task tables 
because it is not portable. All we can do is make sure static 
constructors and destructors are only called once.

> + *
> + * The values are chosen to stop the wait for change in case an alternative
  ... to and wait ... ?
> + * state change occurs during _Per_CPU_Wait_for_state().

Chris