source: soft/giet_vm/sys/ctx_handler.c @ 159

///////////////////////////////////////////////////////////////////////////////////
// File     : ctx_handler.c
// Date     : 01/04/2012
// Authors  : alain greiner & joel porquet
// Copyright (c) UPMC-LIP6
////////////////////////////////////////////////////////////////////////////////////
// The ctx_handler.h and ctx_handler.c files are part of the GIET nano-kernel.
// This code is used to support context switch when several tasks are executing
// in time multiplexing on a single processor.
// The tasks must be statically allocated to a processor in the boot phase, and
// there is one private scheduler per processor:  NB_CLUSTERS * NB_PROCS
// Each sheduler contains up to NB_TASKS_MAX contexts.
////////////////////////////////////////////////////////////////////////////////////
// A task context is an array of 64 words = 256 bytes. 
// It contains copies of processor registers, when the task is not running,
// and some general informations associated to the task.
// - It contains GPR[i], generally stored in slot (i). $0, *26 & $27 are not saved.
// - It contains HI & LO registers.
// - It contains CP0 registers: EPC, SR, CR.
// - It contains CP2 registers : PTPR and MODE.
// - It contains the TTY index for the terminal allocated to the task.
// ctx[0] <- SR   ctx[8] <- $8    ctx[16]<- $16   ctx[24]<- $24   ctx[32]<- EPC
// ctx[1] <- $1   ctx[9] <- $9    ctx[17]<- $17   ctx[25]<- $25   ctx[33]<- CR
// ctx[2] <- $2   ctx[10]<- $10   ctx[18]<- $18   ctx[26]<- LO    ctx[34]<- TTY
// ctx[3] <- $3   ctx[11]<- $11   ctx[19]<- $19   ctx[27]<- HI    ctx[35]<- PTPR
// ctx[4] <- $4   ctx[12]<- $12   ctx[20]<- $20   ctx[28]<- $28   ctx[36]<- MODE
// ctx[5] <- $5   ctx[13]<- $13   ctx[21]<- $21   ctx[29]<- $29   ctx[37]<- reserved
// ctx[6] <- $6   ctx[14]<- $14   ctx[22]<- $22   ctx[30]<- $30   ctx[38]<- reserved
// ctx[7] <- $7   ctx[15]<- $15   ctx[23]<- $23   ctx[31]<- $31   ctx[39]<- reserved
/////////////////////////////////////////////////////////////////////////////////////

#include <giet_config.h>
#include <ctx_handler.h>
#include <sys_handler.h>
#include <drivers.h>
#include <common.h>

extern void _task_switch(unsigned int *, unsigned int *);

/////////////////////////////////////////////////////////////////////////////////
//      Global variables
/////////////////////////////////////////////////////////////////////////////////

static_scheduler_t _scheduler[NB_CLUSTERS * NB_PROCS];

/////////////////////////////////////////////////////////////////////////////////
//      _ctx_switch()
// This function performs a context switch between the running task
// and  another task, using a round-robin sheduling policy.
// It use the global variable scheduler[] : array indexed by the procid,
// that contains NB_CLUSTERS * NB_PROCS entries.
// The return address contained in $31 is saved in the _current task context
// (in the ctx[31] slot), and the function actually returns to the address
// contained in the ctx[31] slot of the new task context. To perform the
// actual switch, it calls the _task_switch function written in assembly language.
/////////////////////////////////////////////////////////////////////////////////
void _ctx_switch()
{
    unsigned char curr_task_id;
    unsigned char next_task_id;

    unsigned int *curr_context;
    unsigned int *next_context;

    unsigned int tasks;
    unsigned int proc_id;

    proc_id = _procid();
    tasks   = _scheduler[proc_id].tasks;

    // return if only one task  */
    if ( tasks <= 1) return;

    // compute the task context base address for the current task
    curr_task_id = _scheduler[proc_id].current;
    curr_context = &(_scheduler[proc_id].context[curr_task_id][0]);

    // select the next task using a round-robin scheduling policy
    next_task_id = (curr_task_id + 1) % tasks;
    
    // compute the task context base address for the next task
    next_context = &(_scheduler[proc_id].context[next_task_id][0]);

    //  update the scheduler state, and makes the task switch
    _scheduler[proc_id].current = next_task_id;
    _task_switch( curr_context, next_context );

#if GIET_DEBUG_SWITCH
unsigned int time = _proctime();
_tty_puts( "\n[GIET] Context switch for processor ");
_tty_putw( proc_id );
_tty_puts( " at cycle ");
_tty_putw( time );
_tty_puts("\n");
_tty_puts( " - tasks        = ");
_tty_putw( tasks );
_tty_puts("\n");
_tty_puts( " - curr_task_id = ");
_tty_putw( curr_task_id );
_tty_puts("\n");
_tty_puts( " - next_task_id = ");
_tty_putw( next_task_id );
_tty_puts("\n");
#endif

} // end _ctx_switch