sched.c

/*
Task scheduling.
*/

#include "defs.h"

#define MAX_TASKS 10
#define STACK_SIZE 1024

uint8 task_stacks[MAX_TASKS][STACK_SIZE];
Context tasks[MAX_TASKS];
Context os_context;
Context *current_context;

int _num_of_tasks = 0;  // number of tasks
int _current_task = -1; // current task id, init to -1 if no task

/*
Initialize scheduler.
*/
void sched_init(void)
{
    // initialize `mscratch` to 0
    write_mscratch(0);

    // enable machine mode software interrupts
    write_mie(read_mie() | MIE_MSIE);
}

/*
Sign up a user task.
Return -1 means fail.
*/
int task_create(void *routine_entry)
{
    // return 0 - success
    // return -1 - fail

    if (_num_of_tasks < MAX_TASKS)
    {
        tasks[_num_of_tasks].sp = (regis)&task_stacks[_num_of_tasks][STACK_SIZE - 1];
        tasks[_num_of_tasks].pc = (regis)routine_entry;
        _num_of_tasks++;
        return 0;
    }
    return -1;
}

/*
User task hand out control to OS actively.
AKA cooperative multitasking.
*/
void task_yield(void)
{
    uint32 id = read_mhartid();
    // write clint.msip to 1
    write_clint_msip_one(id);
    return;
}

/*
Scheduler to decide what task to run next.
Go through all tasks iteratively and serves FIFO.
*/
void task_scheduler(void)
{
    if (_num_of_tasks <= 0)
    {
        panic("No task is running yet!");
        return;
    }
    _current_task = (_current_task + 1) % _num_of_tasks;
    Context *next = &(tasks[_current_task]);
    switch_to(next);
}