ecos kernel 分析

    ecos kernel 是个典型的抢占式多任务的rtos,我这里想从代码上，把它的实现搭个框架出来。
    分时的多任务系统是靠定时时间中断实现的，所以我从这里做切入点
有kernel 的ecos重写了interrupt 处理代码，原来的在drv_api.c里实现的，现在的在kernel/intr/intr.cxx里，时间中断的注册在kernel/common/clock.cxx里

Cyg_RealTimeClock Cyg_RealTimeClock::rtc CYG_INIT_PRIORITY( CLOCK );

// -------------------------------------------------------------------------

Cyg_RealTimeClock::Cyg_RealTimeClock()
    : Cyg_Clock(rtc_resolution),
      interrupt(CYGNUM_HAL_INTERRUPT_RTC,
                CYGNUM_KERNEL_COUNTERS_CLOCK_ISR_PRIORITY,
                (CYG_ADDRWORD)this, isr, dsr)
{
    CYG_REPORT_FUNCTION();

    HAL_CLOCK_INITIALIZE( CYGNUM_KERNEL_COUNTERS_RTC_PERIOD );
    
    interrupt.attach();
    interrupt.unmask_interrupt(CYGNUM_HAL_INTERRUPT_RTC);

    Cyg_Clock::real_time_clock = this;
}
中断的注册很好理解，但这里有个有趣的是这个函数是怎样被调用到的，直接搜索ecos所有的代码是找不到的。一般我们有个概念c++的类在声明后就会被自动调用里面和自己名字一样的那个函数，（很久没有接触c++,忘记叫什么名字了）
这里也是这样，这里第一句就是声明这个实例,然后编译器会把这个函数放到一个特殊的段__CTOR_LIST__里面（target.ld),
然后cyg_hal_invoke_constructors()会遍历__CTOR_LIST__并执行所有的函数，cyg_hal_invoke_constructors() 是在vector.S里面被调用到的。这个“自动调用”就是这样实现的。
    再看时间中断服务程序，ecos 把中断服务分为两块ISR和DSR，ISR里只做些最简单的事情，发生中断后会被直接调到，以保证kernel快速响应的效果。把其他的事情都放到DSR里面，DSR会被稍后调用，先看DSR里面代码
// -------------------------------------------------------------------------
void Cyg_RealTimeClock::dsr(cyg_vector vector, cyg_ucount32 count, CYG_ADDRWORD data)
{
//    CYG_REPORT_FUNCTION();

    Cyg_RealTimeClock *rtc = (Cyg_RealTimeClock *)data;

    CYG_INSTRUMENT_CLOCK( TICK_START,
                          rtc->current_value_lo(),
                          rtc->current_value_hi());
>>这里是提供系统时钟
    rtc->tick( count );
#ifdef CYGSEM_KERNEL_SCHED_TIMESLICE
#if    0 == CYGINT_KERNEL_SCHEDULER_UNIQUE_PRIORITIES

    // If timeslicing is enabled, call the scheduler to
    // handle it. But not if we have unique priorities.
>>分时多任务的处理,它的实现在算法里，我以mlqueue为例
    Cyg_Scheduler::scheduler.timeslice();

#endif
#endif

    CYG_INSTRUMENT_CLOCK( TICK_END,
                          rtc->current_value_lo(),
                          rtc->current_value_hi());
   
}

timeslice()调用timeslice_cpu(),timeslice_cpu里要找出同一priority任务队列中下一个任务,如果有，则设置reschedule的标志:需要做任务切换。
到这里这条路就断了。但是前面我没有讲到DSR是怎样被调到的,这里要看interrupt_end()
在vector.S里被调到，interrupt_end代码在kernel/intr/intr.cxx里

//-------------------------------------
externC void
interrupt_end(
    cyg_uint32          isr_ret,
    Cyg_Interrupt       *intr,
    HAL_SavedRegisters  *regs
    )
{
//    CYG_REPORT_FUNCTION();

#ifdef CYGPKG_KERNEL_SMP_SUPPORT
    Cyg_Scheduler::lock();
#endif
   
    // Sometimes we have a NULL intr object pointer.
    cyg_vector vector = (intr!=NULL)?intr->vector:0;

    CYG_INSTRUMENT_INTR(END, vector, isr_ret);
   
    CYG_UNUSED_PARAM( cyg_vector, vector ); // prevent compiler warning
   
#ifndef CYGIMP_KERNEL_INTERRUPTS_CHAIN

    // Only do this if we are in a non-chained configuration.
    // If we are chained, then chain_isr below will do the DSR
    // posting.
>>这里把当前的DSR post出去，其实就是加入一个DSR 任务链表里去，之后再拿出来处理
    if( isr_ret & Cyg_Interrupt::CALL_DSR && intr != NULL ) intr->post_dsr();

#endif   

  
    // Now unlock the scheduler, which may also call DSRs
    // and cause a thread switch to happen.
>>这里就是多任务处理的入口了,下面再去看里面的实现
    Cyg_Scheduler::unlock();

    CYG_INSTRUMENT_INTR(RESTORE, vector, 0);   
}

unlock()会调用unlock_inner,unlock_inner是kernel最重要的一个函数了，它是多任务切换的
执行者，来看它的实现,代码很长，只挑其中一段

//-------------------------------------
void Cyg_Scheduler::unlock_inner( cyg_ucount32 new_lock )
{

    do {

#ifdef CYGIMP_KERNEL_INTERRUPTS_DSRS
       
        // Call any pending DSRs. Do this here to ensure that any
        // threads that get awakened are properly scheduled.
>>调用前面post的所有的DSR，注意里面会有reschedule flag的设置
>>下面就要用到
        if( new_lock == 0 && Cyg_Interrupt::DSRs_pending() )
            Cyg_Interrupt::call_pending_DSRs();
#endif

        Cyg_Thread *current = get_current_thread();

        // If the current thread is going to sleep, or someone
        // wants a reschedule, choose another thread to run
>>这里有几种情况需要处理，一个是当前的任务主动要求休息(调用yield())，当然就要切换给别的任务;
>>另外一个就是在DSR的timeslice中找到优先级相同的下个任务需要运行;
>>还有一种是当有新的高优先级的任务加入。(所有都会调用set_need_reschedule())        if( current->state != Cyg_Thread::RUNNING || get_need_reschedule() ) {

            CYG_INSTRUMENT_SCHED(RESCHEDULE,0,0);
           
            // Get the next thread to run from scheduler
            Cyg_Thread *next = scheduler.schedule();

            if( current != next )
            {

                CYG_INSTRUMENT_THREAD(SWITCH,current,next);

                // Count this thread switch
                thread_switches[CYG_KERNEL_CPU_THIS()]++;
>>上下文切换，在contexts.S里
                // Switch contexts
                HAL_THREAD_SWITCH_CONTEXT( ¤t->stack_ptr,
                                           &next->stack_ptr );

                // Worry here about possible compiler
                // optimizations across the above call that may try to
                // propogate common subexpresions.  We would end up
                // with the expression from one thread in its
                // successor. This is only a worry if we do not save
                // and restore the complete register set. We need a
                // way of marking functions that return into a
                // different context. A temporary fix would be to
                // disable CSE (-fdisable-cse) in the compiler.
               
                // We return here only when the current thread is
                // rescheduled.  There is a bit of housekeeping to do
                // here before we are allowed to go on our way.
>>一般就不会跑到这里了，cpu pc指针已经切换到别的任务上去了，只有等这个任务再次
>>被reschedule时，才会从这里开始执行
                current_thread[CYG_KERNEL_CPU_THIS()] = current;   // restore current thread pointer
            }

#ifdef CYGSEM_KERNEL_SCHED_TIMESLICE
            // Reset the timeslice counter so that this thread gets a full
            // quantum.
            reset_timeslice_count();
#endif

            clear_need_reschedule();    // finished rescheduling
        }

        return;

    } while( 1 );
}

至此，整个框架已经出来了，对于schedule,thread,semphone,mutex,flag,mailbox等等其他概念，在ecos 发布的文档上
有比较详细的介绍(ecos reference manual)，我就不再赘述了。