目的需求：
为了防止中断的重入，有时候需要对中断处理过程加以保护。
1、Linux中的中断处理机制
为了保护系统执行，防止被其他中断打断，需要在必要的代码段暂时关闭中断，linux关闭中断分为两种方式
a、disable_irq_nosync
关闭中断，函数立刻返回，不存在堵塞问题，一般用在中断处理过程中，防止被其他高优先级中断打断
内核代码：
/**
* disable_irq_nosync - disable an irq without waiting
* @irq: Interrupt to disable
*
* Disable the selected interrupt line. Disables and Enables are
* nested.
* Unlike disable_irq(), this function does not ensure existing
* instances of the IRQ handler have completed before returning.
*
* This function may be called from IRQ context.
*/
void disable_irq_nosync(unsigned int irq)
{
struct irq_desc *desc = irq_to_desc(irq);
unsigned long flags;
if (!desc)
return;
chip_bus_lock(irq, desc);
spin_lock_irqsave(&desc->lock, flags);
__disable_irq(desc, irq, false);
spin_unlock_irqrestore(&desc->lock, flags);
chip_bus_sync_unlock(irq, desc);
}
b、disable_irq
关闭中断并等待正在执行的中断处理完后才返回，一般用于保护非中断代码，防止过程被中断打断
/**
* disable_irq - disable an irq and wait for completion
* @irq: Interrupt to disable
*
* Disable the selected interrupt line. Enables and Disables are
* nested.
* This function waits for any pending IRQ handlers for this interrupt
* to complete before returning. If you use this function while
* holding a resource the IRQ handler may need you will deadlock.
*
* This function may be called - with care - from IRQ context.
*/
void disable_irq(unsigned int irq)
{
struct irq_desc *desc = irq_desc + irq;
if (irq >= NR_IRQS)
return;
disable_irq_nosync(irq);
if (desc->action)
synchronize_irq(irq);
}
关闭中断并等待中断处理完后返回，从代码中可以看到, disable_irq先是调用了disable_irq_nosync, 然后检测desc->action是否为1. 在中断处理程序中, action是置1的, 所以进入synchronize_irq函数中.
/**
* synchronize_irq - wait for pending IRQ handlers (on other CPUs)
* @irq: interrupt number to wait for
*
* This function waits for any pending IRQ handlers for this interrupt
* to complete before returning. If you use this function while
* holding a resource the IRQ handler may need you will deadlock.
*
* This function may be called - with care - from IRQ context.
*/
void synchronize_irq(unsigned int irq)
{
struct irq_desc *desc = irq_to_desc(irq);
unsigned int status;
if (!desc)
return;
do {
unsigned long flags;
/*
* Wait until we're out of the critical section. This might
* give the wrong answer due to the lack of memory barriers.
*/
while (desc->status & IRQ_INPROGRESS)
cpu_relax();
/* Ok, that indicated we're done: double-check carefully. */
spin_lock_irqsave(&desc->lock, flags);
status = desc->status;
spin_unlock_irqrestore(&desc->lock, flags);
/* Oops, that failed? */
} while (status & IRQ_INPROGRESS);
/*
* We made sure that no hardirq handler is running. Now verify
* that no threaded handlers are active.
*/
wait_event(desc->wait_for_threads, !atomic_read(&desc->threads_active));
}
注释中说明该函数是在等待中断处理程序的结束, 这也是disable_irq与disable_irq_nosync不同的主要所在.
如果在中断处理函数中调用disable_irq，进入中断处理函数前IRQ_INPROGRESS会被__setup_irq设置, 所以程序会陷入死锁中，一直陷在while循环, 而此时内核以经被独占, 会导致系统死掉
2、设计CC2541中断处理机制
a、进入中断后暂时关闭中断
很多单片机在处理按键程序时会使用这种方式，原则上不会与太大问题。
#define HAL_ENTER_CRITICAL_SECTION(x) st( x = EA; HAL_DISABLE_INTERRUPTS(); )
#define HAL_EXIT_CRITICAL_SECTION(x) st( EA = x; )
b、问题点
由于编译指令的问题，可能会导致响应速度变慢
/* IAR library uses XCH instruction with EA. It may cause the higher priority interrupt to be
* locked out, therefore, may increase interrupt latency. It may also create a lockup condition.
* This workaround should only be used with 8051 using IAR compiler. When IAR fixes this by
* removing XCH usage in its library, compile the following macros to null to disable them.
*/
#define HAL_ENABLE_INTERRUPTS() st( EA = 1; )
#define HAL_ENTER_ISR() { halIntState_t _isrIntState = EA; HAL_ENABLE_INTERRUPTS();
#define HAL_EXIT_ISR() EA = _isrIntState; }