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Re: irq handling in DDE Linux26

From: Samuel Thibault
Subject: Re: irq handling in DDE Linux26
Date: Wed, 12 May 2010 13:08:27 +0200
User-agent: Mutt/1.5.12-2006-07-14

Da Zheng, le Wed 12 May 2010 17:36:36 +0800, a écrit :
> On 10-5-12 上午6:03, Samuel Thibault wrote:
> > Da Zheng, le Sat 08 May 2010 23:19:08 +0800, a écrit :
> >> A stupid question: when disable_irq_nosync is called, IRQ_DISABLED is set 
> >> in the
> >> irq descriptor in the Linux kernel and the corresponding hardirq line 
> >> should be
> >> masked as well (at least, it seems the kernel for x86 does so).
> > 
> > AIUI, on unmask the hardware will trigger the interrupt that was raised
> > by the hardware in the meanwhile. Which precise x86 file are you looking
> > at? What do you exactly mean by "hardirq": the actual hardware chip or
> > the software hardIRQ handler?
> In disable_irq_nosync, there is code as follow:
>                 desc->status |= IRQ_DISABLED;
>                 desc->chip->disable(irq);
> For the x86 platform, `disable' points to disable_8259A_irq. So when
> disable_irq_nosync is called, IRQ_DISABLED is set and the irq line (the
> hardware) is masked.


> >> How can handle_edge_irq be called?
> > 
> > AIUI, on hardware irq unmasking.
> When hardware irq is unmasked, IRQ_DISABLED should have been removed. I meant,
> how can handle_edge_irq be called when IRQ_DISABLED is still set? It seems to 
> me
> that IRQ_PENDING cannot be set in handle_edge_irq when the irq line is 
> disabled.


        if (unlikely((desc->status & (IRQ_INPROGRESS | IRQ_DISABLED)) ||
                    !desc->action)) {
                desc->status |= (IRQ_PENDING | IRQ_MASKED);
                mask_ack_irq(desc, irq);
                goto out_unlock;

So here, even if IRQ_DISABLED is not cleared before the hardware chip
part is unmasked, IRQ_PENDING flag will be set.


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