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Subject: Re: [RFC patch 2/3] genirq: Add buslock support for irq chips on
 slow busses
From: Peter Zijlstra <peterz@infradead.org>
To: Ingo Molnar <mingo@elte.hu>
Cc: Thomas Gleixner <tglx@linutronix.de>, LKML <linux-kernel@vger.kernel.org>,
       Linus Torvalds <torvalds@linux-foundation.org>,
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Date: Fri, 14 Aug 2009 11:29:35 +0200
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On Fri, 2009-08-14 at 11:18 +0200, Ingo Molnar wrote:
> * Thomas Gleixner <tglx@linutronix.de> wrote:
> 
> > On Fri, 14 Aug 2009, Peter Zijlstra wrote:
> > > On Thu, 2009-08-13 at 19:40 +0000, Thomas Gleixner wrote:
> > > >  
> > > > +/**
> > > > + *	disable_slowbus_irq - disable an slowbus 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 must not be called from IRQ context.
> > > > + */
> > > > +void disable_slowbus_irq(unsigned int irq)
> > > > +{
> > > > +	struct irq_desc *desc = irq_to_desc(irq);
> > > > +
> > > > +	if (!desc || !desc->chip || !desc->chip->bus_lock)
> > > > +		return;
> > > > +
> > > > +	desc->chip->bus_lock(irq);
> > > > +	disable_irq_nosync(irq);
> > > > +	if (desc->action)
> > > > +		synchronize_irq(irq);
> > > > +	desc->chip->bus_sync_unlock(irq);
> > > > +}
> > > > +EXPORT_SYMBOL(disable_slowbus_irq);
> > > 
> > > Should we also not check that desc->chip->bus_lock is not set for the
> > > regular function disable_irq()?
> > > 
> > > It seems to me mixing disable_irq() and disable_slowbus_irq() 
> > > is a recipe for disaster.
> > > 
> > > Same for the other slowbus functions of course.
> > 
> > Yeah, that's what I wanted to avoid with the first version, which 
> > did the conditional locking and did not require a separate API, 
> > but Ingo frowned upon the conditional lock.
> 
> Mind posting that version too?
> 
> Conditional locking is really nasty but if the only other option is 
> nastier there's not much we can do, is there?

Ah what I meant was that we can make the posted version better by adding

  BUG_ON(desc && desc->chip && desc->chip->bus_lock);

to disable_irq/enable_irq/free_irq.