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Date:   Sat, 4 Aug 2018 00:13:16 +0200 (CEST)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Heiner Kallweit <hkallweit1@...il.com>
cc:     Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Marc Zyngier <marc.zyngier@....com>
Subject: Re: [PATCH] genirq: Consider domain hierarchy when checking for
 IRQCHIP_ONESHOT_SAFE

On Fri, 3 Aug 2018, Heiner Kallweit wrote:
> On 03.08.2018 22:00, Thomas Gleixner wrote:
> > On Fri, 3 Aug 2018, Heiner Kallweit wrote:
> > 
> >> In case of a domain hierarchy we may miss the IRQCHIP_ONESHOT_SAFE
> >> flag because we look at top of the stack only. See also discussion
> >> here: https://marc.info/?l=linux-kernel&m=153301773524685&w=2
> > 
> > I think you misunderstood:
> > 
> >> I think the top most chip is the key, the rest of the hierarchy is
> >> irrelevant because the top most chip is the one which is responsible for
> >> not creating an interrupt storm after the interrupt got acknowledged.
> > 
> > The top most chip in the hierarchy, e.g. the PCI MSI one, is the key. If
> > that one is badly implemented and starts to resend after ack/eoi then the
> > interrupt storm happens. The lower layers in the hierarchy down to the
> > vector domain are just transporting what the top level chip does. So it is
> > actively wrong to flag the lower layers.
> > 
> >  	 if (desc->irq_data.chip->flags & IRQCHIP_ONESHOT_SAFE)
> > 
> > is the correct check as it looks at the topmost irq chip which is the one
> > which initiates the interrupt.
> > 
> 
> So you're saying it's correct as it is now in __setup_irq(). Then I don't
> really understand Marc's following comment from earlier in the discussion.
> What else needs to be done if it is correct already?
> 
> "We could also consider extending this to support interrupt
> hierarchies, as __setup_irq() seems only concerned with the top of the
> stack (an IRQ provided by a generic MSI stack and backed by an irqchip
> providing IRQCHIP_ONESHOT_SAFE would go unnoticed)."

I think Marc is wrong there, but I'm not sure what he had in mind.

Thanks,

	tglx

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