linux-kernel - Re: [RFC] PPC: MPIC: necessary readback after EOI?

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Message-ID: <1420641876.5830.32.camel@kernel.crashing.org>
Date:	Wed, 07 Jan 2015 15:44:36 +0100
From:	Benjamin Herrenschmidt <benh@...nel.crashing.org>
To:	Scott Wood <scottwood@...escale.com>
Cc:	Andreas Mohr <andi@...as.de>,
	Purcareata Bogdan <b43198@...escale.com>,
	linuxppc-dev@...ts.ozlabs.org, linux-kernel@...r.kernel.org
Subject: Re: [RFC] PPC: MPIC: necessary readback after EOI?

On Mon, 2015-01-05 at 12:10 -0600, Scott Wood wrote:
> It would have been nice if a code comment explained why it was doing the
> readback...  I don't see any particular need to wait for EOI completion
> here (unlike when masking).

The EOI is what causes the MPIC to drop it's EE output to the CPU, if the
EOI is processed too slowly & asynchronously (posted write + 33Mhz MPIC)
we observe cases of spurrious interrupts. We had some macs basically getting
a spurrious irq for every MPIC interrupts...

Cheers,
Ben.


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