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Subject: Re: polarity inversion on LS1021a
To: Rasmus Villemoes <rasmus.villemoes@prevas.dk>,
        LKML <linux-kernel@vger.kernel.org>
Cc: Thomas Gleixner <tglx@linutronix.de>, Shawn Guo <shawnguo@kernel.org>
References: <67135504-72fb-ef7e-cac4-03331550a38d@prevas.dk>
From: Marc Zyngier <marc.zyngier@arm.com>
Organization: ARM Ltd
Message-ID: <48d2d08c-c57a-ce49-5958-0fd5ad4a2dc7@arm.com>
Date: Mon, 4 Dec 2017 15:23:30 +0000
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Rasmus,

On 04/12/17 15:11, Rasmus Villemoes wrote:
> The LS1021A has a standard GIC-400, but allows inverting the polarity of
> six external interrupt lines via a certain register, effectively
> supporting IRQ_TYPE_LEVEL_LOW and IRQ_TYPE_EDGE_FALLING for those.
> 
> I'm trying to figure out how one would add support for this. The patch
> below works but is obviously just meant to help show what I mean, so
> please don't comment on all the things that are wrong with it.
> 
> It feels wrong to create a whole new irqchip driver copy-pasting the
> entire irg-gic.c, but I can't figure out how and where one could hook
> into the existing one. Any pointers on how to do this properly will be
> greatly appreciated.

You're missing the usual trick, which is to use a stacked irqchip for
these particular interrupts. See drivers/irqchip/irq-mtk-sysirq.c which
does exactly that. Bonus point if you make it a generic driver that can
be reused by other vendors.

Your patch has been erased in order to protect the innocents ;-).

Thanks,

	M.
-- 
Jazz is not dead. It just smells funny...