linux-kernel - Re: polarity inversion on LS1021a

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Message-ID: <2780635.GsDK2O7NgM@ws-stein>
Date:   Mon, 04 Dec 2017 16:31:39 +0100
From:   Alexander Stein <alexander.stein@...tec-electronic.com>
To:     Rasmus Villemoes <rasmus.villemoes@...vas.dk>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Marc Zyngier <marc.zyngier@....com>,
        Shawn Guo <shawnguo@...nel.org>
Subject: Re: polarity inversion on LS1021a

Hi Rasmus,

On Monday, December 4, 2017, 4:11:06 PM CET 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.

In my opinion a new irqchip is still required, but solely for modifying
SCFG_INTPCR depending on IRQ_TYPE_*
You would need to insert it as a cascading interrupt chip in device tree.
You also need to protect accesses to this register using a spinlock.
This is at least my idea how I would have done it, though never got time
for it.

Best regards,
Alexander