From: Alexander Stein <alexander.stein@systec-electronic.com>
To: Marc Zyngier <marc.zyngier@arm.com>
Cc: Rasmus Villemoes <rasmus.villemoes@prevas.dk>,
        LKML <linux-kernel@vger.kernel.org>,
        Thomas Gleixner <tglx@linutronix.de>, Shawn Guo <shawnguo@kernel.org>
Subject: Re: polarity inversion on LS1021a
Date: Mon, 04 Dec 2017 17:04:44 +0100
Message-ID: <6588111.hDmIEKzSZ3@ws-stein>
In-Reply-To: <4f27f4e1-f42c-e931-a31b-067bb62eed77@arm.com>
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On Monday, December 4, 2017, 4:37:20 PM CET Marc Zyngier wrote:
> On 04/12/17 15:31, Alexander Stein wrote:
> > 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.
> Almost. See my earlier reply. You just need a very minimal driver that
> only takes care of the polarity thing. Nobody needs to see yet another
> GIC driver...

Ah, maybe I should have made that more clear. Of course only a rather simple
driver for that single register is needed. The driver needs to handle the
infamous big-little-endian mismatch in ls1021a.

Best regards,
Alexander