linux-kernel - Re: [PATCH v2 2/5] irqchip: Add DT binding doc for dumb demuxer chips

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Date:	Wed, 14 Jan 2015 13:56:19 -0500
From:	Jason Cooper <jason@...edaemon.net>
To:	Boris Brezillon <boris.brezillon@...e-electrons.com>
Cc:	Thomas Gleixner <tglx@...utronix.de>,
	Nicolas Ferre <nicolas.ferre@...el.com>,
	Jean-Christophe Plagniol-Villard <plagnioj@...osoft.com>,
	Alexandre Belloni <alexandre.belloni@...e-electrons.com>,
	"Rafael J. Wysocki" <rafael.j.wysocki@...el.com>,
	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
	Rob Herring <robh+dt@...nel.org>,
	Pawel Moll <pawel.moll@....com>,
	Mark Rutland <mark.rutland@....com>,
	Ian Campbell <ijc+devicetree@...lion.org.uk>,
	Kumar Gala <galak@...eaurora.org>, devicetree@...r.kernel.org
Subject: Re: [PATCH v2 2/5] irqchip: Add DT binding doc for dumb demuxer
 chips

Hey Boris,

On Tue, Jan 13, 2015 at 09:52:07PM +0100, Boris Brezillon wrote:
> On Tue, 13 Jan 2015 14:00:50 -0500 Jason Cooper <jason@...edaemon.net> wrote:
> > On Tue, Jan 13, 2015 at 07:46:18PM +0100, Boris Brezillon wrote:
> > > Add documentation for the dumb demuxer.
> > > 
> > > Signed-off-by: Boris Brezillon <boris.brezillon@...e-electrons.com>
> > > ---
> > >  .../bindings/interrupt-controller/dumb-demux.txt   | 34 ++++++++++++++++++++++
> > >  1 file changed, 34 insertions(+)
> > >  create mode 100644 Documentation/devicetree/bindings/interrupt-controller/dumb-demux.txt
> > > 
> > > diff --git a/Documentation/devicetree/bindings/interrupt-controller/dumb-demux.txt b/Documentation/devicetree/bindings/interrupt-controller/dumb-demux.txt
> > > new file mode 100644
> > > index 0000000..1c777ef
> > > --- /dev/null
> > > +++ b/Documentation/devicetree/bindings/interrupt-controller/dumb-demux.txt
> > > @@ -0,0 +1,34 @@
> > > +* Generic Dumb Interrupt Demultiplexer
> > > +
> > > +This Dumb demultiplixer simply forward all incoming interrupts to its
> > > +enabled/unmasked children.
> > 
> > Please forgive the potentially naïve question, but what hardware is this
> > describing?
> 
> That's not a real hardware per se, but on some hardware (like at91 SoCs)
> some IRQ line are shared by several peripherals, and this dumb
> demultiplex is here to represent such shared irq lines which cannot be
> easily demultiplexed (because they do not provide a 'cause'
> register).
> 
> You can see it as a virtual irqchip provided to address broken hardware
> designs.

Hmm.  Well, given tglx's recent reply, I suppose I'll *not* go down the
rabbit hole of "the DT is for describing hardware."  :-P

However, it would probably be a lot more palatable to the DT maintainers
if we at least change the compatible to prepend "linux,".  This way, if
someone does come up with a better solution down the road, it will be
much easier to deprecate the binding.

I would also be amenable to "virt,", or "hack,", or even
"work-around-piss-poor-hw,".  Basically, anything that would indicate to
consumers of the DT that this is not a true reflection of the hardware,
and that it may be superseded by a better solution later.

It would also be helpful to explain the situation more fully in the
binding document.

thx,

Jason.
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