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

[Boris Brezillon <boris.brezillon-wi1+55ScJUtKEb57/3fJTNBPR1lH4CV8@public.gmane.org>]

Hi Jason,

On Wed, 14 Jan 2015 13:56:19 -0500
Jason Cooper <jason-NLaQJdtUoK4Be96aLqz0jA@public.gmane.org> wrote:

> 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@lakedaemon.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@free-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

Actually I'm a bit surprised no one else already mentioned that :-) (I
thought this would be the first complain regarding this dumb irq
demux chip).

Anyway, IMHO, this can be considered as hardware description since
these irq-lines are really multiplexed into a single one...

> 
> 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.

Actually I thought about changing it to "virtual,dumb-irq-demux" :-).

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

Sure, I'll add a few lines to describe what this irqchip really is.

Regards,

Boris

-- 
Boris Brezillon, Free Electrons
Embedded Linux and Kernel engineering
http://free-electrons.com
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