linux-kernel - Re: [PATCH v3 04/12] of: add J-Core timer bindings

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Date:	Thu, 14 Jul 2016 18:18:08 -0400
From:	Rich Felker <dalias@...c.org>
To:	Rob Herring <robh@...nel.org>
Cc:	"devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	SH-Linux <linux-sh@...r.kernel.org>,
	Ian Campbell <ijc+devicetree@...lion.org.uk>,
	Kumar Gala <galak@...eaurora.org>,
	Mark Rutland <mark.rutland@....com>,
	Pawel Moll <pawel.moll@....com>
Subject: Re: [PATCH v3 04/12] of: add J-Core timer bindings

On Thu, Jun 23, 2016 at 05:16:08PM -0400, Rich Felker wrote:
> On Thu, Jun 02, 2016 at 05:44:25PM -0500, Rob Herring wrote:
> > > > > In theory it would even be possible to just require a DT node per
> > > > > cpulocal timer, but I didn't see a good way to make the bindings
> > > > > represent the relationship to cpus or to make the driver handle irqs
> > > > > correctly for such a setup, so I'd need a viable proposal for how that
> > > > > could be done to even consider such an approach.
> > > > 
> > > > Yeah, there's not really a standard way to map per cpu blocks to cpus.
> > > > We could, but doesn't really seem necessary here.
> > > > 
> > > > For the irqs, percpu irqs doesn't help you?
> > > 
> > > What I mean is that, if there were a separate device node and driver
> > > instance per cpu, they'd all want to register the same irq just to
> > > handle it on their own cpu, so we'd have a lot of spurious handlers
> > > running. The right way to model this, I think, would be as a virtual
> > > irqchip that's the irq parent of all the timer nodes, and that
> > > multiplexes the real irq to one virq per cpu (where the current cpu id
> > > becomes the irq number in its irq domain). But that's a lot of virtual
> > > infrastructure just for the sake of modelling each percpu timer as its
> > > own DT node and I don't think it makes sense to do it that way.
> > 
> > I would have thought your interrupt controller did all this. On the ARM 
> > GIC for example, you have the same irq number but there is a per cpu 
> > interface and really N (== # cpus) physical irq lines.
> 
> I've looked at the ARM GIC code and bindings and I don't see where the
> per-cpu interrupt interfaces are modelled with multiple interrupt
> controller nodes or irq domains. It looks to me like it just uses a
> single interrupt controller/domain with percpu irq. Does that match
> your understanding?

Ping.

I want to submit this again for the upcoming merge window and need to
know whether you still want me to pursue different approaches.

Rich