Date:   Thu, 25 Aug 2016 13:51:35 -0400
From:   Rich Felker <dalias@...c.org>
To:     Mark Rutland <mark.rutland@....com>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Daniel Lezcano <daniel.lezcano@...aro.org>,
        devicetree@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-sh@...r.kernel.org, Rob Herring <robh+dt@...nel.org>,
        Marc Zyngier <Marc.Zyngier@....com>
Subject: Re: [PATCH v6 2/2] clocksource: add J-Core timer/clocksource driver

On Thu, Aug 25, 2016 at 05:38:06PM +0100, Mark Rutland wrote:
> On Thu, Aug 25, 2016 at 10:56:50AM -0400, Rich Felker wrote:
> > On Thu, Aug 25, 2016 at 10:07:08AM +0200, Thomas Gleixner wrote:
> > > On Wed, 24 Aug 2016, Rich Felker wrote:
> > As for this topic, what happened is that, after the first and second
> > time of expressing confusion about how the infrastructure did not make
> > sense and whether I should even be using it, I did not get anything
> > resembling an explanation of why it's the way it is or why I might be
> > wrong in thinking it's a bug, and so I went forward with the
> > assumption that is was just a bug. Now that Mark Rutland has explained
> > it well (and with your additional explanation below in your email), I
> > see what the motivation was, but I still think it could be done in a
> > less-confusing and more-consistent way that doesn't assume ARM-like
> > irq architecture.
> 
> The fun this is that right until the point you sent this patch, it was
> consistent for all hardware that we support and were aware of. For others, it
> is your hardware that is the confusing case. ;)
> 
> In future, it's better to state 'this doesn't seem to match my hardware' rather
> than 'this is misdesigned'. Ideally, with a description of how your hardware
> works, as that's the thing no-one else is familiar with.

OK. What I meant is "it's not sufficiently general". I usually come at
these things not from a particular preconception of how it
is/should-be done on some archs I'm familiar with, but minimizing
unnecessary (i.e. not beneficial to performance or correctness or
simplicity) assumptions at the subsystem level about how hardware
could work.

> > > If your particular hardware has the old scheme of seperate interrupt numbers
> > > for per cpu interrupts, then you can simply use the normal interrupt scheme
> > > and request a seperate interrupt per cpu.
> > 
> > Nominally it uses the same range of hardware interrupt numbers for all
> > (presently both) cpus, but some of them get delivered to a specific
> > cpu associated with the event (presently, IPI and timer; IPI is on a
> > fixed number at synthesis time but timer is runtime configurable)
> > while others are conceptually deliverable to either cpu (presently
> > only delivered to cpu0, but that's treated as an implementation
> > detail).
> 
> Given you say it's delivered to the CPU associated with the event (and you have
> different PIT bases per-cpu), it sounds like your timer interrupt is percpu,
> it's just that the hwirq number can be chosen by software.

It's what I would call percpu in the hardware, but I'm not convinced
that the Linux irq subsystem's "percpu" stuff models it in a way
that fits the hw, nor that it's in any way necessary.

> > It currently works requesting the irq with flags that ensure the
> > handler runs on the same cpu it was delivered on, without using any
> > other percpu irq framework. If you have concerns about ways this could
> > break and want me to make the drivers do something else, I'm open to
> > suggestions.
> 
> As I suggested, I don't think that this is right, and you need some mechanism
> to describe to the kernel that the interrupt is percpu (e.g. a flag in the
> interrupt-specifier in DT).

Thomas seemed to think it's okay as-is. Can you describe what you
expect could go wrong by using request_irq rather than the ARM-style
percpu irq framework?

Rich

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