linux-kernel - Re: [PATCH v6 2/5] irqchip/aspeed-i2c-ic: Add I2C IRQ controller for Aspeed

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Date:   Tue, 28 Mar 2017 10:40:38 +0100
From:   Marc Zyngier <marc.zyngier@....com>
To:     Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Brendan Higgins <brendanhiggins@...gle.com>, wsa@...-dreams.de,
        robh+dt@...nel.org, mark.rutland@....com, tglx@...utronix.de,
        jason@...edaemon.net, joel@....id.au, vz@...ia.com, mouse@...c.ru,
        clg@...d.org
Cc:     linux-i2c@...r.kernel.org, devicetree@...r.kernel.org,
        linux-kernel@...r.kernel.org, openbmc@...ts.ozlabs.org
Subject: Re: [PATCH v6 2/5] irqchip/aspeed-i2c-ic: Add I2C IRQ controller for
 Aspeed

On 28/03/17 10:12, Benjamin Herrenschmidt wrote:
> On Tue, 2017-03-28 at 09:32 +0100, Marc Zyngier wrote:
>> I'm a bit concerned by this. It means that you can't even mask an
>> interrupt. Is that really what you intend to do? Or all that the HW can
>> do? If you cannot mask an interrupt, you're at the mercy of a screaming
>> device...
> 
> This is not really an interrupt controller. It's a "summary" register
> that reflects the state of the 14 i2c controller interrupts.
> 
> This approach does have the advantage of providing separate counters in
> /proc/interrupts which is rather nice, but it does have overhead. On
> those shittly little ARMv9 400Mhz cores it can be significant.

<pedantic>
s/ARMv9/ARM9/, as we're still on variations of the ARMv8 architecture ;-)
</pedantic>

A 400MHz ARM9 (which is either ARMv4 or ARMv5) is not too bad (hey, we
still have a couple of Versatile-ABs here...). Caches are pretty small
though.

> I would personally have some kind of trick to register a single
> interrupt handler that calls directly the handlers of the respective
> i2c busses via a simple indirection for speed, maybe adding my custom
> sysfs or debugfs statistics. But that's just me trying to suck the last
> cycle out of the bloody thing ;-)

I'd hope the irqdomain itself to be pretty light (the revmap should help
here), but of course you're going to do more work. Counters also come at
a cost. It'd be interesting to see if Brendan has any overhead data
about this.

Cheers,

	M.
-- 
Jazz is not dead. It just smells funny...