linux-kernel - Re: [patch 03/32] genirq: Provide generic hwirq allocation facility

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Date:	Thu, 15 May 2014 01:57:58 +0200 (CEST)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Chris Metcalf <cmetcalf@...era.com>
cc:	LKML <linux-kernel@...r.kernel.org>, Tony Lu <zlu@...era.com>,
	Ingo Molnar <mingo@...e.hu>, Peter Anvin <hpa@...or.com>,
	Tony Luck <tony.luck@...el.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Fenghua Yu <fenghua.yu@...el.com>,
	Grant Likely <grant.likely@...aro.org>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>
Subject: Re: [patch 03/32] genirq: Provide generic hwirq allocation
 facility

On Wed, 14 May 2014, Chris Metcalf wrote:

> On 5/7/2014 7:15 PM, Thomas Gleixner wrote:
> > On Wed, 7 May 2014, Chris Metcalf wrote:
> > > We have an internal change that we haven't upstreamed yet that makes
> > > irqs actually (cpu,ipi) pairs, so that more irqs can be allocated.
> > > As a result we allocate some irqs as bound to a specific IPI on a single
> > > cpu, and some irqs get bound to a particular IPI registered on every cpu.
> > > 
> > > I'll have to set aside a bit of time to look more closely at how your
> > > change interacts with the work we've done internally.  I've appended the
> > > per-cpu IRQ change from our internal tree here (and cc'ed the author).
> > > The API change is in the <asm/irq.h> diff at the very start.
> > Sure it'll break it. [...]
> 
> I think the right thing for now is to take my Acked-by for the tile changes
> (given a couple of minor nits replied to in separate emails).
> 
> Acked-by: Chris Metcalf <cmetcalf@...era.com>

Thanks.
 
> When we have the chance to set aside some time for more upstreaming of some
> of the work we've done it does seem like it makes sense to tackle doing some
> kind of matrix mapping in a more generic way.

Please coordinate with x86 folks on that to avoid redundant efforts.
 
> You're probably right that a cpumask approach would have made more sense
> than a cpu integer in the tile API, so we'd certainly go with that in any
> generic matrix mapping code.

Anything else is just an intermediate and non generic solution.
 
> I do have one question about the irq_alloc_hwirqs() API, which is
> the use of zero as an error indicator.  Given that zero can
> plausibly be an IRQ number, it seems cleaner to specify this as
> returning a negative errno failure instead.  Doing so would also
> align with __irq_alloc_descs().  So, what was your thinking around
> using zero?

0 has been for a long time a not allocatable irq. Google will unearth
about a gazillion heated debates about that.

I personally agree, that using a negative error code is the right
thing to do. I'll update the series before I push it into tip.

Thanks for your input and cooperation!

       tglx
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