linux-kernel - Re: [RFC PATCH 1/4] DRIVERS: IRQCHIP: Add crossbar irqchip driver

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Message-ID: <alpine.DEB.2.02.1309190014160.4089@ionos.tec.linutronix.de>
Date:	Thu, 19 Sep 2013 00:31:57 +0200 (CEST)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Santosh Shilimkar <santosh.shilimkar@...com>
cc:	linus.walleij@...aro.org, Sricharan R <r.sricharan@...com>,
	LKML <linux-kernel@...r.kernel.org>, devicetree@...r.kernel.org,
	linux-doc@...r.kernel.org,
	LAK <linux-arm-kernel@...ts.infradead.org>,
	linux-omap@...r.kernel.org, Russell King <linux@....linux.org.uk>,
	Tony Lindgren <tony@...mide.com>, rnayak@...com
Subject: Re: [RFC PATCH 1/4] DRIVERS: IRQCHIP: Add crossbar irqchip driver

On Wed, 18 Sep 2013, Santosh Shilimkar wrote:
> On Friday 13 September 2013 10:55 AM, Santosh Shilimkar wrote:
> > On Friday 13 September 2013 10:24 AM, Thomas Gleixner wrote:
> 
> [...]
> 
> >> Before you dig into MSI, lets talk about irq domains first.
> >>
> >> GIC implements a legacy irq domain, i.e. a linear domain of all
> >> possible GIC interrupts with a 1:1 mapping.
> >>
> >> So why can't you make use of irq domains and have the whole routing
> >> business implemented sanely?
> >>
> >> What's needed is in gic_init_bases():
> >>
> >>        if (of_property_read(node, "routable_irqs", &nr_routable_irqs) {
> >>        	  irq_domain_add_legacy(nr_gic_irqs);
> >>        } else {
> >>        	  irq_domain_add_legacy(nr_per_cpu_irqs);
> >> 	  irq_domain_add_linear(nr_routable_irqs);
> >>        }
> >>
> >> Now that separate domain has an xlate function which grabs a free GIC
> >> irq from a bitmap and returns the hardware irq number in the gic
> >> space. The map/unmap callbacks take care of setting up / tearing down
> >> the route in the crossbar.
> >>
> >> Thoughts?
> >>
> > This sounds pretty good idea. We will explore above option.
> > Thanks Thomas.
> > 
> After further looking into this, the irqdomain approach lets us
> setup the map only once during the init. This is similar to
> the earlier approach of cross-bar driver where at probe time
> the router was setup.  The whole debate started with the fact
> that we shouldn't fix the irq mapping at probe and should
> dynamically change the mapping based on [request/free]_irq()
> to be able to maximize the use of the IP.

Well, that's what irq_of_parse_and_map() resp. irq_create_mapping and
irq_dispose_mapping() are for.

It requires changes to drivers, but you can't get that thing for free.

Thanks,

	tglx
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