linux-kernel - Re: [PATCH] irqchip: gic: Add a cpu map for each GIC instance

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Date:	Thu, 30 Jul 2015 09:20:20 +0100
From:	Marc Zyngier <marc.zyngier@....com>
To:	Jon Hunter <jonathanh@...dia.com>,
	Russell King - ARM Linux <linux@....linux.org.uk>
CC:	Nicolas Pitre <nicolas.pitre@...aro.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Jason Cooper <jason@...edaemon.net>,
	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] irqchip: gic: Add a cpu map for each GIC instance

On 29/07/15 20:27, Jon Hunter wrote:
> 
> On 29/07/15 19:33, Russell King - ARM Linux wrote:
>> On Wed, Jul 29, 2015 at 03:43:04PM +0100, Jon Hunter wrote:
>>> The gic_init_bases() function initialises an array that stores the mapping
>>> between the GIC and CPUs. This array is a global array that is
>>> unconditionally initialised on every call to gic_init_bases(). Although,
>>> it is not common for there to be more than one GIC instance, there are
>>> some devices that do support nested GIC controllers and gic_init_bases()
>>> can be called more than once.
>>>
>>> A 2nd call to gic_init_bases() will clear the previous CPU mapping and
>>> will only setup the mapping again for CPU0. This is because for child GIC
>>> controllers there is most likely only one recipient of the interrupt.
>>>
>>> Fix this by moving the CPU mapping array to the GIC chip data structure
>>> so that it is initialised for each GIC instance separately. It is assumed
>>> that the bL switcher code is only interested in the root or primary GIC
>>> instance.
>>
>> Does it make sense to expose the per-CPU-ness of the non-primary GIC?
>> If they are chained off a primary GIC SPI interrupt, then all IRQs on
>> the secondary GIC are routed to the same CPU that the SPI on the primary
>> GIC is routed to.
> 
> I am looking at a use-case where there is a secondary GIC and the secondary
> GIC is used as a interrupt router between the main CPU cluster and another
> CPU. So in this case the mapping of a secondary is still of interest. This
> patch does not address setting up the secondary mapping, but avoids a
> secondary GIC overwriting the primary map (which we don't want).
>  
>> Other features like the PPIs and SGIs in the secondary CPU should also
>> be ignored - they probably aren't used anyway.
> 
> Yes, agree.
>  
>> I have to say though... are the 1020 IRQs that the primary GIC provides
>> really not enough?  What insane hardware needs more than 1020 IRQs?
> 
> Ha. I guess some realview boards for a start ...
> 
> # grep -r "gic_init(1" arch/arm/
> arch/arm/mach-realview/realview_pb1176.c:	gic_init(1, IRQ_PB1176_GIC_START,
> arch/arm/mach-realview/realview_eb.c:		gic_init(1, 96, __io_address(REALVIEW_EB_GIC_DIST_BASE),
> arch/arm/mach-realview/realview_pb11mp.c:	gic_init(1, IRQ_PB11MP_GIC_START,

Different use case. These are development boards with a relatively
modular design, where the SoC may or may not have a GIC by itself. When
it has one, you end up with the on-board GIC being a secondary one.

I never thought someone would quote these designs as an example to
follow... ;-)

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