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

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Message-ID: <55B9E145.9050202@nvidia.com>
Date:	Thu, 30 Jul 2015 09:33:09 +0100
From:	Jon Hunter <jonathanh@...dia.com>
To:	Marc Zyngier <marc.zyngier@....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 30/07/15 09:20, Marc Zyngier wrote:
> 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.

Yes, I understand that the use-case may be different, but I highlighted
this as a use where the primary map would be overwritten as the driver
is today.

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

I can't say if this example was followed, but nevertheless hardware
designers certainly do get creative ;-)

So we need to ensure the primary cpu map does not get overwritten by a
secondary and I have a case where the secondary map is of interest. So
are ok with this?

Cheers
Jon
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