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

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Message-ID: <55B9E89E.5050905@arm.com>
Date:	Thu, 30 Jul 2015 10:04:30 +0100
From:	Marc Zyngier <marc.zyngier@....com>
To:	Jon Hunter <jonathanh@...dia.com>,
	Russell King - ARM Linux <linux@....linux.org.uk>
CC:	"nicolas.pitre@...aro.org" <nicolas.pitre@...aro.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Jason Cooper <jason@...edaemon.net>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] irqchip: gic: Add a cpu map for each GIC instance

On 30/07/15 09:33, Jon Hunter wrote:
> 
> 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?

My point is that there is no secondary map. That map should only be
written for the primary GIC, because that's the only place it makes
sense. Your "other CPU" is not under the control of Linux (at least, not
as a CPU), so this map is not the right data structure.

Thanks,

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