linux-kernel - Re: [PATCH v5 1/3] initialize each mbigen device node as a interrupt controller.

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Date:	Wed, 14 Oct 2015 10:49:07 +0100
From:	Marc Zyngier <marc.zyngier@....com>
To:	Thomas Gleixner <tglx@...utronix.de>
CC:	"majun (F)" <majun258@...wei.com>, Catalin.Marinas@....com,
	linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
	Will.Deacon@....com, mark.rutland@....com, jason@...edaemon.net,
	lizefan@...wei.com, huxinwei@...wei.com, dingtianhong@...wei.com,
	zhaojunhua@...ilicon.com, liguozhu@...ilicon.com,
	xuwei5@...ilicon.com, wei.chenwei@...ilicon.com,
	guohanjun@...wei.com, wuyun.wu@...wei.com, guodong.xu@...aro.org,
	haojian.zhuang@...aro.org, zhangfei.gao@...aro.org,
	usman.ahmad@...aro.org, klimov.linux@...il.com
Subject: Re: [PATCH v5 1/3] initialize each mbigen device node as a interrupt
 controller.

On 14/10/15 10:17, Thomas Gleixner wrote:
> Marc,
> 
> On Wed, 14 Oct 2015, Marc Zyngier wrote:
>> To me, it feels like we're spreading the complexity across multiple
>> layers instead of keeping it localized. It also means that next time
>> some crazy HW dude comes up with a similar idea (and I have little doubt
>> this will happen sooner than later), we'll have to replicate the same
>> thing again (though we could put all that behind another abstraction layer).
>>
>> I would have preferred a solution where the MSI domain is allowed to be
>> sandwiched between two non-MSI domains, and expose the top level
>> irqchip. This means fixing the following:
>>
>> - Either find a way to prevent DT doing these early IRQ allocations
>> (this could be easily done by simply not registering the irqchip), or be
>> able to elegantly reuse them.
> 
> The reuse part makes me shudder. We really should not go there. It's a
> blatant layering violation.
> 
>> - Add an API allowing an MSI domain to be the parent of another domain.
>>
>> Once we have this, we can use the platform MSI layer for the mbigen
>> without much complexity (well, not more that any other stacked irqchip,
>> the madness of the mbigen programming interface notwithstanding), and
>> drivers stay untouched. It would also give us a 'standard' way to deal
>> with the above HW dude. I'd be happy to prototype it.
> 
> Ok, I have a better understanding of it now.
> 
> I have no objections to your approach as long as it provides us a
> clean way to use a full hierarchy without weird interfaces to reuse
> irq descriptors etc. If you can find a way which just follows the
> proper hierarchy design, I'm certainly not in your way.
> 
> OTOH, the platform msi driver is not a huge amount of code and from my
> understanding of the hardware it looks weird to have this intermediate
> layer. Making mbigen a direct child of ITS feels just more natural to
> me. I'm pretty sure that this can be done without the earlier proposed
> horrible modifications to ITS. It just should fall into place.
> 
> It would be really great just to have shell implementations,
> i.e. without the mbigen specific stuff - for both models so we can
> compare and contrast the results. That means just the interfaces and
> the hookup to the various layers.

OK, let's do that. I'll try to post something before Friday so that we
can really evaluate what the knock-on effect is.

Thanks,

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