linux-kernel - Re: [V10 PATCH 2/2] irqchip: gicv2m: Add supports for ARM GICv2m MSI(-X)

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Date:	Thu, 6 Nov 2014 11:42:32 +0100 (CET)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Suravee Suthikulanit <suravee.suthikulpanit@....com>
cc:	Jiang Liu <jiang.liu@...ux.intel.com>, marc.zyngier@....com,
	mark.rutland@....com, jason@...edaemon.net,
	Catalin.Marinas@....com, Will.Deacon@....com, liviu.dudau@....com,
	Harish.Kasiviswanathan@....com,
	linux-arm-kernel@...ts.infradead.org, linux-pci@...r.kernel.org,
	linux-kernel@...r.kernel.org, linux-doc@...r.kernel.org,
	devicetree@...r.kernel.org
Subject: Re: [V10 PATCH 2/2] irqchip: gicv2m: Add supports for ARM GICv2m
 MSI(-X)

On Thu, 6 Nov 2014, Thomas Gleixner wrote:
> On Wed, 5 Nov 2014, Suravee Suthikulanit wrote:
> > On 11/5/2014 6:05 PM, Suravee Suthikulanit wrote:
> > > - Overall, it seems that msi_domain_alloc() could be quite different
> > > across architectures. Would it be possible to declare this function as
> > > weak, and allow arch to override (similar to arch_setup_msi_irq)?
> > 
> > Actually, declaring "msi_domain_ops" as non-static, and allow other code to
> > override the .alloc and .free?
> 
> Why do you want to do that?

I know why. Because you want to spare a level of hierarchy. But thats
wrong simply because MSI itself is an interrupt chip at the device
level.

[ MSI ] ---> [ GIC-MSI ] ---> [ GIC ]

So the MSI level only cares about the allocation of the virq
space. GIC-MSI allocates out of the bitmap which handles the hard
wired range of MSI capable GIC interrupts and GIC handles the
underlying functionality.

And this makes a lot of sense, if you think about interrupt
remapping. If ARM ever grows that you simply insert it into the chain:

[ MSI ] ---> [ Remap] ---> [ GIC-MSI ] ---> [ GIC ]

If you look at Jiangs x86 implementation it does exactly that.

[ MSI ] ---> [ Vector ]

[ MSI ] ---> [ Remap ] ---> [ Vector ]

And because ARM has this intermediate layer of GIC-MSI you need to
represent it in the hierarchy whether you like it or not. If you'd try
to bolt the GIC-MSI magic into the MSI layer itself, then interrupt
remapping would never work.

Thanks,

	tglx

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