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

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Date:	Thu, 06 Nov 2014 16:34:09 +0000
From:	Marc Zyngier <marc.zyngier@....com>
To:	Thomas Gleixner <tglx@...utronix.de>
CC:	"suravee.suthikulpanit@....com" <suravee.suthikulpanit@....com>,
	Jiang Liu <jiang.liu@...ux.intel.com>,
	Mark Rutland <Mark.Rutland@....com>,
	"jason@...edaemon.net" <jason@...edaemon.net>,
	Catalin Marinas <Catalin.Marinas@....com>,
	Will Deacon <Will.Deacon@....com>,
	Liviu Dudau <Liviu.Dudau@....com>,
	"Harish.Kasiviswanathan@....com" <Harish.Kasiviswanathan@....com>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	"linux-pci@...r.kernel.org" <linux-pci@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-doc@...r.kernel.org" <linux-doc@...r.kernel.org>,
	"devicetree@...r.kernel.org" <devicetree@...r.kernel.org>
Subject: Re: [V10 PATCH 2/2] irqchip: gicv2m: Add supports for ARM GICv2m
 MSI(-X)

Hi Thomas,

On 06/11/14 10:42, Thomas Gleixner wrote:
> 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 ]

I think ARM has reached that stage with the ITS block in GICv3:
- Each device gets programmed with a set of  "event IDs" ranging from 0
to N-1, with N being the number of MSI vectors used by the device
- the ITS uses both the device ID (basically the PCI requester ID) and
the event ID to parse a set of software-managed tables (think page
tables for interrupts).

The x86 remapping thing looks quite similar to that, by reading a couple
of pages from the VT-D document.

So the way I understand the layout (and please correct me if I'm wrong,
which is certainly the case) is that the MSI domain is entirely generic,
allocates the virq, uses Remap as a parent, and uses
irq_chip_compose_msi_msg to call into the parent and generate whatever
goes into the MSI message.

I'm still struggling a bit to see how the remapping layer can access the
requester ID. x86 uses the irq_alloc_info to store that (the result of
an msi_get_hwirq call), but we don't have an equivalent structure on
arm/arm64.

I'll try to hack something with my current ITS driver and come back with
the results.

Thanks,

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

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