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

[Jiang Liu <jiang.liu@linux.intel.com>]

On 2014/11/7 0:34, Marc Zyngier wrote:
> 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.
Hi Marc,
	It works exactly in this way:)

> 
> 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.
irq_alloc_info is newly introduced for hierarchy irqdomain on x86.
Regards!
Gerry

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