Re: [RFT v2 01/24] irqdomain: Introduce new interfaces to support hierarchy irqdomains

From: Abel
Date: Tue Sep 30 2014 - 06:58:26 EST

Hi Thomas,
On 2014/9/29 23:53, Thomas Gleixner wrote:

> On Mon, 29 Sep 2014, Abel wrote:
>> I've been through your patches and noticed that the only domain
>> which does not call irq_domain_alloc_irqs_parent() is
>> x86_vector_domain. And this makes sense *if* we already knew which
>> domain is the nearest one to the CPU.
> Right, and in case of x86 the vector domain _IS_ the one which is
> always the nearest one to the cpu.

Yes, I know that. :)
What I meant is... (please see below)

>> But I don't think a well implemented device driver should assume
>> itself be in a particular position of the interrupt delivery path.
> The device driver has no knowledge of this. The irq domain driver
> definitely has to know to some extent.
>> Actually it should be guaranteed by the core infrastructure that all
>> the domains in the interrupt delivery path should allocate a
>> hardware interrupt for the interrupt source.
> Well, that's what we do. We allocate down the irq domain hierarchy. If
> one level fails the whole operation fails.

Actually the core infrastructure just calls domain->ops->alloc() which is
the one who really guarantees it by calling irq_domain_alloc_irqs_parent().
I think it's enough for a particular domain to pick a hwirq from itself for
that linux irq, and need not to care about its parent.
What I suggest is something like:

for (iter = domain; iter; iter = iter->parent) {
ret = iter->ops->alloc(iter, virq, nr_irqs, arg);
if (ret < 0) {
goto out_free_irq_data;

in this way, the core infrastructure guarantees allocating down the irqdomain
hierarchy, and the implementers of domain_ops->alloc() need not to call
irq_domain_alloc_irqs_parent() any longer, just do the things they have to.

>> And besides the comments/questions I mentioned above, I am also curious about
>> how the chained interrupts been processed.
>> Let's take a 3-level-chained-domains for example.
>> Given 3 interrupt controllers A, B and C, and the interrupt delivery path is:
>> DEV -> A -> B -> C -> CPU
>> After the hierarchy irqdomains are established, the unique linux interrupt of
>> DEV will be mapped with a hardware interrupt in each domain:
>> DomainA: HWIRQ_A => VIRQ_DEV
>> DomainB: HWIRQ_B => VIRQ_DEV
>> DomainC: HWIRQ_C => VIRQ_DEV
>> When the DEV triggered an interrupt signal, the CPU will acknowledge HWIRQ_C,
> Not necessarily. The CPU will process HWIRQ_C. The acknowledge
> mechanism depends on the implementation details of the hierarchy.

Yes, you are right. Thanks for pointing out.

>> and then irq_find_mapping(DomainC, HWIRQ_C) will be called to get the linux
>> interrupt VIRQ_DEV, and after the handler of the VIRQ_DEV has been processed,
>> the interrupt will end with the level (if have) uncleared on B, which will
>> result in the interrupt of DEV cannot be processed again.
>> Or is there anything I misunderstand?
> This heavily depends on the properties of the stacked domains.
> It depends on the hardware requirements and the implementation of
> domain A and B how this is handled.
> It might be sufficient to have the following code in the irq_ack()
> callback of domain A:
> irq_ack_A(struct irq_data *d)
> {
> ack_hw_A();
> }
> Another HW or stacking scenario requires
> irq_ack_A(struct irq_data *d)
> {
> ack_hw_A();
> ack_parent();
> }
> where ack_parent() does:
> if (d->parent_data)
> d->parent_data->chip->ack(d->parent_data);
> and ack_hw_A() can be anything from a nop to some more or less complex
> hw access.
> So we cannot define upfront how deep an ack/mask/unmask/... has to be
> propagated down the chain. This needs a careful consideration in terms
> of functionality and we want to be able to do performance shortcuts as
> well.

Yes, I got it. And one more thing I concerned is that when hierarchy
irqdomains is enabled, shouldn't the ack_parent() be called by default
by the irqchip->irq_ack() of each domain to ensure all the domains in
the delivery path ack this interrupt?


To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at
Please read the FAQ at