linux-kernel - Re: Affinity for GIC irq line

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Date:   Thu, 26 Apr 2018 09:19:40 +0530
From:   valmiki <valmikibow@...il.com>
To:     Marc Zyngier <marc.zyngier@....com>, linux-kernel@...r.kernel.org
Cc:     tglx@...utronix.de, jason@...edaemon.net,
        Catalin Marinas <catalin.marinas@....com>
Subject: Re: Affinity for GIC irq line

> On 25/04/18 04:23, valmiki wrote:
>> Hi all,
>>
>> When an IRQ line is set affinity using irq_set_affinity, which calls
>> irq_do_set_affinity, this API copies affinity mask to affinity variable
>> in irq_common_data of this irq descriptor.
>
> It does a wee bit more. Crucially, it contains the line:
>
>         ret = chip->irq_set_affinity(data, mask, force);
Yes, i overlooked this part.
>
>> But as per ARM GICv2 document in order to drive a interrupt to a
>> specific CPU we need to program GICD_ITARGETSRn register.
>>
>> But irq_set_affinity isn't writing to this register.Please correct me if
>> I'm wrong i did not find code for this register being updated in
>> irq_set_affinity flow.
>> So how affinity is being set for an IRQ line ?
>
> I would have though that a function name such as "gic_set_affinity" was
> clear enough?
>
>> If affinity was set successfully on what CPU does asm_do_IRQ run ?
>
> It runs on the CPU that the interrupt targets, I'd say.
Thanks Marc.
How does affinity for virtual irq work ?
For example pci-xgene-msi.c has xgene_msi_set_affinity function which 
sets affinity for each virq.
As per arm64/boot/dts/apm/apm-storm.dtsi they use GICv2.
They are trying to map each virq to specific cpu but already GIC lines 
affinity is set, when this affinity would be used ?

What does
/* Update MSI number to target the new CPU */
irqdata->hwirq = hwirq_to_canonical_hwirq(irqdata->hwirq) + target_cpu;

the above code do ?
Already mapping for hwirq to virq is done by this time for this domain.
Now if hwirq is updated its mapping doesn't it lead to wrong virq ?

Regards,
Valmiki


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