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Re: [RFC PATCH 1/2] genirq: Add chip hooks for taking CPUs on/off line.

To: Thomas Gleixner <>
Subject: Re: [RFC PATCH 1/2] genirq: Add chip hooks for taking CPUs on/off line.
From: David Daney <>
Date: Mon, 21 Mar 2011 11:26:49 -0700
In-reply-to: <alpine.LFD.2.00.1103192050400.2787@localhost6.localdomain6>
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References: <> <> <alpine.LFD.2.00.1103192050400.2787@localhost6.localdomain6>
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On 03/19/2011 01:51 PM, Thomas Gleixner wrote:
On Fri, 18 Mar 2011, David Daney wrote:
--- a/include/linux/irqdesc.h
+++ b/include/linux/irqdesc.h
@@ -178,6 +178,12 @@ static inline int irq_has_action(unsigned int irq)
        return desc->action != NULL;

+/* Test to see if the irq is currently enabled */
+static inline int irq_desc_is_enabled(struct irq_desc *desc)
+       return desc->depth == 0;

That want's to go into kernel/irq/internal.h

I think I need to use this in my irq_chip.irq_unmask method.

Consider this:

CPU0                   CPU1
    .                  disable_irq

I need to know in my .unmask method if the interrupt has been disabled. If it has, I will not re-enable (unmask)it.

  static inline int irq_balancing_disabled(unsigned int irq)
diff --git a/kernel/irq/chip.c b/kernel/irq/chip.c
index c9c0601..40736f7 100644
--- a/kernel/irq/chip.c
+++ b/kernel/irq/chip.c
@@ -689,3 +689,38 @@ void irq_modify_status(unsigned int irq, unsigned long 
clr, unsigned long set)

        irq_put_desc_unlock(desc, flags);
+void irq_cpu_online(unsigned int irq)

Odd function name. It does not reflect that this is for per cpu
interrupts. So something like irq_xxx_per_cpu_irq(irq)
might be a bit more descriptive.

I am using it for per cpu interrupts, but I didn't want to impose that policy on others.


So that's called on the cpu which goes online, right?


I wonder whether we can add any sanity check to verify this.

Though I would not worry too much about it. Calling that from a cpu
which is not going offline should have enough nasty side effects that
it's noticed during development. :)

+       unsigned long flags;
+       struct irq_chip *chip;
+       struct irq_desc *desc = irq_to_desc(irq);

Needs to check !desc


+       raw_spin_lock_irqsave(&desc->lock, flags);
+       chip = irq_data_get_irq_chip(&desc->irq_data);
+       if (chip&&  chip->irq_cpu_online)
+               chip->irq_cpu_online(&desc->irq_data,
+                                    irq_desc_is_enabled(desc));
+       raw_spin_unlock_irqrestore(&desc->lock, flags);
+void irq_cpu_offline(unsigned int irq)
+       unsigned long flags;
+       struct irq_chip *chip;
+       struct irq_desc *desc = irq_to_desc(irq);

See above.

Style nit: I prefer ordering:

+       struct irq_desc *desc = irq_to_desc(irq);
+       struct irq_chip *chip;
+       unsigned long flags;

For some reason, probably because I'm used to it, that's easier to
parse. But don't worry about that, I'll turn it around before sticking
it into git. :)

Otherwise I'm fine with the approach itself.

Though one question remains: should we just iterate over the irq space
and call the online/offline callbacks when available instead of having
the arch code do the iteration.

That would be good I think, especially for sparse irqs.

In the case of the CPU going offline, the .irq_cpu_offline() may need to adjust the affinity so that the irq no longer has affinity for the off-lined CPU.

This is something needed even for non-per-cpu interrupts. Also I would need a way to call irq_set_affinity() while holding the desc->lock.

David Daney

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