[Xen-devel] Re: [PATCH 1/5] xen: events: use irq_alloc_desc(_at) instead of open-coding an IRQ allocator.

[Jeremy Fitzhardinge <jeremy@xxxxxxxx>]

 On 10/25/2010 10:35 AM, Konrad Rzeszutek Wilk wrote:
> On Mon, Oct 25, 2010 at 05:23:29PM +0100, Ian Campbell wrote:
>> Encapsulate allocate and free in xen_irq_alloc and xen_irq_free.
>>
>> Signed-off-by: Ian Campbell <ian.campbell@xxxxxxxxxx>
>> ---
>>  drivers/xen/events.c |   68 
>> ++++++++++++++++++++-----------------------------
>>  1 files changed, 28 insertions(+), 40 deletions(-)
>>
>> diff --git a/drivers/xen/events.c b/drivers/xen/events.c
>> index 97612f5..c8f3e43 100644
>> --- a/drivers/xen/events.c
>> +++ b/drivers/xen/events.c
>> @@ -394,41 +394,29 @@ static int find_unbound_pirq(void)
>>      return -1;
>>  }
>>  
>> -static int find_unbound_irq(void)
>> +static int xen_irq_alloc(void)
>>  {
>> -    struct irq_data *data;
>> -    int irq, res;
>> -    int start = get_nr_hw_irqs();
>> +    int irq = irq_alloc_desc(0);
>>  
>> -    if (start == nr_irqs)
>> -            goto no_irqs;
>> -
>> -    /* nr_irqs is a magic value. Must not use it.*/
>> -    for (irq = nr_irqs-1; irq > start; irq--) {
>> -            data = irq_get_irq_data(irq);
>> -            /* only 0->15 have init'd desc; handle irq > 16 */
>> -            if (!data)
>> -                    break;
>> -            if (data->chip == &no_irq_chip)
>> -                    break;
>> -            if (data->chip != &xen_dynamic_chip)
>> -                    continue;
>> -            if (irq_info[irq].type == IRQT_UNBOUND)
>> -                    return irq;
>> -    }
>> -
>> -    if (irq == start)
>> -            goto no_irqs;
>> +    if (irq < 0)
>> +            panic("No available IRQ to bind to: increase nr_irqs!\n");
>>  
>> -    res = irq_alloc_desc_at(irq, 0);
>> +    return irq;
>> +}
> So I am curious what the /proc/interrupts looks?The issue (and the reason
> for this implementation above) was that under PV with PCI devices we would
> overlap PCI devices IRQs with Xen event channels. So we could have a USB 
> device
> at IRQ 16 _and_ also a xen_spinlock4 handler. That would throw off the system
> since the xen_spinlock4 was an edge type handler while the USB device was an
> level (at least on my box).

What?  Why?  How?  Surely if we're asking the irq subsystem to allocate
us an irq, it will return a fresh never-before-used (and certainly not
shared) irq?  Shared irqs only make sense if multiple devices are
actually sharing, say, a wire on the board.

Or am I missing something?

    J


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