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xen/irq: Cleanup the find_unbound_irq

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The "find_unbound_irq" is a bit unusual - it allocates
virtual IRQ (event channels) in reverse order. This means
starting at the "top" of the available IRQs (nr_irqs) down
to the GSI/MSI IRQs (nr_irqs_gsi). Lets document this and
also make the variables easier to understand.

Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
This commit is contained in:
Konrad Rzeszutek Wilk 2010-12-09 14:53:29 -05:00
parent cf7d7e5a19
commit d1b758ebc2
1 changed files with 12 additions and 6 deletions
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18
drivers/xen/events.c
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@ -405,15 +405,21 @@ static int find_unbound_irq(void)
{
struct irq_data *data;
int irq, res;
int start = get_nr_hw_irqs();
int bottom = get_nr_hw_irqs();
int top = nr_irqs-1;
if (start == nr_irqs)
if (bottom == nr_irqs)
goto no_irqs;
/* nr_irqs is a magic value. Must not use it.*/
for (irq = nr_irqs-1; irq > start; irq--) {
/* This loop starts from the top of IRQ space and goes down.
* We need this b/c if we have a PCI device in a Xen PV guest
* we do not have an IO-APIC (though the backend might have them)
* mapped in. To not have a collision of physical IRQs with the Xen
* event channels start at the top of the IRQ space for virtual IRQs.
*/
for (irq = top; irq > bottom; irq--) {
data = irq_get_irq_data(irq);
/* only 0->15 have init'd desc; handle irq > 16 */
/* only 15->0 have init'd desc; handle irq > 16 */
if (!data)
break;
if (data->chip == &no_irq_chip)
@ -424,7 +430,7 @@ static int find_unbound_irq(void)
return irq;
}
if (irq == start)
if (irq == bottom)
goto no_irqs;
res = irq_alloc_desc_at(irq, -1);