From d1b758ebc2a82d738092cb42e742470f9d0ea53e Mon Sep 17 00:00:00 2001 From: Konrad Rzeszutek Wilk Date: Thu, 9 Dec 2010 14:53:29 -0500 Subject: xen/irq: Cleanup the find_unbound_irq 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 --- drivers/xen/events.c | 18 ++++++++++++------ 1 file changed, 12 insertions(+), 6 deletions(-) (limited to 'drivers/xen') diff --git a/drivers/xen/events.c b/drivers/xen/events.c index 31af0ac31a98..4d4a23d4c9f6 100644 --- a/drivers/xen/events.c +++ b/drivers/xen/events.c @@ -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); -- cgit v1.2.3-55-g7522