+static int next_irq = 16;
+
+/*
+ * gsi_irq_sharing -- Name overload! "irq" can be either a legacy IRQ
+ * in the range 0-15, a linux IRQ in the range 0-223, or a GSI number
+ * from ACPI, which can reach 800 in large boxen.
+ *
+ * Compact the sparse GSI space into a sequential IRQ series and reuse
+ * vectors if possible.
+ */
+int gsi_irq_sharing(int gsi)
+{
+ int i, tries, vector;
+
+ BUG_ON(gsi >= NR_IRQ_VECTORS);
+
+ if (platform_legacy_irq(gsi))
+ return gsi;
+
+ if (gsi_2_irq[gsi] != 0xFF)
+ return (int)gsi_2_irq[gsi];
+
+ tries = NR_IRQS;
+ try_again:
+ vector = assign_irq_vector(gsi);
+
+ /*
+ * Sharing vectors means sharing IRQs, so scan irq_vectors for previous
+ * use of vector and if found, return that IRQ. However, we never want
+ * to share legacy IRQs, which usually have a different trigger mode
+ * than PCI.
+ */
+ for (i = 0; i < NR_IRQS; i++)
+ if (IO_APIC_VECTOR(i) == vector)
+ break;
+ if (platform_legacy_irq(i)) {
+ if (--tries >= 0) {
+ IO_APIC_VECTOR(i) = 0;
+ goto try_again;
+ }
+ panic("gsi_irq_sharing: didn't find an IRQ using vector 0x%02X for GSI %d", vector, gsi);
+ }
+ if (i < NR_IRQS) {
+ gsi_2_irq[gsi] = i;
+ printk(KERN_INFO "GSI %d sharing vector 0x%02X and IRQ %d\n",
+ gsi, vector, i);
+ return i;
+ }
+
+ i = next_irq++;
+ BUG_ON(i >= NR_IRQS);
+ gsi_2_irq[gsi] = i;
+ IO_APIC_VECTOR(i) = vector;
+ printk(KERN_INFO "GSI %d assigned vector 0x%02X and IRQ %d\n",
+ gsi, vector, i);
+ return i;
+}
+