[PATCH] alpha HARDIRQ_BITS

From Jeff.Wiedemeier@hp.com:

Adjust Alpha HARDIRQ_BITS check to make sure there is enough
room for each IPL, not each interrupt (Marvel can have
too many unique device interrupts for that, and it really
only needs to cover potential nesting of interrupts, which covering
the IPLs does)

include/asm-alpha/hardirq.h

@@ -55,10 +55,12 @@ typedef struct {
 
 /*
  * The hardirq mask has to be large enough to have
- * space for potentially all IRQ sources in the system
- * nesting on a single CPU:
+ * space for potentially nestable IRQ sources in the system
+ * to nest on a single CPU. On Alpha, interrupts are masked at the CPU
+ * by IPL as well as at the system level. We only have 8 IPLs (UNIX PALcode)
+ * so we really only have 8 nestable IRQs, but allow some overhead
  */
-#if (1 << HARDIRQ_BITS) < NR_IRQS
+#if (1 << HARDIRQ_BITS) < 16
 #error HARDIRQ_BITS is too low!
 #endif