irq_handler
engine->irq_handler = nop_irq_handler;
void (*irq_handler)(struct intel_engine_cs *engine, u16 iir);
engine->irq_handler(engine, iir);
smp_store_mb(engine->irq_handler, fn);
intel_engine_set_irq_handler(engine, irq_handler);
if (dev_priv->irq_handler)
return dev_priv->irq_handler(0, dev_priv);
irqreturn_t(*irq_handler) (int, void *);
dev_priv->irq_handler = intel_irq_handler(dev_priv);
dev_priv->irq_handler = NULL;