diff --git a/drivers/iommu/iommu.c b/drivers/iommu/iommu.c
index 3a072596b1b2..bd2d4d2764dd 100644
--- a/drivers/iommu/iommu.c
+++ b/drivers/iommu/iommu.c
@@ -43,6 +43,12 @@ EXPORT_SYMBOL_GPL(iommu_found);
  * iommu_set_fault_handler() - set a fault handler for an iommu domain
  * @domain: iommu domain
  * @handler: fault handler
+ *
+ * This function should be used by IOMMU users which want to be notified
+ * whenever an IOMMU fault happens.
+ *
+ * The fault handler itself should return 0 on success, and an appropriate
+ * error code otherwise.
  */
 void iommu_set_fault_handler(struct iommu_domain *domain,
 					iommu_fault_handler_t handler)
diff --git a/include/linux/iommu.h b/include/linux/iommu.h
index d084e8777e0e..ddad0ae0a433 100644
--- a/include/linux/iommu.h
+++ b/include/linux/iommu.h
@@ -98,11 +98,15 @@ extern void iommu_set_fault_handler(struct iommu_domain *domain,
  * Returns 0 on success and an appropriate error code otherwise (if dynamic
  * PTE/TLB loading will one day be supported, implementations will be able
  * to tell whether it succeeded or not according to this return value).
+ *
+ * Specifically, -ENOSYS is returned if a fault handler isn't installed
+ * (though fault handlers can also return -ENOSYS, in case they want to
+ * elicit the default behavior of the IOMMU drivers).
  */
 static inline int report_iommu_fault(struct iommu_domain *domain,
 		struct device *dev, unsigned long iova, int flags)
 {
-	int ret = 0;
+	int ret = -ENOSYS;
 
 	/*
 	 * if upper layers showed interest and installed a fault handler,