iommu/core: let drivers know if an iommu fault handler isn't installed

Make report_iommu_fault() return -ENOSYS whenever an iommu fault
handler isn't installed, so IOMMU drivers can then do their own
platform-specific default behavior if they wanted.

Fault handlers can still return -ENOSYS in case they want to elicit the
default behavior of the IOMMU drivers.
Signed-off-by: Ohad Ben-Cohen <ohad@wizery.com>
Signed-off-by: Joerg Roedel <joerg.roedel@amd.com>

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)

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,