[PATCH] iommu: explicitly check for NULL in iommu_dma_get_resv_regions()

[Aleksandr Fedorov]

iommu_dma_get_resv_regions() assumes that iommu_fwspec field for
corresponding device is set which is not always true.  Since
iommu_dma_get_resv_regions() seems to be a future-proof generic API
that can be used by any iommu driver, add an explicit check for NULL.

Currently it can work by accident since compiler can eliminate
the 'iommu_fwspec' check altogether when CONFIG_ACPI_IORT=n, but
code elimination from optimizations is not reliable.

Signed-off-by: Aleksandr Fedorov <halcien@xxxxxxxxx>
---
A compilation failure has been observed on a gcc-compatible compiler based on EDG.

diff --git a/drivers/iommu/dma-iommu.c b/drivers/iommu/dma-iommu.c
index d85d54f2b549..474b1b7211d7 100644
--- a/drivers/iommu/dma-iommu.c
+++ b/drivers/iommu/dma-iommu.c
@@ -382,10 +382,10 @@ void iommu_put_dma_cookie(struct iommu_domain *domain)
  */
 void iommu_dma_get_resv_regions(struct device *dev, struct list_head *list)
 {
+	struct iommu_fwspec *iommu_fwspec = dev_iommu_fwspec_get(dev);
 
-	if (!is_of_node(dev_iommu_fwspec_get(dev)->iommu_fwnode))
+	if (iommu_fwspec && !is_of_node(iommu_fwspec->iommu_fwnode))
 		iort_iommu_msi_get_resv_regions(dev, list);
-
 }
 EXPORT_SYMBOL(iommu_dma_get_resv_regions);