iommu: Do not return error code for APIs with size_t return type

Currently, iommu_unmap, iommu_unmap_fast and iommu_map_sg return
size_t.  However, some of the return values are error codes (< 0),
which can be misinterpreted as large size. Therefore, returning size 0
instead to signify failure to map/unmap.

Cc: Joerg Roedel <joro@8bytes.org>
Cc: Alex Williamson <alex.williamson@redhat.com>
Signed-off-by: Suravee Suthikulpanit <suravee.suthikulpanit@amd.com>
Signed-off-by: Joerg Roedel <jroedel@suse.de>

drivers/iommu/amd_iommu.c

@@ -3050,7 +3050,7 @@ static size_t amd_iommu_unmap(struct iommu_domain *dom, unsigned long iova,
 	size_t unmap_size;
 
 	if (domain->mode == PAGE_MODE_NONE)
-		return -EINVAL;
+		return 0;
 
 	mutex_lock(&domain->api_lock);
 	unmap_size = iommu_unmap_page(domain, iova, page_size);

drivers/iommu/iommu.c

@@ -1573,10 +1573,10 @@ static size_t __iommu_unmap(struct iommu_domain *domain,
 
 	if (unlikely(ops->unmap == NULL ||
 		     domain->pgsize_bitmap == 0UL))
-		return -ENODEV;
+		return 0;
 
 	if (unlikely(!(domain->type & __IOMMU_DOMAIN_PAGING)))
-		return -EINVAL;
+		return 0;
 
 	/* find out the minimum page size supported */
 	min_pagesz = 1 << __ffs(domain->pgsize_bitmap);
@@ -1589,7 +1589,7 @@ static size_t __iommu_unmap(struct iommu_domain *domain,
 	if (!IS_ALIGNED(iova | size, min_pagesz)) {
 		pr_err("unaligned: iova 0x%lx size 0x%zx min_pagesz 0x%x\n",
 		       iova, size, min_pagesz);
-		return -EINVAL;
+		return 0;
 	}
 
 	pr_debug("unmap this: iova 0x%lx size 0x%zx\n", iova, size);

include/linux/iommu.h

@@ -465,23 +465,23 @@ static inline int iommu_map(struct iommu_domain *domain, unsigned long iova,
 	return -ENODEV;
 }
 
-static inline int iommu_unmap(struct iommu_domain *domain, unsigned long iova,
-			      size_t size)
+static inline size_t iommu_unmap(struct iommu_domain *domain,
+				 unsigned long iova, size_t size)
 {
-	return -ENODEV;
+	return 0;
 }
 
-static inline int iommu_unmap_fast(struct iommu_domain *domain, unsigned long iova,
-				   int gfp_order)
+static inline size_t iommu_unmap_fast(struct iommu_domain *domain,
+				      unsigned long iova, int gfp_order)
 {
-	return -ENODEV;
+	return 0;
 }
 
 static inline size_t iommu_map_sg(struct iommu_domain *domain,
 				  unsigned long iova, struct scatterlist *sg,
 				  unsigned int nents, int prot)
 {
-	return -ENODEV;
+	return 0;
 }
 
 static inline void iommu_flush_tlb_all(struct iommu_domain *domain)