iommu: Add a kdoc to iommu_unmap()

Describe the most conservative version of the driver implementations.
All drivers should support this.

Many drivers support extending the range if a large page is hit, but
let's not make that officially approved API. The main point is to
document explicitly that split is not supported.

Reviewed-by: Liviu Dudau <liviu.dudau@arm.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>
Link: https://lore.kernel.org/r/3-v3-b3a5b5937f56+7bb-arm_no_split_jgg@nvidia.com
Signed-off-by: Will Deacon <will@kernel.org>

drivers/iommu/iommu.c

@@ -2586,6 +2586,20 @@ static size_t __iommu_unmap(struct iommu_domain *domain,
 	return unmapped;
 }
 
+/**
+ * iommu_unmap() - Remove mappings from a range of IOVA
+ * @domain: Domain to manipulate
+ * @iova: IO virtual address to start
+ * @size: Length of the range starting from @iova
+ *
+ * iommu_unmap() will remove a translation created by iommu_map(). It cannot
+ * subdivide a mapping created by iommu_map(), so it should be called with IOVA
+ * ranges that match what was passed to iommu_map(). The range can aggregate
+ * contiguous iommu_map() calls so long as no individual range is split.
+ *
+ * Returns: Number of bytes of IOVA unmapped. iova + res will be the point
+ * unmapping stopped.
+ */
 size_t iommu_unmap(struct iommu_domain *domain,
 		   unsigned long iova, size_t size)
 {