[PATCH v3 09/17] docs: core-api: document the IOVA-based API

Leon Romanovsky
2024-11-10T13:48:01Z

From: Christoph Hellwig <hch@lst.de>

Add an explanation of the newly added IOVA-based mapping API.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Leon Romanovsky <leonro@nvidia.com>
---
Documentation/core-api/dma-api.rst | 70 ++++++++++++++++++++++++++++++
1 file changed, 70 insertions(+)

diff --git a/Documentation/core-api/dma-api.rst b/Documentation/core-api/dma-api.rst
index 8e3cce3d0a23..61d6f4fe3d88 100644
--- a/Documentation/core-api/dma-api.rst
+++ b/Documentation/core-api/dma-api.rst
@@ -530,6 +530,76 @@ routines, e.g.:::
....
}

+Part Ie - IOVA-based DMA mappings
+---------------------------------
+
+These APIs allow a very efficient mapping when using an IOMMU. They are an
+optional path that requires extra code and are only recommended for drivers
+where DMA mapping performance, or the space usage for storing the DMA addresses
+matter. All the considerations from the previous section apply here as well.
+
+::
+
+ bool dma_iova_try_alloc(struct device *dev, struct dma_iova_state *state,
+ phys_addr_t phys, size_t size);
+
+Is used to try to allocate IOVA space for mapping operation. If it returns
+false this API can't be used for the given device and the normal streaming
+DMA mapping API should be used. The ``struct dma_iova_state`` is allocated
+by the driver and must be kept around until unmap time.
+
+::
+
+ static inline bool dma_use_iova(struct dma_iova_state *state)
+
+Can be used by the driver to check if the IOVA-based API is used after a
+call to dma_iova_try_alloc. This can be useful in the unmap path.
+
+::
+
+ int dma_iova_link(struct device *dev, struct dma_iova_state *state,
+ phys_addr_t phys, size_t offset, size_t size,
+ enum dma_data_direction dir, unsigned long attrs);
+
+Is used to link ranges to the IOVA previously allocated. The start of all
+but the first call to dma_iova_link for a given state must be aligned
+to the DMA merge boundary returned by ``dma_get_merge_boundary())``, and
+the size of all but the last range must be aligned to the DMA merge boundary
+as well.
+
+::
+
+ int dma_iova_sync(struct device *dev, struct dma_iova_state *state,
+ size_t offset, size_t size);
+
+Must be called to sync the IOMMU page tables for IOVA-range mapped by one or
+more calls to ``dma_iova_link()``.
+
+For drivers that use a one-shot mapping, all ranges can be unmapped and the
+IOVA freed by calling:
+
+::
+
+ void dma_iova_destroy(struct device *dev, struct dma_iova_state *state,
+ enum dma_data_direction dir, unsigned long attrs);
+
+Alternatively drivers can dynamically manage the IOVA space by unmapping
+and mapping individual regions. In that case
+
+::
+
+ void dma_iova_unlink(struct device *dev, struct dma_iova_state *state,
+ size_t offset, size_t size, enum dma_data_direction dir,
+ unsigned long attrs);
+
+is used to unmap a range previously mapped, and
+
+::
+
+ void dma_iova_free(struct device *dev, struct dma_iova_state *state);
+
+is used to free the IOVA space. All regions must have been unmapped using
+``dma_iova_unlink()`` before calling ``dma_iova_free()``.

Part II - Non-coherent DMA allocations
--------------------------------------
--
2.47.0