This patch adds struct dma_buf_map and its helpers to the documentation. A
short tutorial is included.
v3:
* update documentation in a separate patch
* expand docs (Daniel)
* carry-over acks from patch 1
Signed-off-by: Thomas Zimmermann <tzimmermann@xxxxxxx>
Reviewed-by: Christian König <christian.koenig@xxxxxxx>
Reviewed-by: Daniel Vetter <daniel.vetter@xxxxxxxx>
Acked-by: Sumit Semwal <sumit.semwal@xxxxxxxxxx>
---
Documentation/driver-api/dma-buf.rst | 9 ++++
include/linux/dma-buf-map.h | 72 ++++++++++++++++++++++++++++
2 files changed, 81 insertions(+)
diff --git a/Documentation/driver-api/dma-buf.rst b/Documentation/driver-api/dma-buf.rst
index 13ea0cc0a3fa..6dbcc4714b0b 100644
--- a/Documentation/driver-api/dma-buf.rst
+++ b/Documentation/driver-api/dma-buf.rst
@@ -115,6 +115,15 @@ Kernel Functions and Structures Reference
.. kernel-doc:: include/linux/dma-buf.h
:internal:
+Buffer Mapping Helpers
+~~~~~~~~~~~~~~~~~~~~~~
+
+.. kernel-doc:: include/linux/dma-buf-map.h
+ :doc: overview
+
+.. kernel-doc:: include/linux/dma-buf-map.h
+ :internal:
+
Reservation Objects
-------------------
diff --git a/include/linux/dma-buf-map.h b/include/linux/dma-buf-map.h
index c173a4abf4ba..fd1aba545fdf 100644
--- a/include/linux/dma-buf-map.h
+++ b/include/linux/dma-buf-map.h
@@ -8,6 +8,78 @@
#include <linux/io.h>
+/**
+ * DOC: overview
+ *
+ * Calling dma-buf's vmap operation returns a pointer to the buffer's memory.
+ * Depending on the location of the buffer, users may have to access it with
+ * I/O operations or memory load/store operations. For example, copying to
+ * system memory could be done with memcpy(), copying to I/O memory would be
+ * done with memcpy_toio().
+ *
+ * .. code-block:: c
+ *
+ * void *vaddr = ...; // pointer to system memory
+ * memcpy(vaddr, src, len);
+ *
+ * void *vaddr_iomem = ...; // pointer to I/O memory
+ * memcpy_toio(vaddr, _iomem, src, len);
+ *
+ * When using dma-buf's vmap operation, the returned pointer is encoded as
+ * :c:type:`struct dma_buf_map <dma_buf_map>`.
+ * :c:type:`struct dma_buf_map <dma_buf_map>` stores the buffer's address in
+ * system or I/O memory and a flag that signals the required method of
+ * accessing the buffer. Use the returned instance and the helper functions
+ * to access the buffer's memory in the correct way.
+ *
+ * Open-coding access to :c:type:`struct dma_buf_map <dma_buf_map>` is
+ * considered bad style. Rather then accessing its fields directly, use one
+ * of the provided helper functions, or implement your own. For example,
+ * instances of :c:type:`struct dma_buf_map <dma_buf_map>` can be initialized
+ * statically with DMA_BUF_MAP_INIT_VADDR(), or at runtime with
+ * dma_buf_map_set_vaddr(). These helpers will set an address in system memory.
+ *
+ * .. code-block:: c
+ *
+ * struct dma_buf_map map = DMA_BUF_MAP_INIT_VADDR(0xdeadbeaf);
+ *
+ * dma_buf_map_set_vaddr(&map. 0xdeadbeaf);
+ *
+ * Test if a mapping is valid with either dma_buf_map_is_set() or
+ * dma_buf_map_is_null().
+ *
+ * .. code-block:: c
+ *
+ * if (dma_buf_map_is_set(&map) != dma_buf_map_is_null(&map))
+ * // always true
+ *
+ * Instances of :c:type:`struct dma_buf_map <dma_buf_map>` can be compared
+ * for equality with dma_buf_map_is_equal(). Mappings the point to different
+ * memory spaces, system or I/O, are never equal. That's even true if both
+ * spaces are located in the same address space, both mappings contain the
+ * same address value, or both mappings refer to NULL.
+ *
+ * .. code-block:: c
+ *
+ * struct dma_buf_map sys_map; // refers to system memory
+ * struct dma_buf_map io_map; // refers to I/O memory
+ *
+ * if (dma_buf_map_is_equal(&sys_map, &io_map))
+ * // always false
+ *
+ * Instances of struct dma_buf_map do not have to be cleaned up, but
+ * can be cleared to NULL with dma_buf_map_clear(). Cleared mappings
+ * always refer to system memory.
+ *
+ * The type :c:type:`struct dma_buf_map <dma_buf_map>` and its helpers are
+ * actually independent from the dma-buf infrastructure. When sharing buffers
+ * among devices, drivers have to know the location of the memory to access
+ * the buffers in a safe way. :c:type:`struct dma_buf_map <dma_buf_map>`
+ * solves this problem for dma-buf and its users. If other drivers or
+ * sub-systems require similar functionality, the type could be generalized
+ * and moved to a more prominent header file.
+ */
+
/**
* struct dma_buf_map - Pointer to vmap'ed dma-buf memory.
* @vaddr_iomem: The buffer's address if in I/O memory
--
2.28.0
