dma-buf: Document struct dma_buf_map

.. 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

-------------------

#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