From ccc22d41bd9acec58cdc7c3b012e1887aba40af4 Mon Sep 17 00:00:00 2001 From: Thomas Zimmermann Date: Fri, 25 Sep 2020 13:56:01 +0200 Subject: [PATCH] dma-buf: Document struct dma_buf_map MIME-Version: 1.0 Content-Type: text/plain; charset=utf8 Content-Transfer-Encoding: 8bit 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 Reviewed-by: Christian König Reviewed-by: Daniel Vetter Acked-by: Sumit Semwal Link: https://patchwork.freedesktop.org/patch/msgid/20200925115601.23955-5-tzimmermann@suse.de --- 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 13ea0cc..6dbcc47 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 c173a4a..fd1aba5 100644 --- a/include/linux/dma-buf-map.h +++ b/include/linux/dma-buf-map.h @@ -9,6 +9,78 @@ #include /** + * 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 `. + * :c:type:`struct 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 ` 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 ` 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 ` 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 ` 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 ` + * 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 * @vaddr: The buffer's address if in system memory -- 2.7.4