[PATCH 4/7] dma-buf: Document DMA_BUF_IOCTL_SYNC

[Jason Ekstrand]

This adds a new "DMA Buffer ioctls" section to the dma-buf docs and adds
documentation for DMA_BUF_IOCTL_SYNC.

Signed-off-by: Jason Ekstrand <jason at jlekstrand.net>
Cc: Daniel Vetter <daniel.vetter at ffwll.ch>
Cc: Christian König <christian.koenig at amd.com>
Cc: Sumit Semwal <sumit.semwal at linaro.org>
---
 Documentation/driver-api/dma-buf.rst |  8 +++++++
 include/uapi/linux/dma-buf.h         | 32 +++++++++++++++++++++++++++-
 2 files changed, 39 insertions(+), 1 deletion(-)

diff --git a/Documentation/driver-api/dma-buf.rst b/Documentation/driver-api/dma-buf.rst
index 7f37ec30d9fd7..784f84fe50a5e 100644
--- a/Documentation/driver-api/dma-buf.rst
+++ b/Documentation/driver-api/dma-buf.rst
@@ -88,6 +88,9 @@ consider though:
 - The DMA buffer FD is also pollable, see `Implicit Fence Poll Support`_ below for
   details.
 
+- The DMA buffer FD also supports a few dma-buf-specific ioctls, see
+  `DMA Buffer ioctls`_ below for details.
+
 Basic Operation and Device DMA Access
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
@@ -106,6 +109,11 @@ Implicit Fence Poll Support
 .. kernel-doc:: drivers/dma-buf/dma-buf.c
    :doc: implicit fence polling
 
+DMA Buffer ioctls
+~~~~~~~~~~~~~~~~~
+
+.. kernel-doc:: include/uapi/linux/dma-buf.h
+
 Kernel Functions and Structures Reference
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
diff --git a/include/uapi/linux/dma-buf.h b/include/uapi/linux/dma-buf.h
index 7f30393b92c3b..1f67ced853b14 100644
--- a/include/uapi/linux/dma-buf.h
+++ b/include/uapi/linux/dma-buf.h
@@ -22,8 +22,38 @@
 
 #include <linux/types.h>
 
-/* begin/end dma-buf functions used for userspace mmap. */
+/**
+ * struct dma_buf_sync - Synchronize with CPU access.
+ *
+ * When a DMA buffer is accessed from the CPU via mmap, it is not always
+ * possible to guarantee coherency between the CPU-visible map and underlying
+ * memory.  To manage coherency, DMA_BUF_IOCTL_SYNC must be used to bracket
+ * any CPU access to give the kernel the chance to shuffle memory around if
+ * needed.
+ *
+ * Prior to accessing the map, the client should call DMA_BUF_IOCTL_SYNC
+ * with DMA_BUF_SYNC_START and the appropriate read/write flags.  Once the
+ * access is complete, the client should call DMA_BUF_IOCTL_SYNC with
+ * DMA_BUF_SYNC_END and the same read/write flags.
+ */
 struct dma_buf_sync {
+	/**
+	 * @flags: Set of access flags
+	 *
+	 * - DMA_BUF_SYNC_START: Indicates the start of a map access
+	 *   session.
+	 *
+	 * - DMA_BUF_SYNC_END: Indicates the end of a map access session.
+	 *
+	 * - DMA_BUF_SYNC_READ: Indicates that the mapped DMA buffer will
+	 *   be read by the client via the CPU map.
+	 *
+	 * - DMA_BUF_SYNC_READ: Indicates that the mapped DMA buffer will
+	 *   be written by the client via the CPU map.
+	 *
+	 * - DMA_BUF_SYNC_RW: An alias for DMA_BUF_SYNC_READ |
+	 *   DMA_BUF_SYNC_WRITE.
+	 */
 	__u64 flags;
 };
 
-- 
2.31.1