This adds a new "DMA Buffer ioctls" section to the dma-buf docs and adds
documentation for DMA_BUF_IOCTL_SYNC.
v2 (Daniel Vetter):
- Fix a couple typos
- Add commentary about synchronization with other devices
- Use item list format for describing flags
Signed-off-by: Jason Ekstrand <jason@xxxxxxxxxxxxxx>
Reviewed-by: Daniel Vetter <daniel.vetter@xxxxxxxx>
Cc: Christian König <christian.koenig@xxxxxxx>
Cc: Sumit Semwal <sumit.semwal@xxxxxxxxxx>
---
Documentation/driver-api/dma-buf.rst | 8 +++++
include/uapi/linux/dma-buf.h | 46 +++++++++++++++++++++++++++-
2 files changed, 53 insertions(+), 1 deletion(-)
diff --git a/Documentation/driver-api/dma-buf.rst b/Documentation/driver-api/dma-buf.rst
index 7f21425d9435a..0d4c13ec1a800 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..1c131002fe1ee 100644
--- a/include/uapi/linux/dma-buf.h
+++ b/include/uapi/linux/dma-buf.h
@@ -22,8 +22,52 @@
#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 must 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.
+ *
+ * The synchronization provided via DMA_BUF_IOCTL_SYNC only provides cache
+ * coherency. It does not prevent other processes or devices from
+ * accessing the memory at the same time. If synchronization with a GPU or
+ * other device driver is required, it is the client's responsibility to
+ * wait for buffer to be ready for reading or writing. If the driver or
+ * API with which the client is interacting uses implicit synchronization,
+ * this can be done via poll() on the DMA buffer file descriptor. If the
+ * driver or API requires explicit synchronization, the client may have to
+ * wait on a sync_file or other synchronization primitive outside the scope
+ * of the DMA buffer API.
+ */
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_WRITE:
+ * 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
