Re: [PATCH 3/6] dma-buf: Document DMA_BUF_IOCTL_SYNC (v2)

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Am 10.06.21 um 23:09 schrieb Jason Ekstrand:
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>
Cc: Daniel Vetter <daniel.vetter@xxxxxxxx>
Cc: Christian König <christian.koenig@xxxxxxx>
Cc: Sumit Semwal <sumit.semwal@xxxxxxxxxx>

Acked-by: Christian König <christian.koenig@xxxxxxx>

---
  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;
  };




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