[RFC v4 17/18] docs-rst: Document precise V4L2_BUF_FLAG_NO_CACHE_SYNC flag behaviour

[Sakari Ailus]

Document when should the user specify V4L2_BUF_FLAG_NO_CACHE_SYNC flag.

Signed-off-by: Sakari Ailus <sakari.ailus at linux.intel.com>
---
 Documentation/media/uapi/v4l/buffer.rst | 21 +++++++++++++++++++++
 1 file changed, 21 insertions(+)

diff --git a/Documentation/media/uapi/v4l/buffer.rst b/Documentation/media/uapi/v4l/buffer.rst
index 9eb42bd..e1f93dd 100644
--- a/Documentation/media/uapi/v4l/buffer.rst
+++ b/Documentation/media/uapi/v4l/buffer.rst
@@ -570,6 +570,27 @@ Buffer Flags
 	then read by another one, in which case the flag should be set in both
 	:ref:`VIDIOC_QBUF` and :ref:`VIDIOC_DQBUF` ioctls. This flag has no
 	effect on some devices / architectures.
+
+	More specifically, this flag causes cache synchronisation to
+	be skipped for OUTPUT buffers when the buffer is queued. The
+	flag can be used if the buffer has been previously written to
+	by hardware but has not been written to by the CPU.
+
+	Additionally, if this flag is specified for a CAPTURE buffer
+	when it is queued, cache synchronisation is skipped. This
+	signals that the application can guarantee that it has not
+	written to the buffer memory since it was last dequeued from
+	the device.
+
+	Specifying this flag for a CAPTURE buffer when
+	dequeueing a buffer will skip cache maintenance for the buffer
+	memory. An application may not access the buffer memory in
+	that case but it may well be passed onwards to another device
+	in the system.
+
+	Specifying this flag has no effect when dequeuing an OUTPUT
+	buffer.
+
     * .. _`V4L2-BUF-FLAG-LAST`:
 
       - ``V4L2_BUF_FLAG_LAST``
-- 
2.7.4