media: videobuf2: add V4L2_MEMORY_FLAG_NON_COHERENT flag

By setting or clearing the V4L2_MEMORY_FLAG_NON_COHERENT flag
user-space should be able to hint vb2 that either non-coherent
(if supported) or coherent memory should be used for the buffer
allocation.
Signed-off-by: Sergey Senozhatsky <senozhatsky@chromium.org>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>

Documentation/userspace-api/media/v4l/buffer.rst

@@ -676,8 +676,6 @@ Buffer Flags
 
     \normalsize
 
-.. _memory-flags:
-
 enum v4l2_memory
 ================
 
@@ -701,6 +699,44 @@ enum v4l2_memory
       - 4
       - The buffer is used for :ref:`DMA shared buffer <dmabuf>` I/O.
 
+.. _memory-flags:
+
+Memory Consistency Flags
+------------------------
+
+.. raw:: latex
+
+    \small
+
+.. tabularcolumns:: |p{7.0cm}|p{2.1cm}|p{8.4cm}|
+
+.. cssclass:: longtable
+
+.. flat-table::
+    :header-rows:  0
+    :stub-columns: 0
+    :widths:       3 1 4
+
+    * .. _`V4L2-MEMORY-FLAG-NON-COHERENT`:
+
+      - ``V4L2_MEMORY_FLAG_NON_COHERENT``
+      - 0x00000001
+      - A buffer is allocated either in coherent (it will be automatically
+	coherent between the CPU and the bus) or non-coherent memory. The
+	latter can provide performance gains, for instance the CPU cache
+	sync/flush operations can be avoided if the buffer is accessed by the
+	corresponding device only and the CPU does not read/write to/from that
+	buffer. However, this requires extra care from the driver -- it must
+	guarantee memory consistency by issuing a cache flush/sync when
+	consistency is needed. If this flag is set V4L2 will attempt to
+	allocate the buffer in non-coherent memory. The flag takes effect
+	only if the buffer is used for :ref:`memory mapping <mmap>` I/O and the
+	queue reports the :ref:`V4L2_BUF_CAP_SUPPORTS_MMAP_CACHE_HINTS
+	<V4L2-BUF-CAP-SUPPORTS-MMAP-CACHE-HINTS>` capability.
+
+.. raw:: latex
+
+    \normalsize
 
 Timecodes
 =========

Documentation/userspace-api/media/v4l/vidioc-reqbufs.rst

@@ -158,8 +158,9 @@ aborting or finishing any DMA in progress, an implicit
       - This capability is set by the driver to indicate that the queue supports
         cache and memory management hints. However, it's only valid when the
         queue is used for :ref:`memory mapping <mmap>` streaming I/O. See
-        :ref:`V4L2_BUF_FLAG_NO_CACHE_INVALIDATE <V4L2-BUF-FLAG-NO-CACHE-INVALIDATE>` and
-        :ref:`V4L2_BUF_FLAG_NO_CACHE_CLEAN <V4L2-BUF-FLAG-NO-CACHE-CLEAN>`.
+        :ref:`V4L2_BUF_FLAG_NO_CACHE_INVALIDATE <V4L2-BUF-FLAG-NO-CACHE-INVALIDATE>`,
+        :ref:`V4L2_BUF_FLAG_NO_CACHE_CLEAN <V4L2-BUF-FLAG-NO-CACHE-CLEAN>` and
+        :ref:`V4L2_MEMORY_FLAG_NON_COHERENT <V4L2-MEMORY-FLAG-NON-COHERENT>`.
 
 .. raw:: latex
 

include/uapi/linux/videodev2.h

@@ -962,6 +962,8 @@ struct v4l2_requestbuffers {
 	__u32			reserved[1];
 };
 
+#define V4L2_MEMORY_FLAG_NON_COHERENT			(1 << 0)
+
 /* capabilities for struct v4l2_requestbuffers and v4l2_create_buffers */
 #define V4L2_BUF_CAP_SUPPORTS_MMAP			(1 << 0)
 #define V4L2_BUF_CAP_SUPPORTS_USERPTR			(1 << 1)