[PATCHv3 5/8] videobuf2: add V4L2_MEMORY_FLAG_NON_COHERENT flag

Sergey Senozhatsky
2021-07-09T09:20:51Z

By setting or clearing V4L2_MEMORY_FLAG_NON_COHERENT flag
user-space should be able to hint vb2 that either a non-coherent
(if supported) or coherent memory should be used for the buffer
allocation.

Signed-off-by: Sergey Senozhatsky <senozhatsky@chromium.org>
---
.../userspace-api/media/v4l/buffer.rst | 40 ++++++++++++++++++-
.../media/v4l/vidioc-reqbufs.rst | 5 ++-
include/uapi/linux/videodev2.h | 2 +
3 files changed, 43 insertions(+), 4 deletions(-)

diff --git a/Documentation/userspace-api/media/v4l/buffer.rst b/Documentation/userspace-api/media/v4l/buffer.rst
index e991ba73d873..4638ec64db00 100644
--- a/Documentation/userspace-api/media/v4l/buffer.rst
+++ b/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
=========
diff --git a/Documentation/userspace-api/media/v4l/vidioc-reqbufs.rst b/Documentation/userspace-api/media/v4l/vidioc-reqbufs.rst
index 50ea72043bb0..e59306aba2b0 100644
--- a/Documentation/userspace-api/media/v4l/vidioc-reqbufs.rst
+++ b/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

diff --git a/include/uapi/linux/videodev2.h b/include/uapi/linux/videodev2.h
index 9260791b8438..9d11e1d9c934 100644
--- a/include/uapi/linux/videodev2.h
+++ b/include/uapi/linux/videodev2.h
@@ -956,6 +956,8 @@ 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)
--
2.32.0.93.g670b81a890-goog