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Subject[PATCHv4 03/11] videobuf2: add V4L2_FLAG_MEMORY_NON_CONSISTENT flag
Date
By setting or clearing V4L2_FLAG_MEMORY_NON_CONSISTENT flag
user-space should be able to set or clear queue's NON_CONSISTENT
->dma_attrs. Queue's ->dma_attrs are passed to the underlying
allocator in __vb2_buf_mem_alloc(), so thus user-space is able
to request vb2 buffer's memory to be either consistent (coherent)
or non-consistent.

The patch set also adds a corresponding capability flag:
fill_buf_caps() reports V4L2_BUF_CAP_SUPPORTS_CACHE_HINTS when
queue supports user-space cache management hints.

Signed-off-by: Sergey Senozhatsky <senozhatsky@chromium.org>
---
Documentation/media/uapi/v4l/buffer.rst | 29 +++++++++++++++++++
.../media/uapi/v4l/vidioc-reqbufs.rst | 7 +++++
.../media/common/videobuf2/videobuf2-v4l2.c | 2 ++
include/uapi/linux/videodev2.h | 3 ++
4 files changed, 41 insertions(+)

diff --git a/Documentation/media/uapi/v4l/buffer.rst b/Documentation/media/uapi/v4l/buffer.rst
index 3112300c2fa0..8084e3f2a58d 100644
--- a/Documentation/media/uapi/v4l/buffer.rst
+++ b/Documentation/media/uapi/v4l/buffer.rst
@@ -681,6 +681,35 @@ Buffer Flags

\normalsize

+.. _memory-flags:
+
+Memory Consistency Flags
+========================
+
+.. tabularcolumns:: |p{7.0cm}|p{2.2cm}|p{8.3cm}|
+
+.. cssclass:: longtable
+
+.. flat-table::
+ :header-rows: 0
+ :stub-columns: 0
+ :widths: 3 1 4
+
+ * .. _`V4L2_FLAG_MEMORY_NON_CONSISTENT`:
+
+ - ``V4L2_FLAG_MEMORY_NON_CONSISTENT``
+ - 0x00000001
+ - A buffer is allocated either in consistent (it will be automatically
+ coherent between the CPU and the bus) or non-consistent 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-consistent 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_CACHE_HINTS` capability.

.. c:type:: v4l2_memory

diff --git a/Documentation/media/uapi/v4l/vidioc-reqbufs.rst b/Documentation/media/uapi/v4l/vidioc-reqbufs.rst
index d0c643db477a..21ecacc72487 100644
--- a/Documentation/media/uapi/v4l/vidioc-reqbufs.rst
+++ b/Documentation/media/uapi/v4l/vidioc-reqbufs.rst
@@ -156,6 +156,13 @@ aborting or finishing any DMA in progress, an implicit
- Only valid for stateless decoders. If set, then userspace can set the
``V4L2_BUF_FLAG_M2M_HOLD_CAPTURE_BUF`` flag to hold off on returning the
capture buffer until the OUTPUT timestamp changes.
+ * - ``V4L2_BUF_CAP_SUPPORTS_CACHE_HINTS``
+ - 0x00000040
+ - Set when the queue/buffer supports memory consistency and cache
+ management hints. See :ref:`V4L2_FLAG_MEMORY_NON_CONSISTENT`,
+ :ref:`V4L2_BUF_FLAG_NO_CACHE_INVALIDATE` and
+ :ref:`V4L2_BUF_FLAG_NO_CACHE_CLEAN`.
+

Return Value
============
diff --git a/drivers/media/common/videobuf2/videobuf2-v4l2.c b/drivers/media/common/videobuf2/videobuf2-v4l2.c
index 2a604bd7793a..c847bcea6e95 100644
--- a/drivers/media/common/videobuf2/videobuf2-v4l2.c
+++ b/drivers/media/common/videobuf2/videobuf2-v4l2.c
@@ -711,6 +711,8 @@ static void fill_buf_caps(struct vb2_queue *q, u32 *caps)
*caps |= V4L2_BUF_CAP_SUPPORTS_DMABUF;
if (q->subsystem_flags & VB2_V4L2_FL_SUPPORTS_M2M_HOLD_CAPTURE_BUF)
*caps |= V4L2_BUF_CAP_SUPPORTS_M2M_HOLD_CAPTURE_BUF;
+ if (q->allow_cache_hints && (q->io_modes & VB2_MMAP))
+ *caps |= V4L2_BUF_CAP_SUPPORTS_CACHE_HINTS;
#ifdef CONFIG_MEDIA_CONTROLLER_REQUEST_API
if (q->supports_requests)
*caps |= V4L2_BUF_CAP_SUPPORTS_REQUESTS;
diff --git a/include/uapi/linux/videodev2.h b/include/uapi/linux/videodev2.h
index 5f9357dcb060..e92c29864730 100644
--- a/include/uapi/linux/videodev2.h
+++ b/include/uapi/linux/videodev2.h
@@ -189,6 +189,8 @@ enum v4l2_memory {
V4L2_MEMORY_DMABUF = 4,
};

+#define V4L2_FLAG_MEMORY_NON_CONSISTENT (1 << 0)
+
/* see also http://vektor.theorem.ca/graphics/ycbcr/ */
enum v4l2_colorspace {
/*
@@ -946,6 +948,7 @@ struct v4l2_requestbuffers {
#define V4L2_BUF_CAP_SUPPORTS_REQUESTS (1 << 3)
#define V4L2_BUF_CAP_SUPPORTS_ORPHANED_BUFS (1 << 4)
#define V4L2_BUF_CAP_SUPPORTS_M2M_HOLD_CAPTURE_BUF (1 << 5)
+#define V4L2_BUF_CAP_SUPPORTS_CACHE_HINTS (1 << 6)

/**
* struct v4l2_plane - plane info for multi-planar buffers
--
2.25.0.265.gbab2e86ba0-goog
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