media: videobuf2: add V4L2_FLAG_MEMORY_NON_CONSISTENT flag

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_MMAP_CACHE_HINTS
when queue supports user-space cache management hints. Note,
however, that MMAP_CACHE_HINTS capability only valid when the
queue is used for memory MMAP-ed streaming I/O.

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>

1 files changed, 38 insertions, 2 deletions

diff --git a/Documentation/userspace-api/media/v4l/buffer.rst b/Documentation/userspace-api/media/v4l/buffer.rst
index 951ae1ed485f..5088393b5a5c 100644
--- a/Documentation/userspace-api/media/v4l/buffer.rst
+++ b/Documentation/userspace-api/media/v4l/buffer.rst
@@ -577,7 +577,10 @@ Buffer Flags
 	applications shall use this flag if the data captured in the
 	buffer is not going to be touched by the CPU, instead the buffer
 	will, probably, be passed on to a DMA-capable hardware unit for
-	further processing or output.
+	further processing or output. This flag is ignored unless the
+	queue is used for :ref:`memory mapping <mmap>` streaming I/O and
+	reports :ref:`V4L2_BUF_CAP_SUPPORTS_MMAP_CACHE_HINTS
+	<V4L2-BUF-CAP-SUPPORTS-MMAP-CACHE-HINTS>` capability.
     * .. _`V4L2-BUF-FLAG-NO-CACHE-CLEAN`:
 
       - ``V4L2_BUF_FLAG_NO_CACHE_CLEAN``
@@ -585,7 +588,10 @@ Buffer Flags
       - Caches do not have to be cleaned for this buffer. Typically
 	applications shall use this flag for output buffers if the data in
 	this buffer has not been created by the CPU but by some
-	DMA-capable unit, in which case caches have not been used.
+	DMA-capable unit, in which case caches have not been used. This flag
+	is ignored unless the queue is used for :ref:`memory mapping <mmap>`
+	streaming I/O and reports :ref:`V4L2_BUF_CAP_SUPPORTS_MMAP_CACHE_HINTS
+	<V4L2-BUF-CAP-SUPPORTS-MMAP-CACHE-HINTS>` capability.
     * .. _`V4L2-BUF-FLAG-M2M-HOLD-CAPTURE-BUF`:
 
       - ``V4L2_BUF_FLAG_M2M_HOLD_CAPTURE_BUF``
@@ -681,6 +687,36 @@ 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_MMAP_CACHE_HINTS
+	<V4L2-BUF-CAP-SUPPORTS-MMAP-CACHE-HINTS>` capability.
 
 .. c:type:: v4l2_memory