media: media/v4l2: remove V4L2_FLAG_MEMORY_NON_CONSISTENT flag

The patch partially reverts some of the UAPI bits of the buffer
cache management hints. Namely, the queue consistency (memory
coherency) user-space hint because, as it turned out, the kernel
implementation of this feature was misusing DMA_ATTR_NON_CONSISTENT.

The patch reverts both kernel and user space parts: removes the
DMA consistency attr functions, rolls back changes to v4l2_requestbuffers,
v4l2_create_buffers structures and corresponding UAPI functions
(plus compat32 layer) and cleans up the documentation.

[hverkuil: fixed a few typos in the commit log]
[hverkuil: fixed vb2_core_reqbufs call in drivers/media/dvb-core/dvb_vb2.c]
[mchehab: fixed a typo in the commit log: revers->reverts]

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
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

@@ -701,23 +701,6 @@ Memory Consistency Flags
     :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
 
 enum v4l2_memory

Documentation/userspace-api/media/v4l/vidioc-create-bufs.rst

@@ -120,13 +120,9 @@ than the number requested.
 	If you want to just query the capabilities without making any
 	other changes, then set ``count`` to 0, ``memory`` to
 	``V4L2_MEMORY_MMAP`` and ``format.type`` to the buffer type.
-    * - __u32
-      - ``flags``
-      - Specifies additional buffer management attributes.
-	See :ref:`memory-flags`.
 
     * - __u32
-      - ``reserved``\ [6]
+      - ``reserved``\ [7]
       - A place holder for future extensions. Drivers and applications
 	must set the array to zero.
 

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

@@ -112,17 +112,10 @@ aborting or finishing any DMA in progress, an implicit
 	``V4L2_MEMORY_MMAP`` and ``type`` set to the buffer type. This will
 	free any previously allocated buffers, so this is typically something
 	that will be done at the start of the application.
-    * - union {
-      - (anonymous)
-    * - __u32
-      - ``flags``
-      - Specifies additional buffer management attributes.
-	See :ref:`memory-flags`.
     * - __u32
       - ``reserved``\ [1]
-      - Kept for backwards compatibility. Use ``flags`` instead.
-    * - }
-      -
+      - A place holder for future extensions. Drivers and applications
+	must set the array to zero.
 
 .. tabularcolumns:: |p{6.1cm}|p{2.2cm}|p{8.7cm}|
 
@@ -169,7 +162,6 @@ 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_FLAG_MEMORY_NON_CONSISTENT <V4L2-FLAG-MEMORY-NON-CONSISTENT>`,
         :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>`.