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SubjectRe: [RFC][PATCHv2 03/12] videobuf2: add V4L2_FLAG_MEMORY_NON_CONSISTENT flag
On Tue, Feb 4, 2020 at 11:57 AM Sergey Senozhatsky
<senozhatsky@chromium.org> wrote:
>
> 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.
>
> Change-Id: Ib333081c482e23c9a89386078293e19c3fd59076
> Signed-off-by: Sergey Senozhatsky <senozhatsky@chromium.org>
> ---
> Documentation/media/uapi/v4l/buffer.rst | 27 +++++++++++++++++++++++++
> include/uapi/linux/videodev2.h | 2 ++
> 2 files changed, 29 insertions(+)
>
> diff --git a/Documentation/media/uapi/v4l/buffer.rst b/Documentation/media/uapi/v4l/buffer.rst
> index 9149b57728e5..af007daf0591 100644
> --- a/Documentation/media/uapi/v4l/buffer.rst
> +++ b/Documentation/media/uapi/v4l/buffer.rst
> @@ -705,6 +705,33 @@ Buffer Flags
>
> .. c:type:: v4l2_memory
>
> +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
> + - vb2 buffer is allocated either in consistent (it will be automatically
> + coherent between CPU and bus) or non-consistent memory. The latter
> + can provide performance gains, for instance CPU cache sync/flush
> + operations can be avoided if the buffer is accesed by the corresponding
> + device only and CPU does not read/write to/from that buffer. However,
> + this requires extra care from the driver -- it must guarantee memory
> + consistency by issuing cache flush/sync when consistency is needed.
> + If this flag is set V4L2 will attempt to allocate vb2 buffer in
> + non-consistent memory. This flag is ignored if queue does not report
> + :ret:`V4L2_BUF_CAP_SUPPORTS_CACHE_HINTS` capability.

nit: Should the patch adding the capability flag precede this one?

> +
> enum v4l2_memory
> ================
>
> diff --git a/include/uapi/linux/videodev2.h b/include/uapi/linux/videodev2.h
> index 5f9357dcb060..72efc1c544cd 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 {
> /*
> --
> 2.25.0.341.g760bfbb309-goog
>

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