Re: [RFC][PATCHv2 03/12] videobuf2: add V4L2_FLAG_MEMORY_NON_CONSISTENT flag

[Tomasz Figa <tfiga@chromium.org>]

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
>