Re: [PATCH 5/8] videobuf2: add V4L2_FLAG_MEMORY_NON_COHERENT flag

[Hans Verkuil <hverkuil-cisco@xs4all.nl>]

On 02/03/2021 01:46, Sergey Senozhatsky wrote:
> By setting or clearing V4L2_FLAG_MEMORY_NON_COHERENT flag
> user-space should be able to hint vb2 that either a non-coherent
> (if supported) or coherent memory should be used for the buffer
> allocation.
> 
> 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.

Hmm, this paragraph is probably outdated (copy and paste?) since this
capability already exists.

> 
> Signed-off-by: Sergey Senozhatsky <senozhatsky@chromium.org>
> ---
>  .../userspace-api/media/v4l/buffer.rst        | 40 ++++++++++++++++++-
>  .../media/v4l/vidioc-reqbufs.rst              |  5 ++-
>  include/uapi/linux/videodev2.h                |  2 +
>  3 files changed, 43 insertions(+), 4 deletions(-)
> 
> diff --git a/Documentation/userspace-api/media/v4l/buffer.rst b/Documentation/userspace-api/media/v4l/buffer.rst
> index 1b0fdc160533..a39852d6174f 100644
> --- a/Documentation/userspace-api/media/v4l/buffer.rst
> +++ b/Documentation/userspace-api/media/v4l/buffer.rst
> @@ -676,8 +676,6 @@ Buffer Flags
>  
>      \normalsize
>  
> -.. _memory-flags:
> -
>  enum v4l2_memory
>  ================
>  
> @@ -701,6 +699,44 @@ enum v4l2_memory
>        - 4
>        - The buffer is used for :ref:`DMA shared buffer <dmabuf>` I/O.
>  
> +.. _memory-flags:
> +
> +Memory Consistency Flags
> +------------------------
> +
> +.. raw:: latex
> +
> +    \small
> +
> +.. tabularcolumns:: |p{7.0cm}|p{2.1cm}|p{8.4cm}|
> +
> +.. cssclass:: longtable
> +
> +.. flat-table::
> +    :header-rows:  0
> +    :stub-columns: 0
> +    :widths:       3 1 4
> +
> +    * .. _`V4L2-FLAG-MEMORY-NON-COHERENT`:
> +
> +      - ``V4L2_FLAG_MEMORY_NON_COHERENT``

Rename this to V4L2_MEMORY_FLAG_NON_COHERENT: this is consistent with
V4L2_FMT_FLAG_ and V4L2_BUF_FLAG_.

> +      - 0x00000001
> +      - A buffer is allocated either in coherent (it will be automatically
> +	coherent between the CPU and the bus) or non-coherent 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-coherent 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.
> +
> +.. raw:: latex
> +
> +    \normalsize
>  
>  Timecodes
>  =========
> diff --git a/Documentation/userspace-api/media/v4l/vidioc-reqbufs.rst b/Documentation/userspace-api/media/v4l/vidioc-reqbufs.rst
> index c1c88e00b106..950e7ec1aac5 100644
> --- a/Documentation/userspace-api/media/v4l/vidioc-reqbufs.rst
> +++ b/Documentation/userspace-api/media/v4l/vidioc-reqbufs.rst
> @@ -154,8 +154,9 @@ 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_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>`.
> +        :ref:`V4L2_BUF_FLAG_NO_CACHE_INVALIDATE <V4L2-BUF-FLAG-NO-CACHE-INVALIDATE>`,
> +        :ref:`V4L2_BUF_FLAG_NO_CACHE_CLEAN <V4L2-BUF-FLAG-NO-CACHE-CLEAN>` and
> +        :ref:`V4L2_FLAG_MEMORY_NON_COHERENT <V4L2-FLAG-MEMORY-NON-COHERENT>`.
>  
>  Return Value
>  ============
> diff --git a/include/uapi/linux/videodev2.h b/include/uapi/linux/videodev2.h
> index 79dbde3bcf8d..b1d4171fe50b 100644
> --- a/include/uapi/linux/videodev2.h
> +++ b/include/uapi/linux/videodev2.h
> @@ -954,6 +954,8 @@ struct v4l2_requestbuffers {
>  	__u32			reserved[1];
>  };
>  
> +#define V4L2_FLAG_MEMORY_NON_COHERENT			(1 << 0)
> +
>  /* capabilities for struct v4l2_requestbuffers and v4l2_create_buffers */
>  #define V4L2_BUF_CAP_SUPPORTS_MMAP			(1 << 0)
>  #define V4L2_BUF_CAP_SUPPORTS_USERPTR			(1 << 1)
> 

Regards,

	Hans