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Hmm, this paragraph is probably outdated (copy and paste?)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@xxxxxxxxxxxx>
> ---
> .../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
