linux-kernel - Re: [RFC][PATCHv2 03/12] videobuf2: add V4L2_FLAG_MEMORY_NON

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Date:   Wed, 19 Feb 2020 09:19:46 +0100
From:   Hans Verkuil <hverkuil@...all.nl>
To:     Sergey Senozhatsky <senozhatsky@...omium.org>,
        Hans Verkuil <hans.verkuil@...co.com>,
        Tomasz Figa <tfiga@...omium.org>,
        Mauro Carvalho Chehab <mchehab@...nel.org>,
        Kyungmin Park <kyungmin.park@...sung.com>,
        Marek Szyprowski <m.szyprowski@...sung.com>
Cc:     Sakari Ailus <sakari.ailus@....fi>,
        Laurent Pinchart <laurent.pinchart@...asonboard.com>,
        Pawel Osciak <posciak@...omium.org>,
        linux-media@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [RFC][PATCHv2 03/12] videobuf2: add
 V4L2_FLAG_MEMORY_NON_CONSISTENT flag

On 2/4/20 3:56 AM, Sergey Senozhatsky 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@...omium.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

This new part should be added *above* the '.. c:type:: v4l2_memory' line.

You also need to add '.. _memory-flags:' just before this section so that
you can link to it from the create_bufs and reqbufs ioctl descriptions.

> +========================
> +
> +.. 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

accesed -> accessed

> +	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.

V4L2_BUF_CAP_SUPPORTS_CACHE_HINTS is not defined. Add that first in a separate
patch.

Also update the current description of the V4L2_BUF_FLAG_NO_CACHE_INVALIDATE/CLEAN
flags to indicate that they are only valid if V4L2_BUF_CAP_SUPPORTS_CACHE_HINTS
is set (that should be done in the patch adding the V4L2_BUF_CAP_SUPPORTS_CACHE_HINTS
capability).

Regards,

	Hans

> +
>  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 {
>  	/*
>