Re: [RFC RESEND 04/11] v4l: Unify cache management hint buffer flags

[Hans Verkuil <hverkuil@xs4all.nl>]

On 09/11/2015 01:50 PM, Sakari Ailus wrote:
> The V4L2_BUF_FLAG_NO_CACHE_INVALIDATE and V4L2_BUF_FLAG_NO_CACHE_CLEAN
> buffer flags are currently not used by the kernel. Replace the definitions
> by a single V4L2_BUF_FLAG_NO_CACHE_SYNC flag to be used by further
> patches.
> 
> Different cache architectures should not be visible to the user space
> which can make no meaningful use of the differences anyway. In case a
> device can make use of non-coherent memory accesses, the necessary cache
> operations depend on the CPU architecture and the buffer type, not the
> requests of the user. The cache operation itself may be skipped on the
> user's request which was the purpose of the two flags.
> 
> On ARM the invalidate and clean are separate operations whereas on
> x86(-64) the two are a single operation (flush). Whether the hardware uses
> the buffer for reading (V4L2_BUF_TYPE_*_OUTPUT*) or writing
> (V4L2_BUF_TYPE_*CAPTURE*) already defines the required cache operation
> (clean and invalidate, respectively). No user input is required.
> 
> Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>

Acked-by: Hans Verkuil <hans.verkuil@cisco.com>

> ---
>  Documentation/DocBook/media/v4l/io.xml | 25 +++++++++++--------------
>  include/trace/events/v4l2.h            |  3 +--
>  include/uapi/linux/videodev2.h         |  7 +++++--
>  3 files changed, 17 insertions(+), 18 deletions(-)
> 
> diff --git a/Documentation/DocBook/media/v4l/io.xml b/Documentation/DocBook/media/v4l/io.xml
> index 7bbc2a4..4facd63 100644
> --- a/Documentation/DocBook/media/v4l/io.xml
> +++ b/Documentation/DocBook/media/v4l/io.xml
> @@ -1112,21 +1112,18 @@ application. Drivers set or clear this flag when the
>  	  linkend="vidioc-qbuf">VIDIOC_DQBUF</link> ioctl is called.</entry>
>  	  </row>
>  	  <row>
> -	    <entry><constant>V4L2_BUF_FLAG_NO_CACHE_INVALIDATE</constant></entry>
> +	    <entry><constant>V4L2_BUF_FLAG_NO_CACHE_SYNC</constant></entry>
>  	    <entry>0x00000800</entry>
> -	    <entry>Caches do not have to be invalidated for this buffer.
> -Typically applications shall use this flag if the data captured in the buffer
> -is not going to be touched by the CPU, instead the buffer will, probably, be
> -passed on to a DMA-capable hardware unit for further processing or output.
> -</entry>
> -	  </row>
> -	  <row>
> -	    <entry><constant>V4L2_BUF_FLAG_NO_CACHE_CLEAN</constant></entry>
> -	    <entry>0x00001000</entry>
> -	    <entry>Caches do not have to be cleaned for this buffer.
> -Typically applications shall use this flag for output buffers if the data
> -in this buffer has not been created by the CPU but by some DMA-capable unit,
> -in which case caches have not been used.</entry>
> +	    <entry>Do not perform CPU cache synchronisation operations
> +	    when the buffer is queued or dequeued. The user is
> +	    responsible for the correct use of this flag. It should be
> +	    only used when the buffer is not accessed using the CPU,
> +	    e.g. the buffer is written to by a hardware block and then
> +	    read by another one, in which case the flag should be set
> +	    in both <link linkend="vidioc-qbuf">VIDIOC_DQBUF</link>
> +	    and <link linkend="vidioc-qbuf">VIDIOC_QBUF</link> IOCTLs.
> +	    The flag has no effect on some devices / architectures.
> +	    </entry>
>  	  </row>
>  	  <row>
>  	    <entry><constant>V4L2_BUF_FLAG_LAST</constant></entry>
> diff --git a/include/trace/events/v4l2.h b/include/trace/events/v4l2.h
> index dbf017b..4cee91d 100644
> --- a/include/trace/events/v4l2.h
> +++ b/include/trace/events/v4l2.h
> @@ -78,8 +78,7 @@ SHOW_FIELD
>  		{ V4L2_BUF_FLAG_ERROR,		     "ERROR" },		      \
>  		{ V4L2_BUF_FLAG_TIMECODE,	     "TIMECODE" },	      \
>  		{ V4L2_BUF_FLAG_PREPARED,	     "PREPARED" },	      \
> -		{ V4L2_BUF_FLAG_NO_CACHE_INVALIDATE, "NO_CACHE_INVALIDATE" }, \
> -		{ V4L2_BUF_FLAG_NO_CACHE_CLEAN,	     "NO_CACHE_CLEAN" },      \
> +		{ V4L2_BUF_FLAG_NO_CACHE_SYNC,	     "NO_CACHE_SYNC" },	      \
>  		{ V4L2_BUF_FLAG_TIMESTAMP_MASK,	     "TIMESTAMP_MASK" },      \
>  		{ V4L2_BUF_FLAG_TIMESTAMP_UNKNOWN,   "TIMESTAMP_UNKNOWN" },   \
>  		{ V4L2_BUF_FLAG_TIMESTAMP_MONOTONIC, "TIMESTAMP_MONOTONIC" }, \
> diff --git a/include/uapi/linux/videodev2.h b/include/uapi/linux/videodev2.h
> index 3228fbe..8d85aac 100644
> --- a/include/uapi/linux/videodev2.h
> +++ b/include/uapi/linux/videodev2.h
> @@ -875,8 +875,11 @@ struct v4l2_buffer {
>  #define V4L2_BUF_FLAG_TIMECODE			0x00000100
>  /* Buffer is prepared for queuing */
>  #define V4L2_BUF_FLAG_PREPARED			0x00000400
> -/* Cache handling flags */
> -#define V4L2_BUF_FLAG_NO_CACHE_INVALIDATE	0x00000800
> +/* Cache sync hint */
> +#define V4L2_BUF_FLAG_NO_CACHE_SYNC		0x00000800
> +/* DEPRECATED. THIS WILL BE REMOVED IN THE FUTURE! */
> +#define V4L2_BUF_FLAG_NO_CACHE_INVALIDATE	V4L2_BUF_FLAG_NO_CACHE_SYNC
> +/* DEPRECATED. THIS WILL BE REMOVED IN THE FUTURE! */
>  #define V4L2_BUF_FLAG_NO_CACHE_CLEAN		0x00001000
>  /* Timestamp type */
>  #define V4L2_BUF_FLAG_TIMESTAMP_MASK		0x0000e000
>