Re: [PATCH v5 3/7] v4l: Add timestamp source flags, mask and document them

[Hans Verkuil <hverkuil@xs4all.nl>]

Hi Sakari,

Some minor comments:

On 02/15/2014 09:53 PM, Sakari Ailus wrote:
> Some devices do not produce timestamps that correspond to the end of the
> frame. The user space should be informed on the matter. This patch achieves
> that by adding buffer flags (and a mask) for timestamp sources since more
> possible timestamping points are expected than just two.
> 
> A three-bit mask is defined (V4L2_BUF_FLAG_TSTAMP_SRC_MASK) and two of the
> eight possible values is are defined V4L2_BUF_FLAG_TSTAMP_SRC_EOF for end of
> frame (value zero) V4L2_BUF_FLAG_TSTAMP_SRC_SOE for start of exposure (next
> value).
> 
> Signed-off-by: Sakari Ailus <sakari.ailus@iki.fi>
> ---
>  Documentation/DocBook/media/v4l/io.xml   |   31 ++++++++++++++++++++++++------
>  drivers/media/v4l2-core/videobuf2-core.c |    4 +++-
>  include/media/videobuf2-core.h           |    2 ++
>  include/uapi/linux/videodev2.h           |    4 ++++
>  4 files changed, 34 insertions(+), 7 deletions(-)
> 
> diff --git a/Documentation/DocBook/media/v4l/io.xml b/Documentation/DocBook/media/v4l/io.xml
> index 46d24b3..fbd0c6e 100644
> --- a/Documentation/DocBook/media/v4l/io.xml
> +++ b/Documentation/DocBook/media/v4l/io.xml
> @@ -653,12 +653,6 @@ plane, are stored in struct <structname>v4l2_plane</structname> instead.
>  In that case, struct <structname>v4l2_buffer</structname> contains an array of
>  plane structures.</para>
>  
> -      <para>For timestamp types that are sampled from the system clock
> -(V4L2_BUF_FLAG_TIMESTAMP_MONOTONIC) it is guaranteed that the timestamp is
> -taken after the complete frame has been received (or transmitted in
> -case of video output devices). For other kinds of
> -timestamps this may vary depending on the driver.</para>
> -
>      <table frame="none" pgwide="1" id="v4l2-buffer">
>        <title>struct <structname>v4l2_buffer</structname></title>
>        <tgroup cols="4">
> @@ -1119,6 +1113,31 @@ in which case caches have not been used.</entry>
>  	    <entry>The CAPTURE buffer timestamp has been taken from the
>  	    corresponding OUTPUT buffer. This flag applies only to mem2mem devices.</entry>
>  	  </row>
> +	  <row>
> +	    <entry><constant>V4L2_BUF_FLAG_TSTAMP_SRC_MASK</constant></entry>
> +	    <entry>0x00070000</entry>
> +	    <entry>Mask for timestamp sources below. The timestamp source
> +	    defines the point of time the timestamp is taken in relation to
> +	    the frame. Logical and operation between the
> +	    <structfield>flags</structfield> field and
> +	    <constant>V4L2_BUF_FLAG_TSTAMP_SRC_MASK</constant> produces the
> +	    value of the timestamp source.</entry>
> +	  </row>
> +	  <row>
> +	    <entry><constant>V4L2_BUF_FLAG_TSTAMP_SRC_EOF</constant></entry>
> +	    <entry>0x00000000</entry>
> +	    <entry>"End of frame." The buffer timestamp has been taken

More a typographical thing than anything else: I prefer this:

"End Of Frame": the buffer...

The capitalization links back to the EOF abbreviation more directly.

> +	    when the last pixel of the frame has been received or the

I would say: "after the last pixel of the frame has been received or after the"

The "when" word suggests that it is exactly "when", which is not true in
practice.

> +	    last pixel of the frame has been transmitted.</entry>
> +	  </row>
> +	  <row>
> +	    <entry><constant>V4L2_BUF_FLAG_TSTAMP_SRC_SOE</constant></entry>
> +	    <entry>0x00010000</entry>
> +	    <entry>"Start of exposure." The buffer timestamp has been taken

Start Of Exposure: the buffer...

> +	    when the exposure of the frame has begun. This is only

Should 'when' be replaced with 'after' here as well? I think Laurent has to decide
that.

Regards,

	Hans

> +	    valid for buffer type
> +	    <constant>V4L2_BUF_TYPE_VIDEO_CAPTURE</constant>.</entry>
> +	  </row>
>  	</tbody>
>        </tgroup>
>      </table>
> diff --git a/drivers/media/v4l2-core/videobuf2-core.c b/drivers/media/v4l2-core/videobuf2-core.c
> index 5a5fb7f..6e314b0 100644
> --- a/drivers/media/v4l2-core/videobuf2-core.c
> +++ b/drivers/media/v4l2-core/videobuf2-core.c
> @@ -2195,7 +2195,9 @@ int vb2_queue_init(struct vb2_queue *q)
>  	    WARN_ON(!q->io_modes)	  ||
>  	    WARN_ON(!q->ops->queue_setup) ||
>  	    WARN_ON(!q->ops->buf_queue)   ||
> -	    WARN_ON(q->timestamp_type & ~V4L2_BUF_FLAG_TIMESTAMP_MASK))
> +	    WARN_ON(q->timestamp_type &
> +		    ~(V4L2_BUF_FLAG_TIMESTAMP_MASK |
> +		      V4L2_BUF_FLAG_TSTAMP_SRC_MASK)))
>  		return -EINVAL;
>  
>  	/* Warn that the driver should choose an appropriate timestamp type */
> diff --git a/include/media/videobuf2-core.h b/include/media/videobuf2-core.h
> index bef53ce..b6b992d 100644
> --- a/include/media/videobuf2-core.h
> +++ b/include/media/videobuf2-core.h
> @@ -312,6 +312,8 @@ struct v4l2_fh;
>   * @buf_struct_size: size of the driver-specific buffer structure;
>   *		"0" indicates the driver doesn't want to use a custom buffer
>   *		structure type, so sizeof(struct vb2_buffer) will is used
> + * @timestamp_type: Timestamp flags; V4L2_BUF_FLAGS_TIMESTAMP_* and
> + *		V4L2_BUF_FLAGS_TSTAMP_SRC_*
>   * @gfp_flags:	additional gfp flags used when allocating the buffers.
>   *		Typically this is 0, but it may be e.g. GFP_DMA or __GFP_DMA32
>   *		to force the buffer allocation to a specific memory zone.
> diff --git a/include/uapi/linux/videodev2.h b/include/uapi/linux/videodev2.h
> index e9ee444..82e8661 100644
> --- a/include/uapi/linux/videodev2.h
> +++ b/include/uapi/linux/videodev2.h
> @@ -695,6 +695,10 @@ struct v4l2_buffer {
>  #define V4L2_BUF_FLAG_TIMESTAMP_UNKNOWN		0x00000000
>  #define V4L2_BUF_FLAG_TIMESTAMP_MONOTONIC	0x00002000
>  #define V4L2_BUF_FLAG_TIMESTAMP_COPY		0x00004000
> +/* Timestamp sources. */
> +#define V4L2_BUF_FLAG_TSTAMP_SRC_MASK		0x00070000
> +#define V4L2_BUF_FLAG_TSTAMP_SRC_EOF		0x00000000
> +#define V4L2_BUF_FLAG_TSTAMP_SRC_SOE		0x00010000
>  
>  /**
>   * struct v4l2_exportbuffer - export of video buffer as DMABUF file descriptor
>