Re: [PATCH v5.1 3/7] v4l: Add timestamp source flags, mask and document them

[Hans Verkuil <hverkuil@xs4all.nl>]

On 02/21/2014 12:30 AM, Laurent Pinchart wrote:
> Hi Hans,
> 
> On Thursday 20 February 2014 21:36:51 Hans Verkuil wrote:
>> On 02/20/2014 08:41 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).
>>
>> Sorry, but I still have two small notes:
>>> Signed-off-by: Sakari Ailus <sakari.ailus@iki.fi>
>>> ---
>>> since v5:
>>> - Add a note on software generated timestamp inaccuracy.
>>>
>>>  Documentation/DocBook/media/v4l/io.xml   |   38 +++++++++++++++++++++----
>>>  drivers/media/v4l2-core/videobuf2-core.c |    4 +++-
>>>  include/media/videobuf2-core.h           |    2 ++
>>>  include/uapi/linux/videodev2.h           |    4 ++++
>>>  4 files changed, 41 insertions(+), 7 deletions(-)
>>>
>>> diff --git a/Documentation/DocBook/media/v4l/io.xml
>>> b/Documentation/DocBook/media/v4l/io.xml index 46d24b3..22b87bc 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 +1113,38 @@ 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
>>> +	    when the last pixel of the frame has been received or the
>>> +	    last pixel of the frame has been transmitted. In practice,
>>> +	    software generated timestamps will typically be read from
>>> +	    the clock a small amount of time after the last pixel has
>>> +	    been received, depending on the system and other activity
>>
>> s/been received/been received or transmitted/
>>
>>> +	    in it.</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 when the exposure of the frame has begun. In
>>> +	    practice, software generated timestamps will typically be
>>> +	    read from the clock a small amount of time after the last
>>> +	    pixel has been received, depending on the system and other
>>> +	    activity in it. This is only valid for buffer type
>>> +	    <constant>V4L2_BUF_TYPE_VIDEO_CAPTURE</constant>.</entry>
>>
>> I would move the last sentence up to just before "In practice...". The
>> way it is now it looks like an afterthought.
>>
>> I am also not sure whether the whole "In practice" sentence is valid
>> here. Certainly the bit about "the last pixel" isn't since this is the
>> "SOE" case and not the End Of Frame. In the case of the UVC driver (and
>> that's the only one using this timestamp source) the timestamps come from
>> the hardware as I understand it, so the "software generated" bit doesn't
>> apply.
>>
>> I would actually be inclined to drop it altogether for this particular
>> timestamp source. But it's up to Laurent.
> 
> What do you mean, drop what altogether ?

The "In practice ... activity in it." sentence.

Regards,

	Hans

> 
>>> +	  </row>
>>>  	</tbody>
>>>        </tgroup>
>>>      </table>
>