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Date: Tue, 25 Feb 2014 19:08:42 +0200
From: Sakari Ailus <sakari.ailus@iki.fi>
To: Hans Verkuil <hverkuil@xs4all.nl>
Cc: linux-media@vger.kernel.org, laurent.pinchart@ideasonboard.com,
	k.debski@samsung.com
Subject: Re: [PATCH v5 7/7] v4l: Document timestamp buffer flag behaviour
Message-ID: <20140225170842.GF15635@valkosipuli.retiisi.org.uk>
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Hi Hans,

On Sun, Feb 23, 2014 at 12:45:28PM +0100, Hans Verkuil wrote:
> On 02/15/2014 09:53 PM, Sakari Ailus wrote:
> > Timestamp buffer flags are constant at the moment. Document them so that 1)
> > they're always valid and 2) not changed by the drivers. This leaves room to
> > extend the functionality later on if needed.
> > 
> > Signed-off-by: Sakari Ailus <sakari.ailus@iki.fi>
> > ---
> >  Documentation/DocBook/media/v4l/io.xml |   10 ++++++++++
> >  1 file changed, 10 insertions(+)
> > 
> > diff --git a/Documentation/DocBook/media/v4l/io.xml b/Documentation/DocBook/media/v4l/io.xml
> > index fbd0c6e..4f76565 100644
> > --- a/Documentation/DocBook/media/v4l/io.xml
> > +++ b/Documentation/DocBook/media/v4l/io.xml
> > @@ -653,6 +653,16 @@ 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>Dequeued video buffers come with timestamps. These
> > +    timestamps can be taken from different clocks and at different
> > +    part of the frame, depending on the driver. Please see flags in
> > +    the masks <constant>V4L2_BUF_FLAG_TIMESTAMP_MASK</constant> and
> > +    <constant>V4L2_BUF_FLAG_TSTAMP_SRC_MASK</constant> in <xref
> > +    linkend="buffer-flags">. These flags are guaranteed to be always
> > +    valid and will not be changed by the driver autonomously. Changes
> > +    in these flags may take place due as a side effect of
> > +    &VIDIOC-S-INPUT; or &VIDIOC-S-OUTPUT; however.</para>
> 
> There is one exception to this: if the timestamps are copied from the output
> buffer to the capture buffer (TIMESTAMP_COPY), then it can change theoretically
> for every buffer since it entirely depends on what is being sent to it. The
> value comes from userspace and you simply don't have any control over that.

Yes; I agree.

And a good point as well --- the timestamp source flags currently come from
__fill_v4l2_buffer() which takes them from q->timestamp. This isn't right
for m2m devices.

I'll fix and resend (3rd patch most likely).

> I'm stress testing vb2 in lots of different ways, including timestamp handling.
> It's not a pretty sight, I'm afraid. Expect a looong list of patches in the
> coming week.

:-)

-- 
Kind regards,

Sakari Ailus
e-mail: sakari.ailus@iki.fi	XMPP: sailus@retiisi.org.uk