From: Sakari Ailus <sakari.ailus@iki.fi>
To: Hans Verkuil <hverkuil@xs4all.nl>
Cc: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>,
linux-media@vger.kernel.org, linux-renesas-soc@vger.kernel.org,
Hans Verkuil <hans.verkuil@cisco.com>,
Mauro Carvalho Chehab <mchehab@osg.samsung.com>
Subject: Re: [PATCH/RFC v2 1/4] v4l: Add metadata buffer type and format
Date: Tue, 24 May 2016 18:28:32 +0300 [thread overview]
Message-ID: <20160524152831.GF26360@valkosipuli.retiisi.org.uk> (raw)
In-Reply-To: <5742D6CC.8040909@xs4all.nl>
Hi Hans,
On Mon, May 23, 2016 at 12:09:16PM +0200, Hans Verkuil wrote:
> On 05/12/2016 02:18 AM, Laurent Pinchart wrote:
> > The metadata buffer type is used to transfer metadata between userspace
> > and kernelspace through a V4L2 buffers queue. It comes with a new
> > metadata capture capability and format description.
>
> Thanks for the patch! I have some comments/suggestions below:
>
> >
> > Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
> > ---
> > Documentation/DocBook/media/v4l/dev-meta.xml | 93 +++++++++++++++++++++++++++
> > Documentation/DocBook/media/v4l/v4l2.xml | 1 +
> > drivers/media/v4l2-core/v4l2-compat-ioctl32.c | 19 ++++++
> > drivers/media/v4l2-core/v4l2-dev.c | 16 +++--
> > drivers/media/v4l2-core/v4l2-ioctl.c | 34 ++++++++++
> > drivers/media/v4l2-core/videobuf2-v4l2.c | 3 +
> > include/media/v4l2-ioctl.h | 8 +++
> > include/uapi/linux/videodev2.h | 14 ++++
> > 8 files changed, 182 insertions(+), 6 deletions(-)
> > create mode 100644 Documentation/DocBook/media/v4l/dev-meta.xml
> >
> > diff --git a/Documentation/DocBook/media/v4l/dev-meta.xml b/Documentation/DocBook/media/v4l/dev-meta.xml
> > new file mode 100644
> > index 000000000000..9b5b1fba2007
> > --- /dev/null
> > +++ b/Documentation/DocBook/media/v4l/dev-meta.xml
> > @@ -0,0 +1,93 @@
> > + <title>Metadata Interface</title>
> > +
> > + <note>
> > + <title>Experimental</title>
> > + <para>This is an <link linkend="experimental"> experimental </link>
>
> No space before/after 'experimental'. It will look ugly in the docbook (I tried
> it :-) ).
>
> > + interface and may change in the future.</para>
> > + </note>
> > +
> > + <para>
> > +Metadata refers to any non-image data that supplements video frames with
> > +additional information. This may include statistics computed over the image
> > +or frame capture parameters supplied by the image source. This interface is
> > +intended for transfer of metadata to userspace and control of that operation.
>
> I think it can also be in the other direction. I'm thinking of metadata needed
> by codecs. I'm not sure if it should be mentioned that this is not supported at
> the moment and that the ML should be contacted for more info if someone wants this.
Good point.
>
> > + </para>
> > +
> > + <para>
> > +The metadata interface is implemented on video capture devices. The device can
>
> s/on/for/?
>
> > +be dedicated to metadata or can implement both video and metadata capture as
> > +specified in its reported capabilities.
> > + </para>
> > +
> > + <section>
> > + <title>Querying Capabilities</title>
> > +
> > + <para>
> > +Devices supporting the metadata interface set the
> > +<constant>V4L2_CAP_META_CAPTURE</constant> flag in the
> > +<structfield>capabilities</structfield> field of &v4l2-capability;
> > +returned by the &VIDIOC-QUERYCAP; ioctl. That flag means the device can capture
> > +metadata to memory.
> > + </para>
>
> I know this is a copy and paste from the other interface descriptions, but it
> is rather outdated. I would write this instead:
>
> <para>
> Device nodes supporting the metadata interface set the
> <constant>V4L2_CAP_META_CAPTURE</constant> flag in the
> <structfield>device_caps</structfield> field of &v4l2-capability;
> returned by the &VIDIOC-QUERYCAP; ioctl. That flag means the device node can capture
> metadata to memory.
> </para>
>
> Any driver using this will be recent and always have a valid device_caps field
> (which, as you know, didn't exist in old kernels).
>
> I find the capabilities field fairly useless in most cases due to the fact that
> it contains the capabilities of all device nodes created by the device driver.
>
> > + <para>
> > +At least one of the read/write or streaming I/O methods must be supported.
> > + </para>
> > + </section>
> > +
> > + <section>
> > + <title>Data Format Negotiation</title>
> > +
> > + <para>
> > +The metadata device uses the <link linkend="format">format</link> ioctls to
> > +select the capture format. The metadata buffer content format is bound to that
> > +selectable format. In addition to the basic
> > +<link linkend="format">format</link> ioctls, the &VIDIOC-ENUM-FMT; ioctl
> > +must be supported as well.
> > + </para>
> > +
> > + <para>
> > +To use the <link linkend="format">format</link> ioctls applications set the
> > +<structfield>type</structfield> field of a &v4l2-format; to
> > +<constant>V4L2_BUF_TYPE_META_CAPTURE</constant> and use the &v4l2-meta-format;
> > +<structfield>meta</structfield> member of the <structfield>fmt</structfield>
> > +union as needed per the desired operation.
> > +Currently there are two fields, <structfield>dataformat</structfield> and
> > +<structfield>buffersize</structfield>, of struct &v4l2-meta-format; that are
> > +used. Content of the <structfield>dataformat</structfield> is the V4L2 FourCC
> > +code of the data format. The <structfield>buffersize</structfield> field is the
> > +maximum buffer size in bytes required for data transfer, set by the driver in
> > +order to inform applications.
>
> Should it be mentioned here that changing the video format might change
> the buffersize? In case the buffersize is always a multiple of the width?
Isn't that the case in general, as with pixel formats? buffersize could also
be something else than a multiple of width (there's no width for metadata
formats) due to e.g. padding required by hardware.
--
Kind regards,
Sakari Ailus
e-mail: sakari.ailus@iki.fi XMPP: sailus@retiisi.org.uk
next prev parent reply other threads:[~2016-05-24 15:29 UTC|newest]
Thread overview: 21+ messages / expand[flat|nested] mbox.gz Atom feed top
2016-05-12 0:17 [PATCH/RFC v2 0/4] Meta-data video device type Laurent Pinchart
2016-05-12 0:18 ` [PATCH/RFC v2 1/4] v4l: Add metadata buffer type and format Laurent Pinchart
2016-05-23 10:09 ` Hans Verkuil
2016-05-24 15:28 ` Sakari Ailus [this message]
2016-05-24 15:36 ` Hans Verkuil
2016-05-24 16:26 ` Sakari Ailus
2016-06-22 16:51 ` Laurent Pinchart
2016-06-24 23:15 ` Sakari Ailus
2016-06-22 16:32 ` Laurent Pinchart
2016-05-12 0:18 ` [PATCH/RFC v2 2/4] v4l: Add metadata video device type Laurent Pinchart
2016-05-12 21:22 ` Sakari Ailus
2016-05-12 0:18 ` [PATCH/RFC v2 3/4] v4l: Define a pixel format for the R-Car VSP1 1-D histogram engine Laurent Pinchart
2016-05-12 0:18 ` [PATCH/RFC v2 4/4] v4l: vsp1: Add HGO support Laurent Pinchart
2016-06-13 15:33 ` Guennadi Liakhovetski
2016-06-24 14:35 ` Laurent Pinchart
2016-05-13 9:26 ` [PATCH/RFC v2 0/4] Meta-data video device type Hans Verkuil
2016-05-13 9:52 ` Sakari Ailus
2016-05-16 9:20 ` Laurent Pinchart
2016-05-23 13:00 ` Guennadi Liakhovetski
2016-05-16 9:21 ` Laurent Pinchart
2016-05-17 9:26 ` Sakari Ailus
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