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@xxxxxxxxxxxxxxxx> > > --- > > 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@xxxxxx XMPP: sailus@xxxxxxxxxxxxxx
