Hi Laurent,
Thank you for the patchset.
On Tue, Apr 14, 2015 at 10:44:48PM +0300, Laurent Pinchart wrote:
> The data_offset field has been introduced along with the multiplane API
> to convey header size information between kernelspace and userspace.
> It's not used by any mainline driver except vivid (for testing purpose).
>
> A different data offset is needed to allow data capture to or data
> output from a userspace-selected offset within a buffer (mainly for the
> DMABUF and MMAP memory types). As the data_offset field already has the
> right name and is unused, repurpose it.
>
> Signed-off-by: Laurent Pinchart <laurent.pinch...@ideasonboard.com>
> ---
> Documentation/DocBook/media/v4l/io.xml | 19 +++++++++++--------
> include/uapi/linux/videodev2.h | 6 ++++--
> 2 files changed, 15 insertions(+), 10 deletions(-)
>
> diff --git a/Documentation/DocBook/media/v4l/io.xml
> b/Documentation/DocBook/media/v4l/io.xml
> index 1c17f80..416c05a 100644
> --- a/Documentation/DocBook/media/v4l/io.xml
> +++ b/Documentation/DocBook/media/v4l/io.xml
> @@ -870,7 +870,7 @@ should set this to 0.</entry>
> If the application sets this to 0 for an output stream, then
> <structfield>bytesused</structfield> will be set to the size of
> the
> plane (see the <structfield>length</structfield> field of this
> struct)
> - by the driver. Note that the actual image data starts at
> + by the driver. Note that the actual plane data content starts at
> <structfield>data_offset</structfield> which may not be 0.</entry>
> </row>
> <row>
> @@ -917,13 +917,16 @@ should set this to 0.</entry>
> <entry>__u32</entry>
> <entry><structfield>data_offset</structfield></entry>
> <entry></entry>
> - <entry>Offset in bytes to video data in the plane.
> - Drivers must set this field when <structfield>type</structfield>
> - refers to an input stream, applications when it refers to an
> output stream.
> - Note that data_offset is included in
> <structfield>bytesused</structfield>.
> - So the size of the image in the plane is
> -
> <structfield>bytesused</structfield>-<structfield>data_offset</structfield> at
> - offset <structfield>data_offset</structfield> from the start of
> the plane.
> + <entry>Offset in bytes from the start of the plane buffer to the
> start of
> + captured or output data. Applications set this field for all
> stream types
> + when calling the <link
> linkend="vidioc-qbuf">VIDIOC_PREPARE_BUF</link> or
> + <link linkend="vidioc-qbuf">VIDIOC_QBUF</link> ioctls to instruct
> the driver
> + to capture or output data starting at an offset in the plane
> buffer. If the
> + requested data offset doesn't match device or driver constraints,
> device
> + drivers must return the &EINVAL; and either leave the field value
> untouched
> + if they support data offsets, or set it to 0 if they don't
> support data
> + offsets at all. Note that <structfield>data_offset</structfield>
> is not
> + included in <structfield>bytesused</structfield>.
At most 80 characters per line would be nice.
How does the user discover what data_offsets are possible if the driver
returns an error if the data_offset does not match hardware capabilities?
I'd rather have the driver to adjust data_offset to match what it can do. If
the user needs to know that the data_offset was not modified, it should
check the field value after QBUF/PREPARE_BUF.
> </entry>
> </row>
> <row>
> diff --git a/include/uapi/linux/videodev2.h b/include/uapi/linux/videodev2.h
> index fa376f7..261fb66 100644
> --- a/include/uapi/linux/videodev2.h
> +++ b/include/uapi/linux/videodev2.h
> @@ -706,8 +706,10 @@ struct v4l2_requestbuffers {
> * pointing to this plane
> * @fd: when memory is V4L2_MEMORY_DMABUF, a userspace
> file
> * descriptor associated with this plane
> - * @data_offset: offset in the plane to the start of data; usually 0,
> - * unless there is a header in front of the data
> + * @data_offset: offset in bytes from the start of the plane buffer to
> + * the start of data; usually 0 unless applications need to
> + * capture data to or output data from elsewhere than the
> + * start of the buffer
> *
> * Multi-planar buffers consist of one or more planes, e.g. an YCbCr buffer
> * with two planes can have one plane for Y, and another for interleaved CbCr
--
Kind regards,
Sakari Ailus
e-mail: sakari.ai...@iki.fi XMPP: sai...@retiisi.org.uk
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