Re: [PATCH v3 08/10] media: v4l: Support line-based metadata capture

[Sakari Ailus <sakari.ailus@linux.intel.com>]

On Wed, Sep 06, 2023 at 12:24:26PM +0000, Sakari Ailus wrote:
> Hi Laurent,
> 
> On Wed, Sep 06, 2023 at 09:21:42AM +0200, Jacopo Mondi wrote:
> > Hi Laurent
> > 
> > On Tue, Sep 05, 2023 at 08:15:33PM +0300, Laurent Pinchart wrote:
> > > Hi Sakari,
> > >
> > > On Mon, Aug 14, 2023 at 11:02:40AM +0000, Sakari Ailus wrote:
> > > > On Thu, Aug 10, 2023 at 05:24:14PM +0200, Jacopo Mondi wrote:
> > > > > On Tue, Aug 08, 2023 at 10:55:36AM +0300, Sakari Ailus wrote:
> > > > > > many camera sensors, among other devices, transmit embedded data and image
> > >
> > > s/many/Many/
> > >
> > > > > > data for each CSI-2 frame. This embedded data typically contains register
> > > > > > configuration of the sensor that has been used to capture the image data
> > > > > > of the same frame.
> > > > > >
> > > > > > The embedded data is received by the CSI-2 receiver and has the same
> > > > > > properties as the image data, including that it is line based: it has
> > > > > > width, height and bytesperline (stride).
> > > > > >
> > > > > > Add these fields to struct v4l2_meta_format and document them.
> > > > > >
> > > > > > Also add V4L2_FMT_FLAG_META_LINE_BASED to tell a given format is
> > > > > > line-based i.e. these fields of struct v4l2_meta_format are valid for it.
> > > > > >
> > > > > > Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
> > > > > > ---
> > > > > >  .../userspace-api/media/v4l/dev-meta.rst          | 15 +++++++++++++++
> > > > > >  .../userspace-api/media/v4l/vidioc-enum-fmt.rst   |  7 +++++++
> > > > > >  .../media/videodev2.h.rst.exceptions              |  1 +
> > > > > >  drivers/media/v4l2-core/v4l2-ioctl.c              |  5 +++--
> > > > > >  include/uapi/linux/videodev2.h                    | 10 ++++++++++
> > > > > >  5 files changed, 36 insertions(+), 2 deletions(-)
> > > > > >
> > > > > > diff --git a/Documentation/userspace-api/media/v4l/dev-meta.rst b/Documentation/userspace-api/media/v4l/dev-meta.rst
> > > > > > index 0e7e1ee1471a..4b24bae6e171 100644
> > > > > > --- a/Documentation/userspace-api/media/v4l/dev-meta.rst
> > > > > > +++ b/Documentation/userspace-api/media/v4l/dev-meta.rst
> > > > > > @@ -65,3 +65,18 @@ to 0.
> > > > > >        - ``buffersize``
> > > > > >        - Maximum buffer size in bytes required for data. The value is set by the
> > > > > >          driver.
> > > > > > +    * - __u32
> > > > > > +      - ``width``
> > > > > > +      - Width of a line of metadata in samples. Valid when :c:type`v4l2_fmtdesc`
> > > > > > +	flag ``V4L2_FMT_FLAG_META_LINE_BASED`` is set, otherwise zero. See
> > > > > > +	:c:func:`VIDIOC_ENUM_FMT`.
> > > > > > +    * - __u32
> > > > > > +      - ``height``
> > > > > > +      - Number of rows of metadata. Valid when :c:type`v4l2_fmtdesc` flag
> > > > > > +	``V4L2_FMT_FLAG_META_LINE_BASED`` is set, otherwise zero. See
> > > > > > +	:c:func:`VIDIOC_ENUM_FMT`.
> > > > > > +    * - __u32
> > > > > > +      - ``bytesperline``
> > > > > > +      - Offset in bytes between the beginning of two consecutive lines. Valid
> > > > > > +	when :c:type`v4l2_fmtdesc` flag ``V4L2_FMT_FLAG_META_LINE_BASED`` is
> > > > > > +	set, otherwise zero. See :c:func:`VIDIOC_ENUM_FMT`.
> > > > > > diff --git a/Documentation/userspace-api/media/v4l/vidioc-enum-fmt.rst b/Documentation/userspace-api/media/v4l/vidioc-enum-fmt.rst
> > > > > > index 000c154b0f98..6d7664345a4e 100644
> > > > > > --- a/Documentation/userspace-api/media/v4l/vidioc-enum-fmt.rst
> > > > > > +++ b/Documentation/userspace-api/media/v4l/vidioc-enum-fmt.rst
> > > > > > @@ -227,6 +227,13 @@ the ``mbus_code`` field is handled differently:
> > > > > >  	The application can ask to configure the quantization of the capture
> > > > > >  	device when calling the :ref:`VIDIOC_S_FMT <VIDIOC_G_FMT>` ioctl with
> > > > > >  	:ref:`V4L2_PIX_FMT_FLAG_SET_CSC <v4l2-pix-fmt-flag-set-csc>` set.
> > > > > > +    * - ``V4L2_FMT_FLAG_META_LINE_BASED``
> > > > > > +      - 0x0200
> > > > > > +      - The metadata format is line-based. In this case the ``width``,
> > > > > > +	``height`` and ``bytesperline`` fields of :c:type:`v4l2_meta_format` are
> > > > > > +	valid. The buffer consists of ``height`` lines, each having ``width``
> > > > > > +	bytes of data and offset between the beginning of each two consecutive
> > > > >
> > > > > Isn't ``width`` in samples ?
> > > >
> > > > Indeed, it's better to refer to samples for clarity. I'll fix for v4.
> > >
> > > How do you define a "sample" in this case ? I wonder if it wouldn't be
> > > simpler for both userspace and kernel drivers if the width was specified
> > > in bytes, including the padding bytes.
> > 
> > Wouldn't this make the image line length (expressed in 'samples')
> > different than the embedded data line length ? Would this confuse
> > userspace or is it fine ?
> 
> If padding is included to width, then the user needs to calculate how many
> bytes of metadata there is, apart from the padding (which is redundant).
> That value is provided to the user for this purpose --- just like for image
> data.

Think of this: the embedded data parser is split into two, the access
function that accesses metadata bytes depending on the generic V4L2
metadata format and the rest, which cares about the specific mbus code. The
latter will need the information how many bytes of metadata there is,
instead it calculates the position in the buffer based on bytesperline,
width and the format itself.

Of course, the amount of padding can be calculated from the total amount,
by adding a second function that does it, but still it's somehow backwards
way to do that. (Makes me think of negabinaries.)

-- 
Regards,

Sakari Ailus