Re: [RFC 6/7] media: uapi: Add generic 8-bit metadata format definitions

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

Hi Laurent,

On Thu, Jun 08, 2023 at 10:54:43AM +0200, Hans Verkuil wrote:
> On 05/05/2023 23:52, Sakari Ailus wrote:
> > Generic 8-bit metadata formats define the in-memory data layout but not
> > the format of the data itself. The reasoning for having such formats is to
> > allow CSI-2 receiver drivers to receive and DMA drivers to write the data
> > to memory without knowing a large number of device specific formats.
> > 
> > These formats may be used only in conjunction of a Media controller
> > pipeline where the internal pad of the source sub-device defines the
> > specific format of the data (using an mbus code).
> > 
> > Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
> > ---
> >  .../userspace-api/media/v4l/meta-formats.rst  |   1 +
> >  .../media/v4l/metafmt-generic.rst             | 317 ++++++++++++++++++
> >  drivers/media/v4l2-core/v4l2-ioctl.c          |   8 +
> >  include/uapi/linux/videodev2.h                |   9 +
> >  4 files changed, 335 insertions(+)
> >  create mode 100644 Documentation/userspace-api/media/v4l/metafmt-generic.rst
> > 
> > diff --git a/Documentation/userspace-api/media/v4l/meta-formats.rst b/Documentation/userspace-api/media/v4l/meta-formats.rst
> > index 0bb61fc5bc00..919f595576b9 100644
> > --- a/Documentation/userspace-api/media/v4l/meta-formats.rst
> > +++ b/Documentation/userspace-api/media/v4l/meta-formats.rst
> > @@ -19,3 +19,4 @@ These formats are used for the :ref:`metadata` interface only.
> >      metafmt-vsp1-hgo
> >      metafmt-vsp1-hgt
> >      metafmt-vivid
> > +    metafmt-generic
> > diff --git a/Documentation/userspace-api/media/v4l/metafmt-generic.rst b/Documentation/userspace-api/media/v4l/metafmt-generic.rst
> > new file mode 100644
> > index 000000000000..09d3fabb1e94
> > --- /dev/null
> > +++ b/Documentation/userspace-api/media/v4l/metafmt-generic.rst
> > @@ -0,0 +1,317 @@
> > +.. SPDX-License-Identifier: GPL-2.0 OR GFDL-1.1-no-invariants-or-later
> > +
> > +**************************************************************************************************************************************************************************************************************************************************************************************************************************
> > +V4L2_META_FMT_GENERIC_8 ('MET8'), V4L2_META_FMT_GENERIC_CSI2_10 ('MC1A'), V4L2_META_FMT_GENERIC_CSI2_12 ('MC1C'), V4L2_META_FMT_GENERIC_CSI2_14 ('MC1E'), V4L2_META_FMT_GENERIC_CSI2_16 ('MC1G'), V4L2_META_FMT_GENERIC_CSI2_20 ('MC1K'), V4L2_META_FMT_GENERIC_CSI2_24 ('MC1O'), V4L2_META_FMT_GENERIC_CSI2_2_24 ('MC2O')
> > +**************************************************************************************************************************************************************************************************************************************************************************************************************************
> > +
> > +
> > +Generic line-based metadata formats
> > +
> > +
> > +Description
> > +===========
> > +
> > +These generic line-based metadata formats define the memory layout of the data
> > +without defining the format or meaning of the metadata itself. These formats may
> > +only be used with a Media controller pipeline where the more specific format is
> > +defined in an :ref:`internal source pad <MEDIA-PAD-FL-INTERNAL-SOURCE>` of the
> > +source sub-device. See also :ref:`source routes <v4l2-subdev-source-routes>`.
> > +
> > +.. _v4l2-meta-fmt-generic-8:
> > +
> > +V4L2_META_FMT_GENERIC_8
> > +-----------------------
> > +
> > +The V4L2_META_FMT_GENERIC_8 format is a plain 8-bit metadata format whereas the
> > +rest of the formats have CSI-2 packing that are typically used on camera sensors
> > +in conjunction with pixel data of the same bit depth.
> 
> You keep referring to CSI-2 packing without ever defining it.
> 
> I'm not sure you should include CSI2 in the name either. It is OK to say something
> here like: "The way the data is stored follows the CSI-2 specification."
> 
> But it is not limited to CSI-2 and can be used with other buses as well.

That's true, certainly, but where this packing is most commonly used is
CSI-2 (or derives from CSI-2).

> 
> > +
> > +This format is also used on CSI-2 on both 8 bits per sample as well as on
> > +16 bits per sample when two bytes of metadata are packed into one sample.
> 
> Little or big endian?

Little endian. I'll address these for v2.

-- 
Kind regards,

Sakari Ailus