Re: [RFC 3/7] media: uapi: v4l: Document source routes

[Tomi Valkeinen <tomi.valkeinen@ideasonboard.com>]

On 06/05/2023 00:52, Sakari Ailus wrote:
> Document how internal pads are used on source routes.
> 
> Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
> ---
>   .../userspace-api/media/v4l/dev-subdev.rst     | 18 ++++++++++++++++++
>   .../media/v4l/vidioc-subdev-g-routing.rst      |  5 +++++
>   include/uapi/linux/v4l2-subdev.h               |  6 +++++-
>   3 files changed, 28 insertions(+), 1 deletion(-)
> 
> diff --git a/Documentation/userspace-api/media/v4l/dev-subdev.rst b/Documentation/userspace-api/media/v4l/dev-subdev.rst
> index a4f1df7093e8..395e06d2f0f2 100644
> --- a/Documentation/userspace-api/media/v4l/dev-subdev.rst
> +++ b/Documentation/userspace-api/media/v4l/dev-subdev.rst
> @@ -551,6 +551,24 @@ A stream at a specific point in the media pipeline is identified by the
>   sub-device and a (pad, stream) pair. For sub-devices that do not support
>   multiplexed streams the 'stream' field is always 0.
>   
> +.. _v4l2-subdev-source-routes:
> +
> +Source routes
> +^^^^^^^^^^^^^
> +
> +Cases where a single sub-device pad is a source of multiple streams are special
> +as there is no sink pad for such a route. In those cases, an internal pad is
> +used as the sink pad. Such pads have the :ref:`MEDIA_PAD_FL_INTERNAL_SOURCE
> +<MEDIA-PAD-FL-INTERNAL-SOURCE>` flag set.
> +
> +Internal pads have all the properties of a sink pad in such case, including
> +formats and selections. The format in this case is the source format of the
> +stream. An internal pad always has a single stream only (0).
> +
> +Generally source routes are not modifiable but they can be activated and
> +inactivated using the :ref:`V4L2_SUBDEV_ROUTE_FL_ACTIVE
> +<v4l2-subdev-routing-flags>` flag, depending on driver capabilities.
> +

I find the above chapter a bit difficult to read, but I think we need to 
conclude on the internal-pad vs internal-source-pad discussion first. 
However, one point/question:

You write that there's only one stream in an internal pad. I wonder if 
that's a good or a necessary limitation... Do you see that allowing 
multiple streams brings extra complexity? It's still up to each driver 
to decide how many streams they support (most would just support a 
single one), so each driver can easily enforce that limit.

I'm fine with starting with only single-stream support, but if we later 
need to loosen that restriction, I wonder if it'll be difficult if we 
have specified that there can be only one stream. I.e. will the drivers 
and the userspace depend on that limit.

  Tomi