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

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

Moi,

On Mon, May 08, 2023 at 01:33:24PM +0300, Tomi Valkeinen wrote:
> 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.

Good question. As we don't seem to have a tangible reason to allow it, I'd
deny it until there's a use case.

Or did you have a use case in mind?

I thought indicating which streams will be bound together (i.e. either are
all disabled or enabled) could be one, but I'm not sure we need that at the
moment at least.

> 
> 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.

We can always allow what wasn't allowed previously so that's a non-issue,
really. But it needs to bring new functionality, otherwise there's no
reason to do that.

-- 
Kind regards,

Sakari Ailus