Hi Hans,

On Tue, Nov 10, 2026 at 01:08:05PM +0100, Hans Verkuil wrote:
> On 23/10/2026 14:26, Mattijs Korpershoek wrote:
> > Hi everyone,
> > 
> > On Thu, Oct 01, 2026 at 14:55, Mattijs Korpershoek <mkorpershoek@kernel.org> wrote:
> > 
> >> On media-centric camera pipelines, every entity is known at boot time
> >> and described statically (for example via device-tree).
> >>
> >> The V4L2 framework assumes that every entity of the camera
> >> pipeline is always present. This is a reasonable assumption for
> >> integrated cameras in laptops or phones. However, in automotive systems,
> >> cameras (e.g. rear-view or surround-view) can be damaged or disconnected.
> >> When the framework assumes all sensors are present, a single missing
> >> camera can prevent the entire pipeline from operating.
> > 
> > This series has been discussed at the Camera & ISP "unconference" in
> > Pragues (during Linux Plumbers)
> > 
> > The notes can be found here:
> > https://lore.kernel.org/linux-media/20261021224119.GA383235@killaraus.ideasonboard.com/T/#u
> 
> What is missing in this discussion is how userspace can indicate that it
> actually can handle partially instantiated pipelines.
> 
> Today if one or more parts do not come up, the devices nodes are also not
> created. This is typically exactly what you want.

That's very arguable? We have plenty of other examples where we allow to
show a software representation of something that isn't available in
hardware, starting with virtually any memory buffer allocation, but also
disabled CPUs, swapped pages, etc.

For things a bit more related to drivers, things like USB root hubs are
also pretty analogous. The "root" device will always be created, even if
devices connected to it are non-functional, missing or whatever. It's
the primary interface that these devices might or might not attach to.

So it's a bit hard to understand why the device nodes not showing up is
exactly what we always want.

> But here you allow partial instantiation, and how does userspace
> signal that? It must be an 'opt-in' mechanism, but how is that done?
> There are no devices yet, so that can't be used, so what alternatives
> are there?
> 
> We've been discussion Fault Tolerant V4L2 for many years now, and every time
> I raise this issue, but I never received any proposals.
> 
> This must be solved.

Is there any way to? You're saying at the same time that we must not
create the device, but that an application would need to signal to that
device that it can indeed be created. It's a circular dependency that
cannot be solved on a per-device basis.

During the meeting, we had a discussion that the media device should be
created as a "skeleton", and then as other devices probe they would fill
in the blanks. How would that not work? Also what is your main concern
exactly that makes you ask to not do that in the first place (or make it
opt-in)?

Maxime