Re: [PATCH RFC 0/5] media: Fault-Tolerant V4L2

[Hans Verkuil <hverkuil+cisco@kernel.org>]

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. 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.

Regards,

	Hans

> 
> I intend to rework V2 based on the feedback I've received on the list
> and at the "unconference".
> 
> I'll try to reuse Sakari's work on media graph events from:
> https://git.linuxtv.org/sailus/media_tree.git/log/?h=partial-reg
> 
> Thanks again for the feedback I've received so far.