Re: [PATCH 12/13] media: v4l2-mc: Introduce v4l2_mc_pipeline_enabled()

[Sakari Ailus <sakari.ailus@iki.fi>]

Hi Jacopo,

Thank you for the comments.

On Fri, Jun 20, 2025 at 10:53:13AM +0200, Jacopo Mondi wrote:
> Hi Sakari
> 
> On Thu, Jun 19, 2025 at 11:15:45AM +0300, Sakari Ailus wrote:
> > v4l2_mc_pipeline_enabled() helps solving a problem known for long but
> > lacked any sort of general solution: with multiple streams, when streaming
> > is started on video nodes one by one, when should streaming be started in
> > the source?
> 
> I tried quite some time to understand this, but if I'm not mistaken,
> a stream-aware subdev, which links to vdev, will always "demux"
> streams to different pads and will connect to the vdev from there
> 
> 
>      Source
>      subdev
>  +-----------------+
>  |                (1/0) ------> vdev0
>  |                 |
> (0)[1,2,3]        (2/0  ------> vdev1
>  |                 |
>  |                (3/0) ------> vdev2
>  +-----------------+
> 
> With
> 
> (0) multiplexed sink pad with 3 streams
> (1) (2) and (3) source pad with a single stream
> 
> Can't we relay on the media-link state between the source pads and the
> video devices with something like what Dan has proposed here ?
> https://patchwork.linuxtv.org/project/linux-media/patch/20250519140403.443915-2-dan.scally@ideasonboard.com/
> 
> What am I missing ?

There are two different concepts here that are probably underdocumented at
the moment: Media entity pipeline state and then when and in which order
streaming control is applied on hardware. But... maybe the latter can be
derived from the former when it comes to video nodes. The pipeline is set
for entities that are upstream from the video device, it's not set on
downstream entities. I'll see if I could use that instead. Otherwise, I'll
check if Dan's approach could be used for the purpose.

-- 
Regards,

Sakari Ailus