Re: [REGRESSION] breakage in sun6i-csi media api

[Laurent Pinchart <laurent.pinchart@ideasonboard.com>]

Hi Paul,

On Fri, Jun 02, 2023 at 11:39:54AM +0200, Paul Kocialkowski wrote:
> Hi,
> 
> (Re-adding folks from the original email, adding Laurent and Hans.)
> 
> On Fri 02 Jun 23, 11:24, Martijn Braam wrote:
> > Hi Paul,
> > 
> > That's basically it yes. My software doesn't expect the bridge block,
> > because it wasn't there. The pipeline always worked "automatically".
> > 
> > This is the workaround that's been made now it run on newer kernels:
> > https://gitlab.com/postmarketOS/megapixels/-/merge_requests/31
> > 
> > I'm pretty sure format propagation would fix this issue.
> 
> Okay that's good to know.
> 
> To be honest it's still not very clear to me if in-driver format propagation is
> a "nice to have" feature or something that all media pipeline drivers are
> supposed to implement.

For MC-based drivers, in-kernel propagation *inside* subdevs is
mandatory, in-kernel propagration *between* subdevs is not allowed. The
latter is the responsibility of userspace.

For traditional (I'd say legacy, but I know not everybody likes that
term :-)) drivers that only expose video device nodes and do not expose
subdev nodes to userspace, the driver is responsible for configuring the
full pipeline internally based on the S_FMT call on the video nodes
only. This isn't applicable to the sun6i-csi driver, as it exposes
subdev nodes to userspace.

> Anyway I feel like this is not really a regression but a result of the driver
> being converted to a newer API.
> 
> Also there's a V4L2_CAP_IO_MC flag which should indicate that the video device
> must be controlled via the media controller API instead of being
> video-device-centric, but I've seen comments asking not to set the flag even
> when MC is used so I'm a bit confused here.

Would you have pointers to those comments ?

> Perhaps the flag is only required when there is no automatic format
> propagation?

The flag is more or less required when you expose subdev nodes to
userspace.

> If anyone has solid answers on these points I'd be happy to read some
> clarification (and act accordingly).
>
> > On 6/2/23 11:16, Paul Kocialkowski wrote:
> > > Hi Martijn,
> > > 
> > > On Thu 01 Jun 23, 23:19, Martijn Braam wrote:
> > > > It seems like this commit:
> > > > 
> > > > media: sun6i-csi: Add bridge v4l2 subdev with port management
> > > > 
> > > > Has changed the way the media pipeline on a64 devices, in my case the PINE64
> > > > PinePhone works. Since this is an API towards userspace and there's active
> > > > applications that use it I think this counts as a regression.
> > > Do you have more details on what changed specifically?
> > > 
> > > The commit added a bridge subdev in addition to the video node, which is
> > > generally a better description of the CSI hardware and also a necessity
> > > to support the ISP data flow.
> > > 
> > > Maybe your userspace application is not configuring the bridge media block with
> > > the right format, which results in a mismatch?
> > > 
> > > Some work was started to achieve automatic format propagation, perhaps it
> > > would be enough to solve your issue.
> > > 
> > > Cheers,
> > > 
> > > Paul
> > > 
> > > > #regzbot introduced: 0d2b746b1bef73de62d2d311e594a7ffed4ca43

-- 
Regards,

Laurent Pinchart