[Ksummit-discuss] [TECH TOPIC] Addressing complex dependencies and semantics (v2)

[Laurent Pinchart]

On Monday 01 Aug 2016 15:21:49 Hans Verkuil wrote:
> On 08/01/2016 03:09 PM, Laurent Pinchart wrote:
> > On Friday 29 Jul 2016 12:13:03 Mark Brown wrote:
> >> On Fri, Jul 29, 2016 at 09:45:55AM +0200, Hans Verkuil wrote:
> >>> My main problem is not so much with deferred probe (esp. for cyclic
> >>> dependencies it is a simple method of solving this, and simple is good).
> >>> My main problem is that you can't tell the system that driver A needs to
> >>> be probed after drivers B, C and D are probed first.
> >>> 
> >>> That would allow us to get rid of v4l2-async.c which is a horrible hack.
> >>> 
> >>> That code allows a bridge driver to wait until all dependent drivers are
> >>> probed. This really should be core functionality.
> >>> 
> >>> Do other subsystems do something similar like
> >>> drivers/media/v4l2-core/v4l2-async.c? Does anyone know?
> >> 
> >> ASoC does, it has an explicit card driver to join things together and
> >> that just defers probe until everything it needs is present.  This was
> >> originally open coded in ASoC but once deferred probe was implemented we
> >> converted to that.
> > 
> > Asynchronous bindings of components, as done in ASoC, DRM and V4L2, is a
> > problem largely solved (or rather hacked around), but I'm curious to know
> > how ASoC handles device unbinding (due to device removal or manual
> > unbinding through sysfs). With asynchronous binding we can more or less
> > easily wait for all components to be present before creating circular
> > dependencies, but breaking them to implement unbinding is an unsolved
> > problem at least in V4L2.
>
> We need to prevent subdevice drivers from being unbound. It's easy enough to
> do that (set suppress_bind_attrs to true), we just never did that. It's
> been on my TODO list for ages to make a patch adding that flag...
> 
> You can only unbind bridge drivers. Unbinding subdevs is pointless in
> general and should be prohibited. Perhaps in the future with dynamically
> reconfigurable video pipelines (FPGA) you want that, but then you need to
> do a lot of additional work. For everything we have today we should just
> set suppress_bind_attrs to true.

If wish reality was so simple :-) In Project Ara our camera modules are hot-
pluggable, and connect to a CSI-2 receiver on the SoC side.

-- 
Regards,

Laurent Pinchart