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

[Mauro Carvalho Chehab]

Em Mon, 1 Aug 2016 15:33:22 +0200
Lars-Peter Clausen <lars at metafoo.de> escreveu:

> On 08/01/2016 03:21 PM, 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.  
> 
> suppress_bind_attrs is the lazy solution and as you pointed out does not
> work too well for all cases.

Agreed. 

What we really need is a kind of "usage count" behavior to suppress
unbinds, e. g. a device driver can be unbound only if any other driver
using resources on it gets unbind first.

That will solve most of unbind issues at the media subsystem.

-- 
Thanks,
Mauro