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

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

Hi Hans,

On Monday 01 Aug 2016 15:35:31 Hans Verkuil wrote:
> On 08/01/2016 03:26 PM, Laurent Pinchart wrote:
> > 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.
> 
> I assume that when a camera module is unplugged the CSI driver is informed
> and it will then unregister the camera subdev. So this is controlled by the
> CSI-2 driver. What you don't want is someone unbinding the camera driver
> without the CSI-2 driver's knowledge. To prevent that you need to set that
> flag in the subdev driver.

The CSI-2 receiver is unaware of the modular nature of the system. The camera 
module unplug event is delivered by the Greybus [0] core directly to the 
camera module subdev driver.

[0] https://lwn.net/Articles/648400/ (slightly outdated, but the overall 
architecture is there)

-- 
Regards,

Laurent Pinchart