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

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

Hi Hans,

On Friday 29 Jul 2016 09:33:03 Hans Verkuil wrote:
> On 07/28/2016 12:41 PM, Laurent Pinchart wrote:
> > On Thursday 28 Jul 2016 02:54:56 Rafael J. Wysocki wrote:
> >> On Wednesday, July 27, 2016 09:20:40 PM Luis R. Rodriguez wrote:
> >>> On Wed, Jul 27, 2016 at 07:03:46PM +0100, Mark Brown wrote:
> >>>> On Wed, Jul 27, 2016 at 07:58:29PM +0200, Luis R. Rodriguez wrote:
> >>>>> On Wed, Jul 27, 2016 at 06:26:36PM +0100, Mark Brown wrote:
> >>>>>>> to help enable asynchronous probe, however for built-in devices
> >>>>>>> this requires very specific platform knowledge otherwise using
> >>>>>>> async probe will blow up your kernel -- if you get it right
> >>>>>>> though, using async probe can help with
> >>>>>> 
> >>>>>> I'm not sure what specific platform knowledge you're thinking of
> >>>>>> here? We have coverage for most things in the form of deferred probe
> >>>>>> (messy though it is).
> >>>>> 
> >>>>> Deferred probe is a complete a hack and sub-optimal. Being able to
> >>>>> address
> >>>> 
> >>>> Sure, I don't think anyone disagrees on that but it does mean we don't
> >>>> actually blow up easily like we used to - it's messy but it does get
> >>>> there safely.
> >>> 
> >>> Good point, without learning from the past we would otherwise expect
> >>> this is just a sloppy situation, so indeed deferred probe had its
> >>> merits, and we're at least now safe. The goal here is then how to
> >>> do this better, optimized and make it a non-hack.
> > 
> > Let's not demonize deferred probing, I think it's a good safety net to be
> > used as a last resort when everything else failed. The problem is that
> > we're using it as our primary mean to order initialization, and that
> > leads to all sorts of inefficient behaviours at boot time.
> > 
> >> Well, my patchset uses deferred probing, so it won't help much here I
> >> guess.
> >>
> >>>> I was specifically querying your statement that things would blow up.
> >>> 
> >>> In practice this varies as it depends on the device driver or component,
> >>> but the general theme seems to be "relying on something which is not
> >>> yet available".
> >> 
> >> Not only that.
> >> 
> >> There also is a "relying on something that is not a direct ancestor of
> >> the device you care about" angle of that.
> >> 
> >> The device hierarchy as we know it is insufficient for representing
> >> dependencies beyond parent-child and that really is part of the problem,
> >> and a significant one IMO.
> > 
> > That's the core of the issue.
> > 
> > Linux has traditionally represented the device hierarchy from a control
> > bus point of view, both in the driver model and in DT. We know other
> > dependencies exist (data bus access, clocks, regulators, power domains,
> > ...), and we try to address them with various heuristics.
> > 
> > One problem with those other dependencies is that they can't always be
> > expressed as a tree and may a graph instead. Worse, in some cases, the
> > graph can be cyclic (I've recently been told about an external I2C-based
> > PLL that takes an input clock and generates an output clock, with the
> > input clock being produced by an on-SoC sound device and the output clock
> > being used by the same sound device). Even when individual resource trees
> > or graphs are not cyclic, combining them in a global dependency graph
> > will often result in cycles. The challenge is to find a proper way to
> > both express the dependency graph and break the cycles.
> 
> I don't think this really matters when it comes to the probing order. At
> least in the media subsystem there is always one driver that should be
> loaded last since that is the controller driver (bridge driver) that hooks
> everything else together.

Pedantic hat on: I assume that by driver loaded last you mean device being 
probed last.

> Being able to model that would allow us to get rid of the v4l2-async.c hack.

Which, for the reader unfamiliar with V4L2, is a framework similar to 
drivers/base/component.c that predates it.

> We'd never would have needed to make that if we could model proper probe
> dependencies.

In OMAP3-based systems the OMAP3 ISP need to be probed first, as it registers 
a clock that sensor drivers need at probe time. The sensor is then bound to 
the ISP through the v4l2-async framework.

> I seriously dislike v4l2-async and anything we can do to get rid of that is
> very welcome.

-- 
Regards,

Laurent Pinchart