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

[Jan Kara]

Hi,

On Thu 28-07-16 14:03:46, Laurent Pinchart wrote:
> On Thursday 28 Jul 2016 12:54:47 Hans Verkuil wrote:
> > > 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.
> > 
> > Do we need to capture 100% of all the weird and wonderful dependencies? I
> > think (speaking for the media subsystem) that the vast majority of the
> > dependencies are pretty simple trees without cycles. Being able to capture
> > that would be a huge help. The remaining more complex devices could still
> > fall back on deferred probe, I'd say.
> 
> When dealing with a single type of resources dependency graphs are very rarely 
> cyclic. However, when merging multiple resource dependency graphs, cycles 
> become quite common.
> 
> One example is the camera device found in OMAP3 chips. The SoC-side camera 
> controller can output a clock to the outside world (and is thus a CCF clock 
> provider). In most hardware designs that clock is used by the external camera 
> sensors, which in turn outputs a video data stream (with a clock and other 
> synchronization signals) fed to the camera controller. We thus get a 
> dependency loop. The situation is even worse on some Atmel platforms where 
> part of the camera controller logic is clocked by the pixel clock received 
> from the sensor. In that case the camera controller can't be software reset 
> without a sensor providing a pixel clock. We can blame the hardware designers, 
> but at the end of the day we need to support such systems. Whether such cases 
> need to be supported by generic code is a discussion we can have, and I'm not 
> pushing in either direction, but I want to make sure we will always have *a* 
> solution.

So I'm complete outsider to media drivers so maybe my question is stupid
but I cannot resist: So how is the device bringup supposed to work for
devices where you have these cycles in the dependency graph? Do you just
bring up the controller and then the sensor and it works because the
controller doesn't need the clock from the sensor for startup?

								Honza
-- 
Jan Kara <jack at suse.com>
SUSE Labs, CR