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

[Hans Verkuil <hverkuil@xs4all.nl>]

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.
> 

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.

I'm never very keen on creating complex code to cater to a small percentage of
problem cases.

Regards,

	Hans