linux-kernel - Re: [RFC PATCH 0/9] dt: dependencies (for deterministic driver initialization order based on the DT)

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Message-ID: <53FC3DBB.7000407@ahsoftware.de>
Date:	Tue, 26 Aug 2014 09:56:43 +0200
From:	Alexander Holler <holler@...oftware.de>
To:	Jon Loeliger <jdl@....com>,
	Thierry Reding <thierry.reding@...il.com>
CC:	Mark Rutland <mark.rutland@....com>,
	"devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
	Russell King <linux@....linux.org.uk>,
	Arnd Bergmann <arnd@...db.de>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Rob Herring <robh+dt@...nel.org>,
	"grant.likely@...aro.org" <grant.likely@...aro.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>
Subject: Re: [RFC PATCH 0/9] dt: dependencies (for deterministic driver initialization
 order based on the DT)

Am 25.08.2014 15:08, schrieb Jon Loeliger:
>>
>
>> Anyway, instead of going back and forth between "deferred probe is good"
>> and "deferred probe is bad", how about we do something useful now and
>> concentrate on how to make use of the information we have in DT with the
>> goal to reduce the number of cases where deferred probing is required?
>
> Good idea.
>
> The proposal on the table is to allow the probe code
> to make a topological sort of the devices based on
> dependency information either implied, explicitly stated
> or both.  That is likely a fundamentally correct approach.
>
> I believe some of the issues that need to be resolved are:
>
>      1) What constitutes a dependency?

In my patches phandles are used. That works pretty good for almost all 
DTs. So almost all dependencies are already declared in a DT and almost 
no changes to the DT are necessary. The only binding I've seen where it 
doesn't work is remote-endpoint, because that binding isn't a directed 
dependency. So one of the two places where such a binding occurs needs a 
"no-dependencies = <phandle>" to avoid circular dependencies which can 
be solved.

>      2) How is that dependency expressed?

Already there in form of phandles.

>      3) How do we add missing dependencies?

My patches offer the possibility to extend or reduce the list of 
(automatically generated) dependencies by using "[no-]dependencies = < 
list of phandles >;"

>      4) Backward compatability problems.

None in my approach. The DT just includes an additional binding to 
circumvent the missing but needed type information for phandles.

Regards,

Alexander Holler
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