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

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Date:	Tue, 26 Aug 2014 13:01:29 +0200
From:	Alexander Holler <holler@...oftware.de>
To:	Thierry Reding <thierry.reding@...il.com>
CC:	Grant Likely <grant.likely@...aro.org>, Jon Loeliger <jdl@....com>,
	Mark Rutland <mark.rutland@....com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
	Russell King <linux@....linux.org.uk>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Rob Herring <robh+dt@...nel.org>,
	Arnd Bergmann <arnd@...db.de>,
	"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 26.08.2014 12:44, schrieb Alexander Holler:
> Am 26.08.2014 12:25, schrieb Thierry Reding:
>> On Tue, Aug 26, 2014 at 11:42:04AM +0200, Alexander Holler wrote:
>
>>> You need either the type information in the DTB (that's why I've add
>>> those
>>> "dependencies" to identify phandles), or you need to know every
>>> binding (at
>>> "dependency-resolve-time" to identify phandles. The latter is
>>> impracticable
>>> to implement in a generic way (for use with every possible binding).
>>
>> Like I already mentioned, this could be done in drivers who contain that
>> information already anyway. Or parts of it could be done in subsystem-
>> specific callbacks where a generic binding is available.
>
> That would end up with almost the same ugly driver-based workarounds as
> now. It's much better if a driver author only has to define it's
> prerequisits (in form of dependencies in the dts) and could be sure the
> driver will only be probed if those are met, than to do that stuff based
> on a subsystem or even driver level.
>
> If you add dependency-information to drivers, you have two problems:
>
> - How do you get these information from the driver (remember, currently
> there is only one initial call, a initcall which might do almost anything)
>
> - These information might become outdated and you would have to change
> all drivers. E.g. if the name of a dependency (driver) changes it
> wouldn't be done with changing the dts (maybe plural), but you would
> have to change the source of all dependant drivers too.

And after having sorted my brain:

A driver depends on a binding (and its API), but not on explicit named 
other drivers. So trying it (again) on driver level is doomed to fail.

Alexander Holler
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