linux-kernel - Re: [PATCH v4 2/8] OF: Introduce DT overlay support.

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Message-Id: <20140514154907.BC65DC40BD2@trevor.secretlab.ca>
Date:	Wed, 14 May 2014 16:49:07 +0100
From:	Grant Likely <grant.likely@...retlab.ca>
To:	Michael Stickel <ms@...able.de>
Cc:	Pantelis Antoniou <pantelis.antoniou@...sulko.com>,
	Rob Herring <robherring2@...il.com>,
	Stephen Warren <swarren@...dotorg.org>,
	Matt Porter <matt.porter@...aro.org>,
	Koen Kooi <koen@...inion.thruhere.net>,
	Alison Chaiken <Alison_Chaiken@...tor.com>,
	Dinh Nguyen <dinh.linux@...il.com>,
	Jan Lubbe <jluebbe@...net.de>,
	Alexander Sverdlin <alexander.sverdlin@....com>,
	Guenter Roeck <linux@...ck-us.net>,
	Dirk Behme <dirk.behme@...il.com>,
	Alan Tull <delicious.quinoa@...il.com>,
	Sascha Hauer <s.hauer@...gutronix.de>,
	Michael Bohan <mbohan@...eaurora.org>,
	Ionut Nicu <ioan.nicu.ext@....com>,
	Michal Simek <monstr@...str.eu>,
	Matt Ranostay <mranostay@...il.com>,
	devicetree@...r.kernel.org, linux-kernel@...r.kernel.org,
	Pete Popov <pete.popov@...sulko.com>,
	Dan Malek <dan.malek@...sulko.com>,
	Georgi Vlaev <georgi.vlaev@...sulko.com>,
	Pantelis Antoniou <panto@...oniou-consulting.com>
Subject: Re: [PATCH v4 2/8] OF: Introduce DT overlay support.

On Wed, 14 May 2014 14:11:52 +0200, Michael Stickel <ms@...able.de> wrote:
> Hi Grant,
> 
> Am 14.05.2014 12:08, schrieb Grant Likely:
> > More generally I am concerned about whether or not overlays
> > will introduce corner cases that can never be handled correctly,
> > particularly in how multiple overlays will get handled. I want to see
> > very clear rules on what happens when multiple overlays are applied, and
> > then removed again. Is it possible to remove overlays out of order? If
> > so, what are the conditions that would not be allowed?
> 
> Yes, it is possible that an overlay depends on another.
> 
> The problem is not, that an overlay is removed other overlays depend on,
> but that nodes of an overlay may depend on the to-be-removed overlay and
> the resulting devicetree can become inconsistent.

So what should the rule be then? It sounds to me that it should be a
hard and fast rule for overlays to always be removed in-order. If two
overlays are applied, and the first one needs to be removed again, then
that forces a removal of the second. The code needs to enforce it too.

The question can be revisited if someone can find a way to validate
overlays do not conflict.

g.

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