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

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Date:	Wed, 14 May 2014 14:11:52 +0200
From:	Michael Stickel <ms@...able.de>
To:	Grant Likely <grant.likely@...retlab.ca>
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.

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.

I have an SPI Bus with two slaves. The second slave is used only on one
of our boards. That is why we split the overlays the following way:

xxxx_spi1.dts:
  Pinmux for SPI-Bus and activation of spi-controller.
  Pinmux for CS0 and definition of first slave.

xxxx_spi1_cs1:
  Pinmux for CS1 and definition of second slave.

When the overlay for the bus is removed, the overlays for the second
slave does not make any sense anymore.

It is even worse in a scenario we have with a test board.
One of the slaves is an spi-io-controller with a few bitbanging i2c
masters. In an extreme case, each component is defined in a separate
overlay and only the overlay with the master is removed. I know, that
this is completely sick. The devices are removed cleanly because of the
device dependency.

Michael

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