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

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Date:	Thu, 15 May 2014 00:14:37 -0700
From:	Pantelis Antoniou <pantelis.antoniou@...sulko.com>
To:	Michael Stickel <ms@...able.de>
Cc:	Grant Likely <grant.likely@...retlab.ca>,
	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>
Subject: Re: [PATCH v4 2/8] OF: Introduce DT overlay support.

Hi Michael,

On May 14, 2014, at 5:11 AM, Michael Stickel 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.
> 
> 
> 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.
> 

Well, shouldn't you be reverting the overlays in reverse sequence?

As I see it, when proper subtree tracking is implemented this use case 
(of removing #0 before #1) will not be allowed.

What is your ideal scenario for this use case?

> Michael
> 

Regards

-- Pantelis

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