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

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Message-ID: <20140526223630.GA32691@earth.universe>
Date:	Tue, 27 May 2014 00:36:30 +0200
From:	Sebastian Reichel <sre@...nel.org>
To:	Grant Likely <grant.likely@...retlab.ca>
Cc:	Pantelis Antoniou <pantelis.antoniou@...sulko.com>,
	Geert Uytterhoeven <geert@...ux-m68k.org>,
	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>,
	Michael Stickel <ms@...able.de>,
	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" <devicetree@...r.kernel.org>,
	"linux-kernel@...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,

On Mon, May 26, 2014 at 10:33:03PM +0100, Grant Likely wrote:
> After thinking about it more, I think it is very likely that removing
> all the overlays is the correct thing to do in the kexec use-case. When
> kexec-ing, it makes sense that we'd want the exact same behaviour from
> the kexec'ed kernel. That means we want the device drivers to do the
> same thing including loading whatever overlays they depend on.
> 
> If the flattened tree was left applied, then the behaviour becomes
> different.
> 
> I say always remove the overlays unless explicitly told not to, but I'm
> struggling to come up with use cases where keeping them applied is
> desirable.

I would assume, that I want them applied in most cases. DT describes
the hardware. If I kexec into a new kernel I change software, not
hardware.

Maybe I'm missing the main purpose of the feature. I currently see
two useful usecases for DT overlays:

1. The dtb the kernel is booted with cannot be changed for some
   reason, but the board has additional hardware attached (e.g.
   the user added a sensor on the i2c bus)
2. The hardware is changed on the fly (e.g. the user flashed the
   FPGA part of a zynq processor), sensors on i2c bus, ...

In both cases the kernel should be booted with the additional
overlay information IMHO. Though for the second case it should
be possible to remove the "programmed" hardware information
somehow.

-- Sebastian

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