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

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Date:	Mon, 26 May 2014 14:08:32 +0300
From:	Pantelis Antoniou <pantelis.antoniou@...sulko.com>
To:	Geert Uytterhoeven <geert@...ux-m68k.org>
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>,
	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 Geert,

On May 26, 2014, at 1:57 PM, Geert Uytterhoeven wrote:

> Hi Grant,
> 
> On Mon, May 26, 2014 at 12:48 PM, Grant Likely
> <grant.likely@...retlab.ca> wrote:
>> On Tue, 20 May 2014 09:38:49 +0200, Geert Uytterhoeven <geert@...ux-m68k.org> wrote:
>>> On Tue, May 20, 2014 at 7:50 AM, Grant Likely <grant.likely@...retlab.ca> wrote:
>>>>> Why has the overlay system been designed for plugging and unpluging whole
>>>>> overlays?
>>>>> That means the kernel has to remember the full stack, causing issues with
>>>>> e.g. kexec.
>>>> 
>>>> Mostly so that drivers don't see any difference in the livetree data
>>>> structure. It also means that userspace sees a single representation of
>>>> the hardware at any given time.
>>> 
>>> Sorry, I don't follow the argument about the "single representation of the
>>> hardware".
>> 
>> Er, s/of the hardware/of the tree/. Right now the overlay design
>> modifies the live tree which at the same time modifies the tree
>> representation in /sys/firmware/devicetree. If the design was changed to
>> keep the overlay logically separate, then I would think we want to
>> expose that information to usespace also. In fact, I think we would need
>> to for usecases like kexec.
> 
> OK, so it does modify the real tree, and doesn't keep the actual overlays.
> 

It modifies the actual tree, and it keeps a log of each modification made to the
tree so that it will be able to revert the changes made.

> I was under the impression the overlay stack was also kept in memory, to allow
> reversal, so there was a misunderstanding.
> 

Yep.

> Hence for kexec, the tree in /sys/firmware/devicetree can just be passed
> to the new kernel, as that's the current representation of the hardware?
> 

Exactly.

> Gr{oetje,eeting}s,
> 
>                        Geert
> 

Regards

-- Pantelis

> --
> Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@...ux-m68k.org
> 
> In personal conversations with technical people, I call myself a hacker. But
> when I'm talking to journalists I just say "programmer" or something like that.
>                                -- Linus Torvalds
> --
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