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

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Date:	Tue, 27 May 2014 15:24:35 +0300
From:	Pantelis Antoniou <pantelis.antoniou@...sulko.com>
To:	Grant Likely <grant.likely@...retlab.ca>
Cc:	Guenter Roeck <linux@...ck-us.net>,
	Sebastian Reichel <sre@...nel.org>,
	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>,
	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 Grant,

On May 27, 2014, at 3:12 PM, Grant Likely wrote:

> On Mon, 26 May 2014 16:42:44 -0700, Guenter Roeck <linux@...ck-us.net> wrote:
>> On 05/26/2014 03:36 PM, Sebastian Reichel wrote:
>>> 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.
>>> 
>> 
>> 3. Some hot-plug device or card is inserted or removed.
>> 
>> I would argue that the kernel should _not_ be booted with the overlay in place.
>> Otherwise the code handling overlays would have to have special handling
>> for the restart case, which is much more complex than just to re-insert
>> the overlay when it is determined that the device or card is still there.
> 
> Exactly.
> 

Looks like we are levitating to the 'remove overlays on kexec' approach.
Is that correct?

> g.
> 

Regards

-- Pantelis


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