Date:	Thu, 7 Nov 2013 09:23:45 +0200
From:	Pantelis Antoniou <panto@...oniou-consulting.com>
To:	Sebastian Andrzej Siewior <sebastian@...akpoint.cc>
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, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 0/3 - V2] Introducing Device Tree Overlays

Hi Sebestian,

On Nov 6, 2013, at 10:31 PM, Sebastian Andrzej Siewior wrote:

> On 06.11.13, Pantelis Antoniou wrote:
>> Hi Sebastian,
> Hi Pantelis,
> 
>> It has been discussed.
>> 
>> We are doing it because
>> 
>> a) We tried to do it in u-boot and it has been a complete disaster.
>> Regular users just can't handle bootloader updates.
> 
> How so? The "additional" dtb piece was deleted by accident as part of
> the u-boot upgrade? Do you maybe a link which describes the disaster?
> 

You're assuming that bootloaders are anything like u-boot or barebox.
In the field, especially on consumer products, bootloaders are custom one-off
jobs that don't do much besides handing control to the kernel as soon as possible.

Even when using u-boot, end users botch updates related to bootloader in
such a way that systems end up RMAed.

Ask Koen Kooi in the CClist about the messy details.

>> b) It is similar to that. It was originally created for the beaglebone,
>> which has a concept of capes (similar to Arduino shields).
>> http://circuitco.com/support/index.php?title=BeagleBone_Capes
>> Turns out it's really useful to anyone doing reconfigurable hardware too,
>> so that's why FPGA people are thinking of using it.
> 
> I am aware of this. My understanding is that those capes have hardware
> information encoded in an eeprom behind i2c _and_ you can't or should
> not replace capes at runtime.
> Naive as I am I *assume* it should be easy to read this eeprom in u-boot
> as part of the boot setup and extend the dtb before passing it to the
> kernel. In case this works well, the problem here is a) ?
> 

It is just better system design to have it all done in the kernel.
Other people in the list can chime in, but it's hard to get a feel of the
problem if you haven't dealt with it before.

>> c) There are people that want to tinker with Linux based hardware boards
>> but are not kernel developers. This gives them a way to do so without
>> having to recompile the kernel and/or reboot while tinkering.
> 
> I understand that they want to avoid reboot. But they still need to
> recompile the device tree, don't they? Or does this allow to change the
> HW description without knowing the syntax of .dts?
> 

They understand the syntax of the DTS (barely).
They can't hack compiling the kernel and updating it, not by a long shot.
Not everyone is a kernel hacker (neither it needs be).

Imagine people coming over from Arduino trying to hack a 4K board file to
add support for the thing they're working on.

>> Regards
>> 
>> -- Pantelis
> 
> Sebastian

Regards

-- Pantelis

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