linux-kernel - Re: [PATCH] scripts/dtc: pad DTBs to facilitate later modification

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Date:	Thu, 29 May 2014 09:55:34 -0600
From:	Stephen Warren <swarren@...dotorg.org>
To:	Kevin Hilman <khilman@...aro.org>,
	linux-arm-kernel@...ts.infradead.org, devicetree@...r.kernel.org
CC:	Arnd Bergmann <arnd@...db.de>, Olof Johansson <olof@...om.net>,
	linaro-kernel@...ts.linaro.org, Nicolas Pitre <nico@...aro.org>,
	Thomas Petazzoni <thomas.petazzoni@...e-electrons.com>,
	Michal Marek <mmarek@...e.cz>,
	"open list:KERNEL BUILD + fi..." <linux-kbuild@...r.kernel.org>,
	open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] scripts/dtc: pad DTBs to facilitate later modification

On 05/23/2014 05:41 PM, Kevin Hilman wrote:
> By default, add some padding to the DT blobs to facilitate later
> patching.
> 
> An example need for DTB patching is the need to modifiy the command
> line on platforms where ATAGS are not (or cannot) be used to pass the
> commandline.  For example, we do not support a big-endian kernel
> reading ATAGS from a little-endian u-boot, so the only way to pass a
> command line in the DT.
> 
> Also, without ATAG support (or if u-boot was built without
> CONFIG_INITRD_TAG) the only way to pass an initrd is by adding an
> initrd= option to command line (in the DT).
> 
> Therefore, to facilitate adding to the DT command line directly in the
> DTB, add some padding.
> 
> Cc: Nicolas Pitre <nico@...aro.org>
> Cc: Stephen Warren <swarren@...dotorg.org>
> Cc: Thomas Petazzoni <thomas.petazzoni@...e-electrons.com>
> Signed-off-by: Kevin Hilman <khilman@...aro.org>
> ---
> I kinda pulled 64 bytes out of the air here since it's enough to add
> some common things to the commandline like debug, earlyprink
> initrd=<addr>,<size>, etc., but I'm certainlly not opposed to more
> padding.

Conceptually,
Acked-by: Stephen Warren <swarren@...dia.com>

But I would expect a pad of something like 4KB to be more future-proof.
U-Boot appears to use 4KB on ARM at least:

./arch/arm/dts/Makefile:37:DTC_FLAGS += -R 4 -p 0x1000
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