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

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Message-ID: <CAGa+x84SkGzC3cPCvnZHJEK2uJion=0fowBNdkydjF1GnLLsTw@mail.gmail.com>
Date:	Thu, 29 May 2014 10:20:16 -0700
From:	Kevin Hilman <khilman@...aro.org>
To:	Stephen Warren <swarren@...dotorg.org>
Cc:	linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>,
	"devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
	Arnd Bergmann <arnd@...db.de>, Olof Johansson <olof@...om.net>,
	"linaro-kernel@...ts.linaro.org" <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 Thu, May 29, 2014 at 8:55 AM, Stephen Warren <swarren@...dotorg.org> wrote:
> 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

Great.  As I mentioned above, I'm certainly not opposed to more
padding, and agree that it would be more future proof.  I was simply
being rather conservative in case folks didn't want to see every .dtb
grow by 4k.

I'll wait a bit to see if there are any other opinions, and the respin
with 4k padding.

Kevin
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