linux-kernel - Re: [PATCH RFC 15/17] microblaze: Convert from _fdt_start to __dtb

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Date:	Wed, 13 Nov 2013 10:14:43 -0600
From:	Rob Herring <robherring2@...il.com>
To:	monstr@...str.eu, Geert Uytterhoeven <geert@...ux-m68k.org>
CC:	Andrew Morton <akpm@...ux-foundation.org>,
	linux-arch@...r.kernel.org, linux-kernel@...r.kernel.org,
	Rob Herring <rob.herring@...xeda.com>,
	microblaze-uclinux@...e.uq.edu.au, devicetree@...r.kernel.org
Subject: Re: [PATCH RFC 15/17] microblaze: Convert from _fdt_start to __dtb_start

On 11/13/2013 07:40 AM, Michal Simek wrote:
> On 11/13/2013 11:19 AM, Michal Simek wrote:
>> On 11/12/2013 08:42 PM, Geert Uytterhoeven wrote:
>>> Kill the microblaze-specific __fdt_blob section, and start
>>> using .dtb.init.rodata from <asm-generic/sections.h> for
>>> built-in DTBs, like most other DT enabled architectures.
>>> 
>>> Signed-off-by: Geert Uytterhoeven <geert@...ux-m68k.org> Cc:
>>> Michal Simek <monstr@...str.eu> Cc: Rob Herring
>>> <rob.herring@...xeda.com> Cc:
>>> microblaze-uclinux@...e.uq.edu.au Cc:
>>> devicetree@...r.kernel.org --- Untested, but the resulting
>>> vmlinux after "make simpleImage.system" looks sane.
>>> 
>>> QUESTION: The padding of the DTB is gone, is this OK?
>> 
>> Removing this kernel padding should be fine. I can't remember why
>> it was there from the beginning.
> 
> I know why is this here. The reason is process how dtb is passed to
> the kernel via command line. head.S code just copy that dtb to the
> same location where compiled-in dts is expected and kernel doesn't
> need to care about position of dtb because this copy is done
> without MMU. And the full kernel is covered by 2 tlbs and we don't
> need to use another TLB for dtb mapping.
> 
> It means pad matters a lot. Because u-boot ITS format doesn't use 
> simpleImage target but the kernel need to have a space for copying 
> dtb to this kernel location.
> 
> Let me think about if there is an easy way to handle dtbs which are
> passed from bootloader.

Isn't the built-in dtb in a RO init section? Seems like a bad idea to
write to it. Also, it will be a double copy because a built-in dtb has
to be copied out of init section.

Rob
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