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

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Date:	Wed, 13 Nov 2013 15:02:49 +0100
From:	Geert Uytterhoeven <geert@...ux-m68k.org>
To:	Michal Simek <monstr@...str.eu>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Linux-Arch <linux-arch@...r.kernel.org>,
	"linux-kernel@...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 Wed, Nov 13, 2013 at 2:40 PM, Michal Simek <monstr@...str.eu> wrote:
>>> 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.

You can add to asm-generic/sections.h:

#ifndef ARCH_DTB_PADDING
#define ARCH_DTB_PADDING
#endif

and change KERNEL_DTB() to:

#define KERNEL_DTB()                                                    \
       STRUCT_ALIGN();                                                 \
       VMLINUX_SYMBOL(__dtb_start) = .;                                \
       *(.dtb.init.rodata)                                             \
       ARCH_DTB_PADDING                                \
       VMLINUX_SYMBOL(__dtb_end) = .;

Then microblaze can predefine ARCH_DTB_PADDING in its
<asm/sections.h>.

Still, it will crash badly if the external DTB is larger than
ARCH_DTB_PADDING.

Gr{oetje,eeting}s,

                        Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@...ux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds
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