Subject: ARM big-endian on current kernels for linux-3.8

From: [email-masked] (Ben Dooks)
Date: Wed, 13 Feb 2013 11:00:01 +0000

On 12/02/13 22:13, Nicolas Pitre wrote:
> On Tue, 12 Feb 2013, Ben Dooks wrote:
>
>> If we ignore the ATAG issue, then most of the support is currently in
>> the kernel to do this and there's not a lot of changes.
>
> You could make CPU_BE8_BOOT_LE depend on !ATAG.

That would be one consideration. For my next series I've removed the
CPU_BE8_BOOT_LE and added the 'setend be' as conditional on running BE8
as this is such as small code addition it seems to make sense to do it
that way.

I will put together a new series that deals with the endian-ness of the
bootloader.

>> I think that some of the boot changes could be dealt with by a
>> post-process of the zImage (such as the zImage magic) which would make
>> this series less intrusive to the kernel.
>
> No, please don't do that.  Especially if the kernel binary _expects_ to
> be booted from a LE environment.  Otherwise we might end up with
> kernels that don't boot because there are environment expectation
> mismatch, etc.
>
> Here's what could be done for the zImage header:

I will keep this in mind, it looks like a pretty good solution to the
whole issue.

> diff --git a/arch/arm/boot/compressed/head.S b/arch/arm/boot/compressed/head.S
> index 4d5f4192ee..2077ebb734 100644
> --- a/arch/arm/boot/compressed/head.S
> +++ b/arch/arm/boot/compressed/head.S
> @@ -129,9 +129,9 @@ start:
>    THUMB(		adr	r12, BSYM(1f)	)
>    THUMB(		bx	r12		)
>
> -		.word	0x016f2818		@ Magic numbers to help the loader
> -		.word	start			@ absolute load/run zImage address
> -		.word	_edata			@ zImage end address
> +		.word	_magic_sig	@ Signature to help the loader (0x016f2818)
> +		.word	_magic_start	@ zImage load/run address (0 if relocatable)
> +		.word	_magic_end	@ zImage end address
>    THUMB(		.thumb			)
>   1:
>   		mrs	r9, cpsr
> diff --git a/arch/arm/boot/compressed/Makefile b/arch/arm/boot/compressed/Makefile
> index a517153a13..52539f46c1 100644
> --- a/arch/arm/boot/compressed/Makefile
> +++ b/arch/arm/boot/compressed/Makefile
> @@ -200,8 +200,11 @@ CFLAGS_font.o := -Dstatic=
>   $(obj)/font.c: $(FONTC)
>   	$(call cmd,shipped)
>
> -$(obj)/vmlinux.lds: $(obj)/vmlinux.lds.in arch/arm/boot/Makefile $(KCONFIG_CONFIG)
> +$(obj)/vmlinux.lds.S: $(obj)/vmlinux.lds.in arch/arm/boot/Makefile $(KCONFIG_CONFIG)
>   	@sed "$(SEDFLAGS)"<  $<  >  $@
>
> +$(obj)/vmlinux.lds: $(obj)/vmlinux.lds.S
> +	$(call if_changed,cpp_lds_S)
> +
>   $(obj)/hyp-stub.S: $(srctree)/arch/$(SRCARCH)/kernel/hyp-stub.S
>   	$(call cmd,shipped)
> diff --git a/arch/arm/boot/compressed/vmlinux.lds.in b/arch/arm/boot/compressed/vmlinux.lds.in
> index 4919f2ac8b..567250fb7d 100644
> --- a/arch/arm/boot/compressed/vmlinux.lds.in
> +++ b/arch/arm/boot/compressed/vmlinux.lds.in
> @@ -7,6 +7,16 @@
>    * it under the terms of the GNU General Public License version 2 as
>    * published by the Free Software Foundation.
>    */
> +
> +#ifdef CONFIG_CPU_BE8_BOOT_LE
> +#define ZIMAGE_MAGIC(x) ( (((x)>>  24)&  0x000000ff) | \
> +			  (((x)>>   8)&  0x0000ff00) | \
> +			  (((x)<<   8)&  0x00ff0000) | \
> +			  (((x)<<  24)&  0xff000000) )
> +#else
> +#define ZIMAGE_MAGIC(x) (x)
> +#endif
> +
>   OUTPUT_ARCH(arm)
>   ENTRY(_start)
>   SECTIONS
> @@ -57,6 +67,10 @@ SECTIONS
>     .pad			: { BYTE(0); . = ALIGN(8); }
>     _edata = .;
>
> +  _magic_sig = ZIMAGE_MAGIC(0x016f2818);
> +  _magic_start = ZIMAGE_MAGIC(_start);
> +  _magic_end = ZIMAGE_MAGIC(_edata);
> +
>     . = BSS_START;
>     __bss_start = .;
>     .bss			: { *(.bss) }

-- 
Ben Dooks				http://www.codethink.co.uk/
Senior Engineer				Codethink - Providing Genius