linux-kernel - Re: [PATCH 1/8] arm/dt: Make __vet

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Date:	Tue, 18 Jan 2011 16:26:17 -0500 (EST)
From:	Nicolas Pitre <nicolas.pitre@...aro.org>
To:	Grant Likely <grant.likely@...retlab.ca>
cc:	linux-arm-kernel@...ts.infradead.org,
	Russell King <linux@....linux.org.uk>,
	Catalin Marinas <catalin.marinas@....com>,
	devicetree-discuss@...ts.ozlabs.org,
	lkml <linux-kernel@...r.kernel.org>,
	Olof Johansson <olof@...om.net>,
	Jeremy Kerr <jeremy.kerr@...onical.com>,
	John Linn <John.Linn@...inx.com>,
	Lennert Buijtenhek <buytenh@...tstofly.org>
Subject: Re: [PATCH 1/8] arm/dt: Make __vet_atags also accept a dtb image

On Tue, 18 Jan 2011, Grant Likely wrote:

> The dtb is passed to the kernel via register r2, which is the same
> method that is used to pass an atags pointer.  This patch modifies
> __vet_atags to not clear r2 when it encounters a dtb image.
> 
> Signed-off-by: Grant Likely <grant.likely@...retlab.ca>
> ---
>  arch/arm/kernel/head-common.S |   19 +++++++++++++------
>  arch/arm/kernel/head.S        |    8 ++++----
>  2 files changed, 17 insertions(+), 10 deletions(-)
> 
> diff --git a/arch/arm/kernel/head-common.S b/arch/arm/kernel/head-common.S
> index 8f57515..d9a9105 100644
> --- a/arch/arm/kernel/head-common.S
> +++ b/arch/arm/kernel/head-common.S
> @@ -14,6 +14,7 @@
>  #define ATAG_CORE 0x54410001
>  #define ATAG_CORE_SIZE ((2*4 + 3*4) >> 2)
>  #define ATAG_CORE_SIZE_EMPTY ((2*4) >> 2)
> +#define OF_DT_MAGIC 0xedfe0dd0 /* 0xd00dfeed in big-endian */

ARM can be big endian too.  Would be nice to make this endian 
independent, or at least list this limitation in the commit log.

>  /*
>   * Exception handling.  Something went wrong and we can't proceed.  We
> @@ -105,22 +106,28 @@ __lookup_machine_type_data:
>  
>  /* Determine validity of the r2 atags pointer.  The heuristic requires
>   * that the pointer be aligned, in the first 16k of physical RAM and
> - * that the ATAG_CORE marker is first and present.  Future revisions
> + * that the ATAG_CORE marker is first and present.  If CONFIG_OF_FLATTREE
> + * is selected, then it will also accept a dtb pointer.  Future revisions
>   * of this function may be more lenient with the physical address and
>   * may also be able to move the ATAGS block if necessary.
>   *
>   * r8  = machinfo
>   *
>   * Returns:
> - *  r2 either valid atags pointer, or zero
> + *  r2 either valid atags pointer, valid dtb pointer, or zero
>   *  r5, r6 corrupted
>   */
>  __vet_atags:
>  	tst	r2, #0x3			@ aligned?
>  	bne	1f
>  
> -	ldr	r5, [r2, #0]			@ is first tag ATAG_CORE?
> -	cmp	r5, #ATAG_CORE_SIZE
> +#ifdef CONFIG_OF_FLATTREE
> +	ldr	r5, [r2, #0]			@ is it a DTB?
> +	ldr	r6, =OF_DT_MAGIC
> +	cmp	r5, r6
> +	beq	2f
> +#endif
> +	cmp	r5, #ATAG_CORE_SIZE		@ is first tag ATAG_CORE?

The "ldr r5 ..." is now done only within #ifdef CONFIG_OF_FLATTREE.  So 
if CONFIG_OF_FLATTREE is undefined then r5 will contain garbage.


Nicolas
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