Subject: [RFC] arm: pick the r2 passed DTB over the appended one

From: [email-masked] (Valentin Longchamp)
Date: Thu, 30 Apr 2015 14:39:03 +0200

On 04/29/2015 06:43 PM, Nicolas Pitre wrote:
> On Wed, 29 Apr 2015, Valentin Longchamp wrote:
> 
>> On 04/29/2015 03:58 PM, Nicolas Pitre wrote:
>>> Now, to fix your test, you'd simply have to augment it with:
>>>
>>> +		cmp	r6, r8		@ is r8 pointing to the appended DTB?
>>> +		beq	1f
>>> 		ldr	lr, [r8, #0]	@ conventionaly passed dtb ?
>>> 		cmp	lr, r1
>>> 		beq	dtb_check_done	@ yes, do not manage it
>>> +1:
>>>
>>
>> I had thought the same and implemented a similar test as you propose (patch
>> attached, with some more debug code - please excuse my poor assembler). However
>> it does not work ! The reason for it is that on the second run, r6 contains
>> another value. As the output below seems to show, this 2nd run r6 value seems to
>> point to a DTB since the first jump to dtb_check_done is not performed. However,
>> since r8 now points to the "initial" appended DTB, the 2nd test jumps to
>> dtb_check_done. Please see the output below.
> 
> Right.  On the second run, r6 points at the relocated DTB.  That's the 
> one that should be used. r8 points at the initial, non relocated and 
> about to be overwritten DTB. By giving priority to r8 with your patch, 
> you're picking the wrong one.
> 
> The following should fix this issue:
> 
> diff --git a/arch/arm/boot/compressed/head.S b/arch/arm/boot/compressed/head.S
> index c41a793b51..bbce6a0f0d 100644
> --- a/arch/arm/boot/compressed/head.S
> +++ b/arch/arm/boot/compressed/head.S
> @@ -399,6 +399,16 @@ dtb_check_done:
>  1:
>  #endif
>  
> +#ifdef CONFIG_ARM_APPENDED_DTB
> +		/*
> +		 * If r8 refers to an appended DTB, it is no longer valid
> +		 * and should be revalidated once relocated.
> +		 */
> +		cmp	r8, r5
> +		cmpcs	r6, r8
> +		movcs	r8, #0
> +#endif
> +
>  		sub	r9, r6, r5		@ size to copy
>  		add	r9, r9, #31		@ rounded up to a multiple
>  		bic	r9, r9, #31		@ ... of 32 bytes
> 
> 
> Nicolas
> 

Thank you very much for your help Nicolas, this fixed the issue indeed. I now
have the behavior I wanted.

Do you think it makes sense to send the updated patch for mainline inclusion or
is this a too "exotic" use case ?

Valentin