[PATCH] ARM:boot:device tree: Allow the device tree binary to be appended to zImage

[John Bonesio]

Hi Nicolas,

Thanks for the comments. I have a question (below).

Thanks for your help,

- John

On 03/17/2011 11:42 AM, Nicolas Pitre wrote:
> On Wed, 9 Mar 2011, John Bonesio wrote:
> 
>> This patch provides the ability to boot using a device tree that is appended
>> to the raw binary zImage (e.g. cat zImage <filename>.dtb > zImage_w_dtb).
>>
>> Signed-off-by: John Bonesio <bones at secretlab.ca>
> 
> You're almost there, but there are still remaining issues with this 
> patch.
> 
>> ---
>>
>>  arch/arm/Kconfig                |    7 ++++
>>  arch/arm/boot/compressed/head.S |   75 +++++++++++++++++++++++++++++++++++++--
>>  2 files changed, 79 insertions(+), 3 deletions(-)
>>
>> diff --git a/arch/arm/Kconfig b/arch/arm/Kconfig
>> index d8a330f..1a55e3e 100644
>> --- a/arch/arm/Kconfig
>> +++ b/arch/arm/Kconfig
>> @@ -1593,6 +1593,13 @@ config USE_OF
>>  	help
>>  	  Include support for flattened device tree machine descriptions.
>>  
>> +config ARM_APPENDED_DTB
>> +	bool "Use appended device tree blob"
>> +	depends on OF
>> +	help
>> +	  With this option, the boot code will look for a device tree binary
>> +	  (dtb) appended to zImage.
>> +
> 
> I'd put this option below the ZBOOT_ROM option, and make it dependent on 
> !ZBOOT_ROM, e.g.:
> 
> 	depends on OF && !ZBOOT_ROM
> 
>> diff --git a/arch/arm/boot/compressed/head.S b/arch/arm/boot/compressed/head.S
>> index 200625c..611719e 100644
>> --- a/arch/arm/boot/compressed/head.S
>> +++ b/arch/arm/boot/compressed/head.S
>> @@ -210,6 +210,58 @@ restart:	adr	r0, LC0
>>  		 */
>>  		mov	r10, r6
>>  #endif
>> +#ifdef CONFIG_ARM_APPENDED_DTB
>> +/*
>> + *   r0  = delta
>> + *   r2  = BSS start
>> + *   r3  = BSS end
>> + *   r4  = final kernel address
>> + *   r5  = start of this image
>> + *   r6  = _edata
>> + *   r7  = architecture ID
>> + *   r8  = atags/device tree pointer
>> + *   r9  = size of decompressed image
>> + *   r10 = end of this image, including  bss/stack/malloc space if non XIP
>> + *   r11 = GOT start
>> + *   r12 = GOT end
>> + *
>> + * if there are device trees (dtb) appended to zImage, advance r10 so that the
>> + * dtb data will get relocated along with the kernel if necessary.
>> + */
>> +
>> +		ldr	lr, [r6, #0]
>> +#ifndef __ARMEB__
>> +		ldr	r1, =0xedfe0dd0		@ sig is 0xd00dfeed big endian
>> +#else
>> +		ldr	r1, =0xd00dfeed
>> +#endif
>> +		cmp	lr, r1
>> +		bne	dtb_check_done
>> +
>> +		mov	r8, r6			@ use the appended device tree
>> +
>> +		/* Get the dtb's size */
>> +		ldr	lr, [r6, #4]
>> +
>> +#ifndef __ARMEB__
>> +		/* convert lr (dtb size) to little endian */
>> +		eor	r1, lr, lr, ror #16
>> +		bic	r1, r1, #0x00ff0000
>> +		mov	lr, lr, ror #8
>> +		eor	lr, lr, r1, lsr #8
>> +#endif
>> +		/*
>> +		 * dtb size rounded up to a multiple of 8 bytes so a
>> +		 * misaligned GOT entry doesn't cause the end of the
>> +		 * dtb binary to be overwritten.
>> +		 */
>> +		add	lr, lr, #7
>> +		bic	lr, lr, #7
>> +
>> +		add	r10, r10, lr
>> +		add	r6, r6, lr
>> +dtb_check_done:
>> +#endif
> 
> Now if no dtb was found, or if that code is not configured, then lr 
> contains garbage and that may have random consequences with the code 
> that follows, which would explain the reported bad behaviors with this 
> patch applied even if not configured in the kernel.

I'm probably not understanding something here. When I look at the code
prior to my patch, it looks like lr is not initialized by head.S. It
also looks like this code makes use of lr when it relocates the code and
leaves it uninitialized when it jumps back to 'restart'.

Is this a bug in the previous code too?

What should lr be initialized to, or should we just be preserving it's
value?

I saw the comment about the performance is different with this patch -
something about a pause between 'Uncompressing Linux...' and the kernel
boot output. I'm not sure what's going on here. By the time
'Uncompressing Linux...' comes out all relocations and dtb discovery is
complete. Do you really think having lr uninitialized would cause this?

> 
> 

[snipped rest of patch out]

> 
> Nicolas