[PATCH] arm64: Add support for bigger u-boot when CONFIG_POSITION_INDEPENDENT=y

[André Przywara]

On 03/09/2020 14:35, Michal Simek wrote:
> 
> 
> On 02. 09. 20 18:34, Stephen Warren wrote:
>> On 9/2/20 5:15 AM, Michal Simek wrote:
>>> From: "Edgar E. Iglesias" <edgar.iglesias at xilinx.com>
>>>
>>> When U-Boot binary exceeds 1MB with CONFIG_POSITION_INDEPENDENT=y
>>> compilation error is shown:
>>> /mnt/disk/u-boot/arch/arm/cpu/armv8/start.S:71:(.text+0x3c): relocation
>>> truncated to fit: R_AARCH64_ADR_PREL_LO21 against symbol `__rel_dyn_end'
>>> defined in .bss_start section in u-boot.
>>>
>>> It is caused by adr instruction which permits the calculation of any byte
>>> address within +- 1MB of the current PC.
>>> Because U-Boot is bigger then 1MB calculation is failing.
>>>
>>> The patch is using adrp/add instructions where adrp shifts a signed, 21-bit
>>> immediate left by 12 bits (4k page), adds it to the value of the program
>>> counter with the bottom 12 bits cleared to zero. Then add instruction
>>> provides the lower 12 bits which is offset within 4k page.
>>> These two instructions together compose full 32bit offset which should be
>>> more then enough to cover the whole u-boot size.
>>
>>> diff --git a/arch/arm/cpu/armv8/start.S b/arch/arm/cpu/armv8/start.S
>>
>>> @@ -67,8 +67,10 @@ pie_fixup:
>>>  	adr	x0, _start		/* x0 <- Runtime value of _start */
>>>  	ldr	x1, _TEXT_BASE		/* x1 <- Linked value of _start */
>>>  	sub	x9, x0, x1		/* x9 <- Run-vs-link offset */
>>> -	adr	x2, __rel_dyn_start	/* x2 <- Runtime &__rel_dyn_start */
>>> -	adr	x3, __rel_dyn_end	/* x3 <- Runtime &__rel_dyn_end */
>>> +	adrp    x2, __rel_dyn_start     /* x2 <- Runtime &__rel_dyn_start */
>>> +	add     x2, x2, #:lo12:__rel_dyn_start
>>> +	adrp    x3, __rel_dyn_end       /* x3 <- Runtime &__rel_dyn_end */
>>> +	add     x3, x3, #:lo12:__rel_dyn_end
>>>  pie_fix_loop:
>>>  	ldp	x0, x1, [x2], #16	/* (x0, x1) <- (Link location, fixup) */
>>>  	ldr	x4, [x2], #8		/* x4 <- addend */
>>
>> There are likely a bunch of other places in the code that need updating
>> too; take a look at commit 49e93875a62f "arm64: support running at addr
>> other than linked to" (which introduced the code above) to find other
>> places with similar instruction sequences that almost certainly need
>> updating.
>>
> 
> I didn't hit any issue to have a need to update them. Definitely worth
> to check that locations too.

So I thought the same, so I checked at least this file. And while there
are other "adr" instructions, they only go to nearby labels, so don't
need to be pumped up.
But I will try to grep for more cases as well.

Cheers,
Andre