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

[Michal Simek]

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.

Thanks,
Michal


-- 
Michal Simek, Ing. (M.Eng), OpenPGP -> KeyID: FE3D1F91
w: www.monstr.eu p: +42-0-721842854
Maintainer of Linux kernel - Xilinx Microblaze
Maintainer of Linux kernel - Xilinx Zynq ARM and ZynqMP ARM64 SoCs
U-Boot custodian - Xilinx Microblaze/Zynq/ZynqMP/Versal SoCs