Re: {standard input}:577: Error: unsupported relocation against base

[Michael Ellerman]

Feng Tang <feng.tang@xxxxxxxxx> writes:
> Hi Christophe and Michael,
>
> On Mon, Jan 18, 2021 at 10:24:08PM +0800, Christophe Leroy wrote:
>> 
>> Le 05/01/2021 ? 11:58, kernel test robot a 閏rit :
>> > tree:   https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git master
>> > head:   e71ba9452f0b5b2e8dc8aa5445198cd9214a6a62
>> > commit: 8b8319b181fd9d6821703fef1228b4dcde613a16 powerpc/44x: Don't support 440 when CONFIG_PPC_47x is set
>> 
>> I see no link with that commit. Looks like the problem has been existing for some time.
>> It exists on the commit before that one, it exists on v5.9 and it exists on v5.10 with that commit
>> reverted.
>  
> Yes, this seems to be a long-standing issue, and we just double checked
> this compile error. 
>
> It happend when compiling arch/powerpc/platforms/44x/fsp2.c, macro
> 'mfdcr' requirs an instant number as parameter, while is not met by
> show_plbopb_regs(). Changing show_plbopb_regs() from function to
> a macro fixes the error, as the patch below:
>
> Thanks,
> Feng
>
>
> From 3bcb9638afc873d0e803aea1aad4f77bf1c2f6f6 Mon Sep 17 00:00:00 2001
> From: Feng Tang <feng.tang@xxxxxxxxx>
> Date: Fri, 5 Feb 2021 16:08:43 +0800
> Subject: [PATCH] powerpc/44x/fsp2: fix a compiling error regarding macro
>  'mdfcr'
>
> 0day's kbuild test found error:
>
> "
>   CC      arch/powerpc/platforms/44x/fsp2.o
>
>   {standard input}:577: Error: unsupported relocation against base
>   {standard input}:580: Error: unsupported relocation against base
>   {standard input}:583: Error: unsupported relocation against base
> "
>
> The reason is macro 'mfdcr' requirs an instant number as parameter,
> which is not met by show_plbopb_regs().

It doesn't require a constant, it checks if the argument is constant:

#define mfdcr(rn)						\
	({unsigned int rval;					\
	if (__builtin_constant_p(rn) && rn < 1024)		\
		asm volatile("mfdcr %0," __stringify(rn)	\
		              : "=r" (rval));			\
	else if (likely(cpu_has_feature(CPU_FTR_INDEXED_DCR)))	\
		rval = mfdcrx(rn);				\
	else							\
		rval = __mfdcr(rn);				\
	rval;})

But the error you're seeing implies the compiler is choosing the first
leg of the if, even when rn == "base + x", which is surprising.

We've had cases in the past of __builtin_constant_p() returning false
for things that a human can see are constant at build time, but I've
never seen the reverse.

cheers