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

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Message-ID: <87a6s3uu3y.fsf@mpe.ellerman.id.au>
Date:   Wed, 17 Feb 2021 16:43:45 +1100
From:   Michael Ellerman <mpe@...erman.id.au>
To:     Segher Boessenkool <segher@...nel.crashing.org>
Cc:     Feng Tang <feng.tang@...el.com>,
        Christophe Leroy <christophe.leroy@...roup.eu>,
        lkp <lkp@...el.com>,
        "kbuild-all@...ts.01.org" <kbuild-all@...ts.01.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: {standard input}:577: Error: unsupported relocation against base

Segher Boessenkool <segher@...nel.crashing.org> writes:
> Hi!
>
> On Tue, Feb 16, 2021 at 08:36:02PM +1100, Michael Ellerman wrote:
>> Feng Tang <feng.tang@...el.com> writes:
>> >   {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;})
>
> It requires a constant number with known (at compile time) value, while
> __builtin_constant_p checks for any constant.  The address of some
> defined symbol is a constant as well normally, for example.
>
> It's better to write that asm as
> 	asm volatile("mfdcr %0,%1" : "=r" (rval) : "n"(rn));
> btw (the "n" constraint means "constant integer with known value" (it
> stands for "numeric"), while the "i" constraint means just "constant
> integer").

Actually that fixes it.

diff --git a/arch/powerpc/include/asm/dcr-native.h b/arch/powerpc/include/asm/dcr-native.h
index 7141ccea8c94..d143308b0f95 100644
--- a/arch/powerpc/include/asm/dcr-native.h
+++ b/arch/powerpc/include/asm/dcr-native.h
@@ -53,8 +53,8 @@ static inline void mtdcrx(unsigned int reg, unsigned int val)
 #define mfdcr(rn)                                              \
        ({unsigned int rval;                                    \
        if (__builtin_constant_p(rn) && rn < 1024)              \
-               asm volatile("mfdcr %0," __stringify(rn)        \
-                             : "=r" (rval));                   \
+               asm volatile("mfdcr %0, %1" : "=r" (rval)       \
+                            : "n" (rn));                       \
        else if (likely(cpu_has_feature(CPU_FTR_INDEXED_DCR)))  \
                rval = mfdcrx(rn);                              \
        else                                                    \


I guess because we give the compiler time to work out the constant,
rather than stringifying it immediately.

cheers