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

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Date:   Wed, 17 Feb 2021 09:37:04 -0600
From:   Segher Boessenkool <segher@...nel.crashing.org>
To:     Michael Ellerman <mpe@...erman.id.au>
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

Hi!

On Wed, Feb 17, 2021 at 04:43:45PM +1100, Michael Ellerman wrote:
> Segher Boessenkool <segher@...nel.crashing.org> writes:
> > 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.

Huh interesting.  I was going to suggest to use __is_constexpr instead,
but that should return true for slightly fewer expressions (but probably
still okay in this case).

> 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.

Yeah, this is not preprocessor magic, the compiler actually does its
thing here.  It still won't work if the compiler cannot reduce the
expression to a number (but it does see it is constant), but that is the
best you can do here probably.


Segher