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

Segher Boessenkool
2021-02-17T15:41:16Z

Hi!

On Wed, Feb 17, 2021 at 04:43:45PM +1100, Michael Ellerman wrote:
> Segher Boessenkool <segher@kernel.crashing.org> writes:
> > On Tue, Feb 16, 2021 at 08:36:02PM +1100, Michael Ellerman wrote:
> >> Feng Tang <feng.tang@intel.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