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

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Message-ID: <20210216220619.GL28121@gate.crashing.org>
Date:   Tue, 16 Feb 2021 16:06:19 -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 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").

> 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.

And it doesn't here :-)

But, you need some way to figure out an arg is a constant known number
here.  We don't have a builtin for that I think.  Maybe some trick can
be done?  Maybe simply test "rn >= 0" as well, does that work?


Segher