From: Segher Boessenkool <segher@kernel.crashing.org>
To: Michael Ellerman <mpe@ellerman.id.au>
Cc: Feng Tang <feng.tang@intel.com>,
Christophe Leroy <christophe.leroy@csgroup.eu>,
lkp <lkp@intel.com>,
"kbuild-all@lists.01.org" <kbuild-all@lists.01.org>,
"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
Subject: Re: {standard input}:577: Error: unsupported relocation against base
Date: Tue, 16 Feb 2021 16:06:19 -0600 [thread overview]
Message-ID: <20210216220619.GL28121@gate.crashing.org> (raw)
In-Reply-To: <87lfbouzgd.fsf@mpe.ellerman.id.au>
Hi!
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").
> 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
next prev parent reply other threads:[~2021-02-16 22:11 UTC|newest]
Thread overview: 9+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-01-05 10:58 {standard input}:577: Error: unsupported relocation against base kernel test robot
2021-01-18 14:24 ` Christophe Leroy
2021-02-05 10:08 ` Feng Tang
2021-02-16 9:36 ` Michael Ellerman
2021-02-16 22:06 ` Segher Boessenkool [this message]
2021-02-17 5:43 ` Michael Ellerman
2021-02-17 15:37 ` Segher Boessenkool
2021-02-17 10:49 ` Feng Tang
2021-02-18 11:08 ` Michael Ellerman
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=20210216220619.GL28121@gate.crashing.org \
--to=segher@kernel.crashing.org \
--cc=christophe.leroy@csgroup.eu \
--cc=feng.tang@intel.com \
--cc=kbuild-all@lists.01.org \
--cc=linux-kernel@vger.kernel.org \
--cc=lkp@intel.com \
--cc=mpe@ellerman.id.au \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).