linux-kernel - Re: [RFC PATCH] ARM: Use generic BUG() handler

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Date:	Tue, 1 Mar 2011 08:34:37 -0800
From:	Simon Glass <sjg@...omium.org>
To:	Russell King - ARM Linux <linux@....linux.org.uk>
Cc:	Mikael Pettersson <mikpe@...uu.se>,
	Nicolas Pitre <nicolas.pitre@...aro.org>,
	Phil Carmody <ext-phil.2.carmody@...ia.com>,
	Tony Lindgren <tony@...mide.com>,
	Catalin Marinas <catalin.marinas@....com>,
	linux-kernel@...r.kernel.org, Rabin Vincent <rabin@....in>,
	Alexander Shishkin <virtuoso@...nd.org>,
	Laurent Pinchart <laurent.pinchart@...asonboard.com>,
	Joe Perches <joe@...ches.com>,
	linux-arm-kernel@...ts.infradead.org
Subject: Re: [RFC PATCH] ARM: Use generic BUG() handler

On Tue, Mar 1, 2011 at 2:03 AM, Russell King - ARM Linux
<linux@....linux.org.uk> wrote:
> On Tue, Mar 01, 2011 at 10:12:40AM +0100, Mikael Pettersson wrote:
>> Russell King - ARM Linux writes:
>>  > On Tue, Mar 01, 2011 at 08:49:49AM +0000, Russell King - ARM Linux wrote:
>>  > > On Mon, Feb 28, 2011 at 04:27:43PM -0800, Simon Glass wrote:
>>  > > > +      asm volatile("1:\t.word %c3\n"                          \
>>  > > > +                   ".pushsection __bug_table,\"a\"\n"         \
>>  > > > +                   "2:\t.word 1b, %c0\n"                      \
>>  > > > +                   "\t.hword %c1, 0\n"                        \
>>  > > > +                   "\t.org 2b+%c2\n"                          \
>>  > >
>>  > > %c doesn't work on lots of versions of gcc, which is why we can't use
>>  > > the generic bug support.  There's no way to reliably generate constants
>>  > > without many compiler versions spitting out a '#' before them.
>>  >
>>  > gcc 4.3.2:
>>  >
>>  > asm(".word %c0" : : "i" (0));
>>  >
>>  > produces:
>>  >
>>  >         .word #0
>>  >
>>  > which gas chokes on:
>>  >
>>  > /tmp/cc2hGOHd.s:12: Error: bad expression
>>  > /tmp/cc2hGOHd.s:12: Error: junk at end of line, first unrecognized character is `0'
>>  >
>>  > So what this means is that it's impossible to generate constants in
>>  > assembly with GCC targetting ARM without having them prefixed by '#',
>>  > which in turn makes it impossible to use the generic BUG support.
>>  >
>>  > I reported this bug to gcc folk many years ago.  I've no idea which
>>  > version it has been fixed in or if it's even been fixed.
>>
>> What's the gcc bugzilla bug number?
>
> No idea off hand - I'll have to search for that.
>

It seems I am lucky with the gcc I am using. I would have thought this
would be a pretty fundamental feature, but yes I did notice that %c
wasn't used anywhere.

Would this kernel feature be acceptable as a selectable config option
on ARM then?

Thanks,
Simon
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