linux-kernel - Re: [PATCH v4] drivers/tty: Folding Android's keyreset driver in sysRQ

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Date:	Wed, 27 Feb 2013 10:49:53 -0800
From:	Dmitry Torokhov <dmitry.torokhov@...il.com>
To:	Mathieu Poirier <mathieu.poirier@...aro.org>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Dave Airlie <airlied@...il.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	arve@...roid.com, kernel-team@...roid.com,
	John Stultz <john.stultz@...aro.org>,
	Alan Cox <alan@...rguk.ukuu.org.uk>
Subject: Re: [PATCH v4] drivers/tty: Folding Android's keyreset driver in
 sysRQ

On Wed, Feb 27, 2013 at 10:58:44AM -0700, Mathieu Poirier wrote:
> On 13-02-27 09:57 AM, Linus Torvalds wrote:
> > On Tue, Feb 26, 2013 at 11:33 PM, Dave Airlie <airlied@...il.com> wrote:
> >>
> >> It looks to me like the weak bit isn't working so well
> >>
> >>         if (platform_sysrq_reset_seq) {
> >>                 for (i = 0; i < ARRAY_SIZE(sysrq_reset_seq); i++) {
> >>                         key = platform_sysrq_reset_seq[i];
> >>   6d:   66 8b 8c 00 00 00 00    mov    0x0(%eax,%eax,1),%cx
> >>   74:   00
> >>
> >> is around where it craps out.
> >> gcc version 4.7.2 20121109 (Red Hat 4.7.2-8) (GCC)
> >> Fedora 18 machine.
> > 
> > Hmm. I would love to blame gcc, but no, I think the code is crap.
> > 
> > The whole 'platform_sysrq_reset_seq[]' thing is broken in current git,
> > and it apparently only happens to work by mistake for most of us.
> > 
> > Doing a "grep" for it shows all three uses:
> > 
> >    git grep platform_sysrq_reset_seq
> > 
> >   extern unsigned short platform_sysrq_reset_seq[] __weak;
> >   if (platform_sysrq_reset_seq) {
> >             key = platform_sysrq_reset_seq[i];
> > 
> > and the thing is, if it is declared as an array (not a pointer), then
> > I think it is perfectly understandable that when then testing the
> > *address* of that array, gcc just says "you're stupid, you're testing
> > something that cannot possibly be NULL, so I'll throw your idiotic
> > test away".
> > 
> > And gcc would be completely correct. That test is moronic. You just
> > said that platform_sysrq_reset_seq[] was an external array, there is
> > no way in hell that is NULL.
> > 
> > Now, if it was a _pointer_, that would be a different thing entirely.
> > A pointer can have a NULL value. A named array, not so much.
> > 
> > So I *think* the fix might be something like the attached. Totally
> > untested. It may compile, or it may not.
> > 
> >                 Linus
> > 
> 
> Your fix is compiling, running and yielding the correct results -
> apologies about that.
> 

Actually defining platform_sysrq_reset_seq as __weak array was my doing
so the fault is actually mine. Mathieu had a function there originally.

Thanks.

-- 
Dmitry
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