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

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Message-ID: <512E4954.6060601@linaro.org>
Date:	Wed, 27 Feb 2013 10:58:44 -0700
From:	Mathieu Poirier <mathieu.poirier@...aro.org>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
CC:	Dave Airlie <airlied@...il.com>,
	Dmitry Torokhov <dmitry.torokhov@...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 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.

Acked-by: Mathieu Poirier <mathieu.poirier@...aro.org>
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