linux-kernel - Re: [RFC PATCH V2] ARM: Use generic BUG() handler

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Date:	Thu, 10 Mar 2011 14:19:20 -0800
From:	Simon Glass <sjg@...omium.org>
To:	Dave Martin <dave.martin@...aro.org>
Cc:	linux-arm-kernel@...ts.infradead.org,
	Russell King <linux@....linux.org.uk>,
	Tony Lindgren <tony@...mide.com>,
	Nicolas Pitre <nicolas.pitre@...aro.org>,
	Catalin Marinas <catalin.marinas@....com>,
	Joe Perches <joe@...ches.com>,
	Laurent Pinchart <laurent.pinchart@...asonboard.com>,
	Alexander Shishkin <virtuoso@...nd.org>,
	Phil Carmody <ext-phil.2.carmody@...ia.com>,
	Rabin Vincent <rabin@....in>, linux-kernel@...r.kernel.org,
	Mikael Pettersson <mikpe@...uu.se>
Subject: Re: [RFC PATCH V2] ARM: Use generic BUG() handler

On Wed, Mar 9, 2011 at 1:51 AM, Dave Martin <dave.martin@...aro.org> wrote:
> On Wed, Mar 9, 2011 at 2:55 AM, Simon Glass <sjg@...omium.org> wrote:
>> +#ifdef CONFIG_THUMB2_KERNEL
>> +#define BUG_INSTR_VALUE 0xf7f2
>
> This isn't an undefined instruction... it's actually half of some
> 32-bit Thumb instruction. Did you mean 0xdexx instead?

Yes, will fix.

>> +#define BUG_INSTR_TYPE ".hword "
>> +#else
>> +#define BUG_INSTR_VALUE 0xe7f001f2
>> +#define BUG_INSTR_TYPE ".word "
>> +#endif
>> +
>
> This works, but do we really need to split the definition of the bug
> instruction into two macros?

This is so that I can test for the correct instruction word in valid_bug_addr().

>
> Unless you can see a use for calling _BUG() directly with a different
> instruction value, of course...
>

>> +       /*
>> +       * .exit.text is discarded at runtime, not link time, to deal with
>> +       *  references from bug_table
>> +       */
>> +       .exit.text : AT(ADDR(.exit.text)) {
>> +               EXIT_TEXT
>> +       }
>> +
>> +       .exit.data : AT(ADDR(.exit.data)) {
>> +               EXIT_DATA
>> +       }
>
> This is the same unfortunate hack we have to do for other fixups,
> because there's no way to separate out some of these entries in such a
> way that they can be discarded at link time...  I did have an idea for
> an extention to gas which would allow fixups and similar data to be
> emitted in separate ancillary sections which the linker can handle
> sensibly, so that bug entries referencing .exit.text (for example) can
> be discarded at link time.
>
> See http://sourceware.org/ml/binutils/2011-02/msg00004.html
>
> I haven't received any support for the idea so far though...

Well yes, and some might even argue that calling BUG() in __exit code
is not a great idea. Here we are adding in all exit data again which
is about 8.5KB in my kernel. These two pages are then reclaimed at
runtime.

However, at present I don't see any alternative to this, and this is
what is done on other architectures to support BUG().

>
> Cheers
> ---Dave
>

I will tidy this up, test it properly on Thumb and submit a patch.

Regards,
Simon
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