linux-kernel - Re: [PATCH] MTD: Fix Orion NAND driver compilation with ARM OABI

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Date:	Sat, 20 Mar 2010 13:20:50 +0200
From:	Paulius Zaleckas <paulius.zaleckas@...il.com>
To:	David Woodhouse <dwmw2@...radead.org>
Cc:	nico@...xnic.net, akpm@...ux-foundation.org,
	u.kleine-koenig@...gutronix.de, simon.kagstrom@...insight.net,
	linux-mtd@...ts.infradead.org,
	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] MTD: Fix Orion NAND driver compilation with ARM OABI

On Sat, Mar 20, 2010 at 11:41 AM, David Woodhouse <dwmw2@...radead.org> wrote:
> On Sat, 2010-03-20 at 10:55 +0200, Paulius Zaleckas wrote:
>> We must tell GCC to use even register for variable passed
>> to ldrd instruction. Without this patch GCC 4.2.1 puts this
>> variable to r2/r3 on EABI and r3/r4 on OABI, so force it to
>> r2/r3. This does not change anything when EABI and OABI
>> compilation works OK.
>>
>> Without this patch and with OABI I get:
>> /tmp/ccMkwOCs.s:63: Error: first destination register must be even -- `ldrd r3,[ip]'
>> make[5]: *** [drivers/mtd/nand/orion_nand.o] Error 1
>
>  ...
>> -             uint64_t x;
>> +             /*
>> +              * force x variable to r2/r3 registers since ldrd instruction
>> +              * requires first register to be even.
>> +              */
>> +             register uint64_t x asm ("r2");
>> +
>>               asm volatile ("ldrd\t%0, [%1]" : "=&r" (x) : "r" (io_base));
>
> Hm, isn't there an asm constraint which will force it into an
> appropriate register pair?

Not that I know of...

> Failing that, "=&2,4,6,8" ought to work.

No, fails with error: matching constraint not valid in output operand

> (Um, and why the earlyclobber? Why can't io_base be passed in in one of
> the same registers?)
>
> We should try to avoid making our constraints more restrictive than they
> need to be.
>
> --
> David Woodhouse                            Open Source Technology Centre
> David.Woodhouse@...el.com                              Intel Corporation
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