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

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Date:	Sat, 20 Mar 2010 09:41:23 +0000
From:	David Woodhouse <dwmw2@...radead.org>
To:	Paulius Zaleckas <paulius.zaleckas@...il.com>
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, 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? Failing that, "=&2,4,6,8" ought to work.
(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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