From mboxrd@z Thu Jan  1 00:00:00 1970
From: Graeme Russ <graeme.russ@gmail.com>
Date: Mon, 15 Nov 2010 06:34:37 +1100
Subject: [U-Boot] RFC: Aligning arch initialisation sequences
In-Reply-To: <20101114192331.62ECA134F54@gemini.denx.de>
References: <4CD67A22.9040802@gmail.com>
	<201011091835.38581.vapier@gentoo.org>	<4CDE1107.80108@gmail.com>
	<4CDE4A53.4070106@free.fr>	<4CDE741C.1020507@gmail.com>
	<4CDF6F2E.9040900@gmail.com>	<4CDF7821.4080300@gmail.com>
	<4CDFA8E9.3050803@free.fr>	<20101114103001.7EE2014EA7E@gemini.denx.de>	<4CDFD1AE.1070409@free.fr>	<20101114150102.DAAF914EA7E@gemini.denx.de>	<4CE0221A.7030502@free.fr>
	<20101114192331.62ECA134F54@gemini.denx.de>
Message-ID: <4CE039CD.90706@gmail.com>
List-Id: <u-boot.lists.denx.de>
MIME-Version: 1.0
Content-Type: text/plain; charset="us-ascii"
Content-Transfer-Encoding: 7bit
To: u-boot@lists.denx.de

On 15/11/10 06:23, Wolfgang Denk wrote:
> Dear Albert ARIBAUD,
> 
> In message <4CE0221A.7030502@free.fr> you wrote:
>>
>> Alright, then I think we should document how we comply, or do not
>> comply, with GNU EABI / AAPCS (maybe a README.arm that people could read
>> up) -- and I think if there is a way to access GD both before and after
>> relocation without making a register unavailable to the whole u-boot
>> code, then we should use it.
> 
> By the way - it should be not difficult to use a normal extern pointer
> to reference the global data; see
> "arch/powerpc/include/asm/global_data.h":
> 
> 194 #if 1
> 195 #define DECLARE_GLOBAL_DATA_PTR     register volatile gd_t *gd asm ("r2")
> 196 #else /* We could use plain global data, but the resulting code is bigger */
> 197 #define XTRN_DECLARE_GLOBAL_DATA_PTR    extern
> 198 #define DECLARE_GLOBAL_DATA_PTR     XTRN_DECLARE_GLOBAL_DATA_PTR \
> 199                                     gd_t *gd
> 200 #endif

I think you will find this peculiar to PowerPC

What you are talking about is exactly how x86 defines gd, but for x86, gd
is not accessible until after relocation

> 
> When I implemented this code I tested both versions. There is not
> much of a difference, except that the register based version results
> in smaller code.
> 

Probably due to one less register load from memory for each gd access

Regards,

Graeme