linux-kernel - Re: [PATCH soc] ARM: use ARM_SINGLE

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Date:	Fri, 22 May 2015 17:36:44 +0200
From:	Stefan Agner <stefan@...er.ch>
To:	Maxime Coquelin <mcoquelin.stm32@...il.com>
Cc:	Arnd Bergmann <arnd@...db.de>,
	linux-arm-kernel@...ts.infradead.org,
	Russell King <linux@....linux.org.uk>, manabian@...il.com,
	linux-kernel@...r.kernel.org,
	Uwe Kleine-König <u.kleine-koenig@...gutronix.de>,
	olof@...om.net
Subject: Re: [PATCH soc] ARM: use ARM_SINGLE_ARMV7M for ARMv7-M platforms

On 2015-05-22 17:29, Maxime Coquelin wrote:
> 2015-05-22 16:50 GMT+02:00 Arnd Bergmann <arnd@...db.de>:
>> [one small request as I have four armv7-m folks on Cc already:
>>  could one of you try to fix the warning that I get with every
>>  single build: "/git/arm-soc/arch/arm/kernel/head-nommu.S: Assembler
>> messages: /git/arm-soc/arch/arm/kernel/head-nommu.S:167: Warning:
>> Use of r13 as a source register is deprecated when r15 is the
>> destination register."]
> 
> Moving r13 to r12 and returning r12 seems to do the job (see below).
> But I don't know if there is a more elegant way, and if it is also
> valid for other architectures than armv7-m.
> I can propose a patch if someone can confirm it is valid.

> 
> -------------------------------------------------------------------------------------------------------------
> 
> diff --git a/arch/arm/kernel/head-nommu.S b/arch/arm/kernel/head-nommu.S
> index aebfbf7..e84bdad 100644
> --- a/arch/arm/kernel/head-nommu.S
> +++ b/arch/arm/kernel/head-nommu.S
> @@ -164,7 +164,8 @@ __after_proc_init:
>  #endif
>         mcr     p15, 0, r0, c1, c0, 0           @ write control reg
>  #endif /* CONFIG_CPU_CP15 */
> -       ret     r13
> +       mov     r12, r13
> +       ret     r12
>  ENDPROC(__after_proc_init)
>         .ltorg

That is actually a patch I have here too, altough I used r11 back
then... :-)

However, I don't think this is a nice solution. We should avoid using
r13 for that address in the first place, e.g. using a different register
to get the value when calling __mmap_switched. However, for that one
need to know what registers are guaranteed to be not used within
PROCINFO_INITFUNC...

--
Stefan


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