linux-kernel - Re: [PATCH RFC] ARM: vexpress: Use assembler mnemonic for WFI in platform_do

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Date:	Fri, 19 Aug 2011 19:15:14 +0100
From:	Jamie Iles <jamie@...ieiles.com>
To:	Nick Bowler <nbowler@...iptictech.com>
Cc:	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
	Russell King <linux@....linux.org.uk>
Subject: Re: [PATCH RFC] ARM: vexpress: Use assembler mnemonic for WFI in
 platform_do_lowpower.

Hi Nick,

On Fri, Aug 19, 2011 at 02:03:24PM -0400, Nick Bowler wrote:
> Current Versatile Express CPU hotplug code includes a hardcoded WFI
> instruction, in ARM encoding.  When the kernel is compiled in Thumb-2
> mode, this is invalid and causes the machine to hang hard when a CPU
> is offlined.
> 
> Using the assembler mnemonic instead of hardcoding it causes the correct
> instruction to be emitted in either case.
> 
> Signed-off-by: Nick Bowler <nbowler@...iptictech.com>
> ---
>  arch/arm/mach-vexpress/hotplug.c |    5 +----
>  1 files changed, 1 insertions(+), 4 deletions(-)
> 
> OK.  I assume that the reason for the hardcoded instruction is that
> older versions of binutils did not support the mnemonic.  Looking at the
> binutils CVS history, support seems to have been added in 2005, and was
> included in the 2.16 release.  If we care about those old assembler
> versions, I can resubmit this patch using ARM/THUMB macros instead,
> which should work just as well.
> 
> I imagine that other platforms which use this same construct, such as
> realview, will have similar problems in thumb mode.  I can't test
> changes on those platforms, unfortunately.
> 
> diff --git a/arch/arm/mach-vexpress/hotplug.c b/arch/arm/mach-vexpress/hotplug.c
> index ea4cbfb..d6be197 100644
> --- a/arch/arm/mach-vexpress/hotplug.c
> +++ b/arch/arm/mach-vexpress/hotplug.c
> @@ -62,10 +62,7 @@ static inline void platform_do_lowpower(unsigned int cpu, int *spurious)
>  	 * code will have already disabled interrupts
>  	 */
>  	for (;;) {
> -		/*
> -		 * here's the WFI
> -		 */
> -		asm(".word	0xe320f003\n"
> +		asm("wfi\n"
>  		    :
>  		    :
>  		    : "memory", "cc");

There's a wfi() macro defined in arch/arm/include/asm/system.h that 
could be used here.

Jamie
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