[PATCH 2/6] ARM: gic: fix big endian support

[Nicolas Pitre]

On Tue, 15 Nov 2011, Junxiao Bi wrote:

> The irq state from gic is in little-endian mode. We need do endian
> conversion for it in big-endian machine. Usually byte swapping of
> 32-bits word needs 4 assembler instructions for arm machine which
> arch < 6. But since the high order half word of the irq state is
> zero, there is a way that can use lesser instrunctions to improve
> the performance since we only need a bytes swaping of half word.
> That's what swab16_low_half and swab16_high_half do.
> 
> Signed-off-by: Junxiao Bi <junxiao.bi at windriver.com>

Comments below.

> ---
>  arch/arm/include/asm/assembler.h                |   27 +++++++++++++++++++++++
>  arch/arm/include/asm/hardware/entry-macro-gic.S |    7 ++++++
>  2 files changed, 34 insertions(+), 0 deletions(-)
> 
> diff --git a/arch/arm/include/asm/assembler.h b/arch/arm/include/asm/assembler.h
> index 29035e8..3ddee22 100644
> --- a/arch/arm/include/asm/assembler.h
> +++ b/arch/arm/include/asm/assembler.h
> @@ -302,4 +302,31 @@
>  	.size \name , . - \name
>  	.endm
>  
> +/*
> + * swab16_low_half is used to swap the bytes order of the low order
> + * half word, it assumes that the high order half word is zero.
> + * swab16_high_half is used to swap the bytes order of the high order
> + * half word, it assumes that the low order half word is zero.
> + */
> +#if __LINUX_ARM_ARCH__ >= 6
> +	.macro swab16_low_half, reg
> +	rev \reg, \reg
> +	.endm
> +
> +	.macro swab16_high_half, reg
> +	rev \reg, \reg
> +	.endm
> +#else
> +	.macro swab16_low_half, reg
> +	mov \reg, \reg, ror #8
> +	orr \reg, \reg, \reg, lsr #16
> +	bic \reg, \reg, \reg, lsl #16
> +	.endm

I fail to see how this macro would be generally useful.  Presuming that 
the top or bottom half has to be zero is not particularly practical.  

> +	.macro swab16_high_half, reg
> +	swab16_low_half \reg
> +	mov \reg, \reg, lsl #16
> +	.endm

And this doesn't end up cheaper than a plain swab32.

Furthermore, you're using those macros 1) in the gic code, and 2) onli 
in the BE8 case.  Both of those conditions are only possible on ARMv6 
and above anyway, so I'd simply open code a rev instruction inline.


Nicolas