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

[junxiao.bi@windriver.com (Bi Junxiao)]

on 11/22/2011 12:10 PM Nicolas Pitre wrote:
> On Tue, 22 Nov 2011, Bi Junxiao wrote:
>
>    
>> on 11/22/2011 03:11 AM Nicolas Pitre wrote:
>>      
>>> 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@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.
>>>
>>>        
>> We can use it to swap bytes order of a 16-bits number in 32-bits register in
>> which case the top or bottom half will be zero. For the \irqstat register, as
>> the original implementation assumes its top half is zero, so I make the same
>> assumption.
>>      
> The problem is that you're loading some assumption about a particular
> register onto a generic macro.  Unless this is a very common pattern, I
> don't think this needs to be made generic.
>
>    
>>>> +	.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.
>>>
>>>        
>> This macro has an advantage that it only needs one register. For swab32, it
>> will need another temporary register besides the target register.
>>      
> Sure, but I don't think we're so short of registers.  And BE on
> pre-ARMv6 is not that common either.  So, unless this particular pattern
> is heavily used on pre-ARMv6 BE systems, I don't think it is reasonable
> to create a generic macro with such specific assumptions.
>
>    
>>> 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.
>>>
>>>        
>> This is not be8 specific. It will be needed for both be8 and be32. So the
>> machine with arch<  6 also needs to be supported.
>>      
> There is no gic on machines with arch<  6, no?
>
> If you meant for those macros to be used for other purposes then they
> deserve a patch of their own.
>    
I didn't find other need of the macro yet. I will leave it aside.
> And Russell just merged in his devel branch a patch from Marc Zyngier
> completely removing entry-macro-gic.S anyway, so this patch as it is
> will be moot soon.
>
>    
OK, thank you for reviewing it. I will drop this patch.
> Nicolas
>
>