alignment handler instruction endian-ness

[Ben Dooks]

On 19/07/13 13:32, Russell King - ARM Linux wrote:
> On Fri, Jul 19, 2013 at 01:15:33PM +0100, Ben Dooks wrote:
>> On 19/07/13 12:09, Russell King - ARM Linux wrote:
>>> On Fri, Jul 19, 2013 at 11:58:45AM +0100, Ben Dooks wrote:
>>>> I ran in to an issue with the alignment handler when running BE8 where
>>>> it loads instructions and fails to swap.
>>>>
>>>> Is there a better way of swapping instructions for ARM when loading
>>>> from arbitrary places? Have I missed any other places this could happen?
>>>
>>> Maybe we need a macro which deals with this automatically?
>>>
>>> 	arm_instr_to_cpu(x)
>>> 	thumb_instr_to_cpu(x)
>>>
>>> These should probably make use of the swab*() macros when an endian swap
>>> is needed.
>>>
>>>> The following patch is my first attempt at solving the problem for the
>>>> alignment handler:
>>>
>>> I'd suggest checking this with sparse too - you have some type errors here.
>>
>> How about:
>>
>> diff --git a/arch/arm/include/uapi/asm/byteorder.h
>> b/arch/arm/include/uapi/asm/b
>> index 7737974..9e63a00 100644
>> --- a/arch/arm/include/uapi/asm/byteorder.h
>> +++ b/arch/arm/include/uapi/asm/byteorder.h
>> @@ -21,5 +21,13 @@
>>   #include<linux/byteorder/little_endian.h>
>>   #endif
>>
>>
>> +#ifdef CONFIG_CPU_ENDIAN_BE8
>> +#define cpu_to_arm_instr(__instr)      swab32(__instr)
>> +#define cpu_to_thum_instr(__instr)     swab16(__instr)
>> +#else
>> +#define cpu_to_arm_instr(__instr)      (__instr)
>> +#define cpu_to_thum_instr(__instr)     (__instr)
>> +#endif
>
> ARGH.
>
> First point: does userspace have any knowledge how the kernel is
> configured?  Do they include the kernel configuration header file?  No.
> Therefore the use of CONFIG_* in UAPI header files is wrong.
>
> Second point: does userspace need these macros and should the kernel
> provide them to userspace?  No on both counts.  Therefore, this must
> not be placed in UAPI.
>
> UAPI was created explicitly to separate the kernel private definitions
> from the parts of the kernel interface which the user API depends on.
>
> If you value your soul, NEVER EVER EVER place stuff which is not part
> of the user API in a header file in a UAPI subdirectory.  And never
> modify a header file in a UAPI subdirectory without first having a
> good long think about how that modification may impact _userspace_.

Sorry, searched for byteorder.h and just did not notice.

Where is the best place for these to go?

> Third point: the name is totally and utterly wrong.  I think you've
> misunderstood the naming conventions of the cpu_to_xxx() and xxx_to_cpu()
> macros.  "cpu" is the CPUs native data endianness - the endianness that
> you can do standard arithmetic on and you get the right answer.  "xxx"
> is the "foreign" endianness.  In this case, it's the instructions which
> are not the CPUs native data endianness, so they are the foreign state -
> they are the "xxx".
>
> So, I've no idea why you decided to reverse the name of the macro I gave
> you...  I am astounded at this.

I meant to put both versions in as I thought they'd be needed/.

-- 
Ben Dooks				http://www.codethink.co.uk/
Senior Engineer				Codethink - Providing Genius