[RFT/RFC PATCH 3/6] ARM: add macro to perform far branches (b/bl)

[Ard Biesheuvel]

On 12 March 2015 at 21:32, Nicolas Pitre <nicolas.pitre at linaro.org> wrote:
> On Thu, 12 Mar 2015, Ard Biesheuvel wrote:
>
>> These macros execute PC-relative branches, but with a larger
>> reach than the 24 bits that are available in the b and bl opcodes.
>>
>> Signed-off-by: Ard Biesheuvel <ard.biesheuvel at linaro.org>
>> ---
>>  arch/arm/include/asm/assembler.h | 29 +++++++++++++++++++++++++++++
>>  1 file changed, 29 insertions(+)
>>
>> diff --git a/arch/arm/include/asm/assembler.h b/arch/arm/include/asm/assembler.h
>> index f67fd3afebdf..bd08c3c1b73f 100644
>> --- a/arch/arm/include/asm/assembler.h
>> +++ b/arch/arm/include/asm/assembler.h
>> @@ -108,6 +108,35 @@
>>       .endm
>>  #endif
>>
>> +     /*
>> +      * Macros to emit relative branches that may exceed the range
>> +      * of the 24-bit immediate of the ordinary b/bl instructions.
>> +      * NOTE: this doesn't work with locally defined symbols, as they
>> +      * might lack the ARM/Thumb annotation (even if they are annotated
>> +      * as functions)
>
> I really hope you won't need a far call with local symbols ever!
>

Well, if you use pushsection/popsection, then local, numbered labels
you refer to can be quite far away in the output image, and those will
not have the thumb bit set.

>> +      */
>> +     .macro  b_far, target, tmpreg
>> +#if defined(CONFIG_CPU_32v7) || defined(CONFIG_CPU_32v7M)
>> + ARM(        movt    \tmpreg, #:upper16:(\target - (8888f + 8))      )
>> + ARM(        movw    \tmpreg, #:lower16:(\target - (8888f + 8))      )
>> + THUMB(      movt    \tmpreg, #:upper16:(\target - (8888f + 4))      )
>> + THUMB(      movw    \tmpreg, #:lower16:(\target - (8888f + 4))      )
>> +8888:        add     pc, pc, \tmpreg
>> +#else
>> +     ldr     \tmpreg, 8889f
>> +8888:        add     pc, pc, \tmpreg
>> +     .align  2
>> +8889:
>> + ARM(        .word   \target - (8888b + 8)           )
>
> The Thumb relocation value is missing here.
>

Yes, this is bogus. But Thumb2 implies v7 or v7m, so it is not
actually incorrect in this case.
But I will fix it in the next version

>> +#endif
>> +     .endm
>> +
>> +     .macro  bl_far, target, tmpreg=ip
>> +     adr     lr, 8887f

BTW just realised this needs a BSYM()

>> +     b_far   \target, \tmpreg
>> +8887:
>> +     .endm
>> +
>>       .macro asm_trace_hardirqs_off
>>  #if defined(CONFIG_TRACE_IRQFLAGS)
>>       stmdb   sp!, {r0-r3, ip, lr}
>> --
>> 1.8.3.2
>>
>>