RE: [PATCH 01/10] MIPS: Add core files for MIPS SEAD-3 development platform.

["Maciej W. Rozycki" <macro@linux-mips.org>]

On Mon, 7 May 2012, Hill, Steven wrote:

> I will certainly remove CONFIG_CPU_HAS_LLSC, thank you. I attempted to 
> enable 'cpu_has_clo_clz' for SEAD-3, but it breaks my microMIPS-only 
> kernel builds. Specifically, since microMIPS LL/SC instructions do not 
> have 16-bit address offsets, in the '__cmpxchg_asm' macro function I get 
> constraint errors because then the assembler has to use the %LO register 
> in order to calculate the offset address. I am going to hold off on 
> enabling the option until after the 3.5 release and then revisit for a 
> solution. Thank you.

 It's OK to ask. :)  Try using the "YC" constraint, this piece:

int test(void *p)
{
        int i;

        __asm__ __volatile__("ll\t%0,%1" : "=r" (i) : "YC" (*p));

        return i;
}

assembles to:

        ll      $2,0($4)
        jrc     $31

for me (with "-O2 -mmicromips").

 You may have to factor in backwards compatibility -- while "YC" is 
supported both in the microMIPS and the standard MIPS mode, switching the 
width of the immediate offset between 12 and 16 bits as appropriate, it's 
certainly a new constraint older compilers did not support.  I suggest 
using a macro, as usually, you can use some existing examples as a 
reference, e.g. <asm/compiler.h> (you know where to place this stuff now 
too :) ).

  Maciej