ABSTRACT: broken contents in FPU registers while switching context.
 REFERENCE_BOARD: almost mips using FPU.
     LSP_VERSION: both MVL-2.1 and 3.0 PE for nec-ddb-vr5xxxa-mips_fp_le.
REPORTED_CUSTMER: Sony SCE.
     DESCRIPTION: It might be broken any values which are in FPU register 
                  while it's running any process with enabled Preemption kernel.
   CONSIDERATION: It might be caused this issue when any process are swiched their context 
                  in handler 'do_cpu() (arch/mips/kernel/traps.c)'
                  which allows to interrupt when exception for FPU co-processoser is caused.
     REPRODUCING: 
     1. building program.
	 > cc -O -mips2 -o fputest fputest.c
     2. run script to reproduce on kernel with 'CONFIG_PREEMPT' was enabled.
	 > sh ./run.sh
	 3. Appears the warning messages as follows.
	 a: nan, b: nan
     a: 1.895935, b: 0.733478

I tried both lsps for MVL-2.1 and 3.0 PE for nec-ddb-vr5xxxa-mips_fp_le then it's caused this issue on both lsps.
The following returns are in case of MVL-3.0.
root@192.168.0.11:~# sh ./run.sh 
a: nan, b: nan
a: nan, b: nan
a: nan, b: nan
a: nan, b: nan
a: nan, b: nan
a: nan, b: nan
a: nan, b: nan
a: nan, b: 0.086879
a: nan, b: 2.482951
a: nan, b: 0.364709
a: nan, b: 0.364709
a: nan, b: 1.523164
a: nan, b: 1.523164
a: nan, b: 0.695692
a: 0.033763, b: 2.958411
a: nan, b: 1.523164
a: nan, b: 0.223309
a: nan, b: 1428800553.000000
a: nan, b: 12.789888
a: 0.486427, b: 0.033763
a: 1.713750, b: 7.251947
a: 1.491375, b: 0.982025
a: 0.769617, b: 1.689032
a: nan, b: 1.557239
a: nan, b: 1.557239
a: nan, b: 0.109266
a: 7.251947, b: 0.231507
a: 0.982025, b: 0.917356
a: nan, b: nan
a: nan, b: nan
a: nan, b: nan
a: nan, b: nan
a: nan, b: nan
a: nan, b: nan
a: nan, b: nan
a: nan, b: nan
a: nan, b: nan
a: nan, b: nan
a: nan, b: 0.681027
a: nan, b: 0.906195
a: nan, b: 213801961.000000

<!-- fputest.c
#include <stdio.h>
#include <stdlib.h>
#include <time.h>

#define N       (1024 * 1024)

void test(double, double);

struct timespec wait;

int main(int argc, char *argv[]) {
     int      i;
     double   d;

     wait.tv_sec  = 0;
     wait.tv_nsec = 1;   /* 1ns */

     srandom(0);
     for (;;) {
         d = (double)random() / (double)random();
           test(d, d);
           }
           return 0;
}

void test(double a, double b) {
      nanosleep(&wait, NULL);
      if (a != b) {
         printf("a: %f, b: %f\n", a, b);
                    exit(1);
                    }
}
-->
<!-- run.sh
#!/bin/sh
i=0
while [ $i -lt 360 ]
do 
  ./fputest &
  i=`expr $i + 1`
done
-->
---
(TAKESHI - taihana@mvista.com)