If you read the manual, how can you have missed the _naked keyword?

unsigned char myfunc(unsigned char x) _naked
{
  _asm
  mov a,dpl         ; pass the parameter
  lcall MY_FUNCTION ; better use call if you want to come back
  mov dpl,a         ; put my return value where SDCC expects it
  ret               ; must insert ret for naked functions too
  _endasm;
}