arch/arc/kernel/fpu.c

   1 /*
   2  * fpu.c - save/restore of Floating Point Unit Registers on task switch
   3  *
   4  * Copyright (C) 2004, 2007-2010, 2011-2012 Synopsys, Inc. (www.synopsys.com)
   5  *
   6  * This program is free software; you can redistribute it and/or modify
   7  * it under the terms of the GNU General Public License version 2 as
   8  * published by the Free Software Foundation.
   9  */
  10
  11 #include <linux/sched.h>
  12 #include <asm/switch_to.h>
  13
  14 /*
  15  * To save/restore FPU regs, simplest scheme would use LR/SR insns.
  16  * However since SR serializes the pipeline, an alternate "hack" can be used
  17  * which uses the FPU Exchange insn (DEXCL) to r/w FPU regs.
  18  *
  19  * Store to 64bit dpfp1 reg from a pair of core regs:
  20  *   dexcl1 0, r1, r0  ; where r1:r0 is the 64 bit val
  21  *
  22  * Read from dpfp1 into pair of core regs (w/o clobbering dpfp1)
  23  *   mov_s    r3, 0
  24  *   daddh11  r1, r3, r3   ; get "hi" into r1 (dpfp1 unchanged)
  25  *   dexcl1   r0, r1, r3   ; get "low" into r0 (dpfp1 low clobbered)
  26  *   dexcl1    0, r1, r0   ; restore dpfp1 to orig value
  27  *
  28  * However we can tweak the read, so that read-out of outgoing task's FPU regs
  29  * and write of incoming task's regs happen in one shot. So all the work is
  30  * done before context switch
  31  */
  32
  33 void fpu_save_restore(struct task_struct *prev, struct task_struct *next)
  34 {
  35         unsigned int *saveto = &prev->thread.fpu.aux_dpfp[0].l;
  36         unsigned int *readfrom = &next->thread.fpu.aux_dpfp[0].l;
  37
  38         const unsigned int zero = 0;
  39
  40         __asm__ __volatile__(
  41                 "daddh11  %0, %2, %2\n"
  42                 "dexcl1   %1, %3, %4\n"
  43                 : "=&r" (*(saveto + 1)), /* early clobber must here */
  44                   "=&r" (*(saveto))
  45                 : "r" (zero), "r" (*(readfrom + 1)), "r" (*(readfrom))
  46         );
  47
  48         __asm__ __volatile__(
  49                 "daddh22  %0, %2, %2\n"
  50                 "dexcl2   %1, %3, %4\n"
  51                 : "=&r"(*(saveto + 3)), /* early clobber must here */
  52                   "=&r"(*(saveto + 2))
  53                 : "r" (zero), "r" (*(readfrom + 3)), "r" (*(readfrom + 2))
  54         );
  55 }