powerpc: Make flush_fp_to_thread() nop when CONFIG_PPC_FPU is disabled

In the current kernel, the function flush_fp_to_thread() is not
dependent on CONFIG_PPC_FPU. So most invocations of this function
is not wrapped by CONFIG_PPC_FPU. Even through we don't really
save the FPRs to the thread struct if CONFIG_PPC_FPU is not enabled,
but there does have some runtime overhead such as the check for
tsk->thread.regs and preempt disable and enable. It really make
no sense to do that. So make it a nop when CONFIG_PPC_FPU is
disabled. Also remove the wrapped #ifdef CONFIG_PPC_FPU
when invoking this function.

Signed-off-by: Kevin Hao <haokexin@gmail.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>

arch/powerpc/include/asm/switch_to.h

@@ -19,7 +19,6 @@ extern struct task_struct *_switch(struct thread_struct *prev,
 extern void giveup_fpu(struct task_struct *);
 extern void load_up_fpu(void);
 extern void enable_kernel_fp(void);
-extern void flush_fp_to_thread(struct task_struct *);
 extern void enable_kernel_altivec(void);
 extern void load_up_altivec(struct task_struct *);
 extern int emulate_altivec(struct pt_regs *);
@@ -37,6 +36,12 @@ static inline void discard_lazy_cpu_state(void)
 }
 #endif
 
+#ifdef CONFIG_PPC_FPU
+extern void flush_fp_to_thread(struct task_struct *);
+#else
+static inline void flush_fp_to_thread(struct task_struct *t) { }
+#endif
+
 #ifdef CONFIG_ALTIVEC
 extern void flush_altivec_to_thread(struct task_struct *);
 extern void giveup_altivec(struct task_struct *);

arch/powerpc/kernel/process.c

@@ -74,6 +74,7 @@ struct task_struct *last_task_used_vsx = NULL;
 struct task_struct *last_task_used_spe = NULL;
 #endif
 
+#ifdef CONFIG_PPC_FPU
 /*
  * Make sure the floating-point register state in the
  * the thread_struct is up to date for task tsk.
@@ -107,6 +108,7 @@ void flush_fp_to_thread(struct task_struct *tsk)
 	}
 }
 EXPORT_SYMBOL_GPL(flush_fp_to_thread);
+#endif
 
 void enable_kernel_fp(void)
 {

arch/powerpc/math-emu/math.c

@@ -7,6 +7,7 @@
 
 #include <asm/uaccess.h>
 #include <asm/reg.h>
+#include <asm/switch_to.h>
 
 #include <asm/sfp-machine.h>
 #include <math-emu/double.h>
@@ -433,9 +434,7 @@ do_mathemu(struct pt_regs *regs)
 	 * is flushed into the thread_struct before attempting
 	 * emulation
 	 */
-#ifdef CONFIG_PPC_FPU
 	flush_fp_to_thread(current);
-#endif
 
 	eflag = func(op0, op1, op2, op3);