MIPS: Simplify FP context initialization

MIPS has up until now had 3 different ways for a task's floating point
context to be initialized:

  - If the task's first use of FP involves it gaining ownership of an
    FPU then _init_fpu() is used to initialize the FPU's registers such
    that they all contain ~0, and the FPU registers will be stored to
    struct thread_info later (eg. when context switching).

  - If the task first uses FP on a CPU without an associated FPU then
    fpu_emulator_init_fpu() initializes the task's floating point
    register state in struct thread_info such that all floating point
    register contain the bit pattern 0x7ff800007ff80000, different to
    the _init_fpu() behaviour.

  - If a task's floating point context is first accessed via ptrace then
    init_fp_ctx() initializes the floating point register state in
    struct thread_info to ~0, giving equivalent state to _init_fpu().

The _init_fpu() path has 2 separate implementations - one for r2k/r3k
style systems & one for r4k style systems. The _init_fpu() path also
requires that we be careful to clear & restore the value of the
Config5.FRE bit on modern systems in order to avoid inadvertently
triggering floating point exceptions.

None of this code is in a performance critical hot path - it runs only
the first time a task uses floating point. As such it doesn't seem to
warrant the complications of maintaining the _init_fpu() path.

Remove _init_fpu() & fpu_emulator_init_fpu(), instead using
init_fp_ctx() consistently to initialize floating point register state
in struct thread_info. Upon a task's first use of floating point this
will typically mean that we initialize state in memory & then load it
into FPU registers using _restore_fp() just as we would on a context
switch. For other paths such as __compute_return_epc_for_insn() or
mipsr2_decoder() this results in a significant simplification of the
work to be done.

Signed-off-by: Paul Burton <paul.burton@mips.com>
Patchwork: https://patchwork.linux-mips.org/patch/21002/
Cc: linux-mips@linux-mips.org

arch/mips/include/asm/fpu.h

@@ -33,7 +33,6 @@
 struct sigcontext;
 struct sigcontext32;
 
-extern void _init_fpu(unsigned int);
 extern void _save_fp(struct task_struct *);
 extern void _restore_fp(struct task_struct *);
 
@@ -198,42 +197,36 @@ static inline void lose_fpu(int save)
 	preempt_enable();
 }
 
-static inline int init_fpu(void)
+/**
+ * init_fp_ctx() - Initialize task FP context
+ * @target: The task whose FP context should be initialized.
+ *
+ * Initializes the FP context of the target task to sane default values if that
+ * target task does not already have valid FP context. Once the context has
+ * been initialized, the task will be marked as having used FP & thus having
+ * valid FP context.
+ *
+ * Returns: true if context is initialized, else false.
+ */
+static inline bool init_fp_ctx(struct task_struct *target)
 {
-	unsigned int fcr31 = current->thread.fpu.fcr31;
-	int ret = 0;
+	/* If FP has been used then the target already has context */
+	if (tsk_used_math(target))
+		return false;
 
-	if (cpu_has_fpu) {
-		unsigned int config5;
+	/* Begin with data registers set to all 1s... */
+	memset(&target->thread.fpu.fpr, ~0, sizeof(target->thread.fpu.fpr));
 
-		ret = __own_fpu();
-		if (ret)
-			return ret;
+	/* FCSR has been preset by `mips_set_personality_nan'.  */
 
-		if (!cpu_has_fre) {
-			_init_fpu(fcr31);
+	/*
+	 * Record that the target has "used" math, such that the context
+	 * just initialised, and any modifications made by the caller,
+	 * aren't discarded.
+	 */
+	set_stopped_child_used_math(target);
 
-			return 0;
-		}
-
-		/*
-		 * Ensure FRE is clear whilst running _init_fpu, since
-		 * single precision FP instructions are used. If FRE
-		 * was set then we'll just end up initialising all 32
-		 * 64b registers.
-		 */
-		config5 = clear_c0_config5(MIPS_CONF5_FRE);
-		enable_fpu_hazard();
-
-		_init_fpu(fcr31);
-
-		/* Restore FRE */
-		write_c0_config5(config5);
-		enable_fpu_hazard();
-	} else
-		fpu_emulator_init_fpu();
-
-	return ret;
+	return true;
 }
 
 static inline void save_fp(struct task_struct *tsk)

arch/mips/include/asm/fpu_emulator.h

@@ -188,17 +188,6 @@ int isBranchInstr(struct pt_regs *regs, struct mm_decoded_insn dec_insn,
 int mm_isBranchInstr(struct pt_regs *regs, struct mm_decoded_insn dec_insn,
 		     unsigned long *contpc);
 
-#define SIGNALLING_NAN 0x7ff800007ff80000LL
-
-static inline void fpu_emulator_init_fpu(void)
-{
-	struct task_struct *t = current;
-	int i;
-
-	for (i = 0; i < 32; i++)
-		set_fpr64(&t->thread.fpu.fpr[i], 0, SIGNALLING_NAN);
-}
-
 /*
  * Mask the FCSR Cause bits according to the Enable bits, observing
  * that Unimplemented is always enabled.