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

1 files changed, 2 insertions, 10 deletions

diff --git a/arch/mips/kernel/branch.c b/arch/mips/kernel/branch.c
index e48f6c0a9e4a..74f12a91bfb4 100644
--- a/arch/mips/kernel/branch.c
+++ b/arch/mips/kernel/branch.c
@@ -674,16 +674,8 @@ int __compute_return_epc_for_insn(struct pt_regs *regs,
 		if (cpu_has_mips_r6 &&
 		    ((insn.i_format.rs == bc1eqz_op) ||
 		     (insn.i_format.rs == bc1nez_op))) {
-			if (!used_math()) { /* First time FPU user */
-				ret = init_fpu();
-				if (ret && NO_R6EMU) {
-					ret = -ret;
-					break;
-				}
-				ret = 0;
-				set_used_math();
-			}
-			lose_fpu(1);    /* Save FPU state for the emulator. */
+			if (!init_fp_ctx(current))
+				lose_fpu(1);
 			reg = insn.i_format.rt;
 			bit = get_fpr32(&current->thread.fpu.fpr[reg], 0) & 0x1;
 			if (insn.i_format.rs == bc1eqz_op)