MIPS: Support for 64-bit FP with O32 binaries

CPUs implementing MIPS32 R2 may include a 64-bit FPU, just as MIPS64 CPUs
do. In order to preserve backwards compatibility a 64-bit FPU will act
like a 32-bit FPU (by accessing doubles from the least significant 32
bits of an even-odd pair of FP registers) when the Status.FR bit is
zero, again just like a mips64 CPU. The standard O32 ABI is defined
expecting a 32-bit FPU, however recent toolchains support use of a
64-bit FPU from an O32 MIPS32 executable. When an ELF executable is
built to use a 64-bit FPU a new flag (EF_MIPS_FP64) is set in the ELF
header.

With this patch the kernel will check the EF_MIPS_FP64 flag when
executing an O32 binary, and set Status.FR accordingly. The addition
of O32 64-bit FP support lessens the opportunity for optimisation in
the FPU emulator, so a CONFIG_MIPS_O32_FP64_SUPPORT Kconfig option is
introduced to allow this support to be disabled for those that don't
require it.

Inspired by an earlier patch by Leonid Yegoshin, but implemented more
cleanly & correctly.

Signed-off-by: Paul Burton <paul.burton@imgtec.com>
Cc: linux-mips@linux-mips.org
Cc: Paul Burton <paul.burton@imgtec.com>
Patchwork: https://patchwork.linux-mips.org/patch/6154/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>

1 files changed, 5 insertions, 5 deletions

diff --git a/arch/mips/math-emu/cp1emu.c b/arch/mips/math-emu/cp1emu.c
index 0e47ae2aa96b..506925b2c3f3 100644
--- a/arch/mips/math-emu/cp1emu.c
+++ b/arch/mips/math-emu/cp1emu.c
@@ -859,20 +859,20 @@ static int isBranchInstr(struct pt_regs *regs, struct mm_decoded_insn dec_insn,
  * In the Linux kernel, we support selection of FPR format on the
  * basis of the Status.FR bit.	If an FPU is not present, the FR bit
  * is hardwired to zero, which would imply a 32-bit FPU even for
- * 64-bit CPUs so we rather look at TIF_32BIT_REGS.
+ * 64-bit CPUs so we rather look at TIF_32BIT_FPREGS.
  * FPU emu is slow and bulky and optimizing this function offers fairly
  * sizeable benefits so we try to be clever and make this function return
  * a constant whenever possible, that is on 64-bit kernels without O32
- * compatibility enabled and on 32-bit kernels.
+ * compatibility enabled and on 32-bit without 64-bit FPU support.
  */
 static inline int cop1_64bit(struct pt_regs *xcp)
 {
 #if defined(CONFIG_64BIT) && !defined(CONFIG_MIPS32_O32)
 	return 1;
-#elif defined(CONFIG_64BIT) && defined(CONFIG_MIPS32_O32)
-	return !test_thread_flag(TIF_32BIT_REGS);
-#else
+#elif defined(CONFIG_32BIT) && !defined(CONFIG_MIPS_O32_FP64_SUPPORT)
 	return 0;
+#else
+	return !test_thread_flag(TIF_32BIT_FPREGS);
 #endif
 }