MIPS: Support for hybrid FPRs

Hybrid FPRs is a scheme where scalar FP registers are 64b wide, but
accesses to odd indexed single registers use bits 63:32 of the
preceeding even indexed 64b register. In this mode all FP code
except that built for the plain FP64 ABI can execute correctly. Most
notably a combination of FP64A & FP32 code can execute correctly,
allowing for existing FP32 binaries to be linked with new FP64A binaries
that can make use of 64 bit FP & MSA.

Hybrid FPRs are implemented by setting both the FR & FRE bits, trapping
& emulating single precision FP instructions (via Reserved Instruction
exceptions) whilst allowing others to execute natively. It therefore has
a penalty in terms of execution speed, and should only be used when no
fully native mode can be. As more binaries are recompiled to use either
the FPXX or FP64(A) ABIs, the need for hybrid FPRs should diminish.
However in the short to mid term it allows for a gradual transition
towards that world, rather than a complete ABI break which is not
feasible for some users & not desirable for many.

A task will be executed using the hybrid FPR scheme when its
TIF_HYBRID_FPREGS flag is set & TIF_32BIT_FPREGS is clear. A further
patch will set the flags as necessary, this patch simply adds the
infrastructure necessary for the hybrid FPR mode to work.

Signed-off-by: Paul Burton <paul.burton@imgtec.com>
Cc: linux-mips@linux-mips.org
Cc: Alexander Viro <viro@zeniv.linux.org.uk>
Cc: linux-fsdevel@vger.kernel.org
Cc: linux-kernel@vger.kernel.org
Patchwork: https://patchwork.linux-mips.org/patch/7683/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>

1 files changed, 41 insertions, 8 deletions

diff --git a/arch/mips/include/asm/fpu.h b/arch/mips/include/asm/fpu.h
index dd562414cd5..994d2193967 100644
--- a/arch/mips/include/asm/fpu.h
+++ b/arch/mips/include/asm/fpu.h
@@ -36,14 +36,16 @@ extern void _restore_fp(struct task_struct *);
 
 /*
  * This enum specifies a mode in which we want the FPU to operate, for cores
- * which implement the Status.FR bit. Note that FPU_32BIT & FPU_64BIT
- * purposefully have the values 0 & 1 respectively, so that an integer value
- * of Status.FR can be trivially casted to the corresponding enum fpu_mode.
+ * which implement the Status.FR bit. Note that the bottom bit of the value
+ * purposefully matches the desired value of the Status.FR bit.
  */
 enum fpu_mode {
 	FPU_32BIT = 0,		/* FR = 0 */
-	FPU_64BIT,		/* FR = 1 */
+	FPU_64BIT,		/* FR = 1, FRE = 0 */
 	FPU_AS_IS,
+	FPU_HYBRID,		/* FR = 1, FRE = 1 */
+
+#define FPU_FR_MASK		0x1
 };
 
 static inline int __enable_fpu(enum fpu_mode mode)
@@ -57,6 +59,14 @@ static inline int __enable_fpu(enum fpu_mode mode)
 		enable_fpu_hazard();
 		return 0;
 
+	case FPU_HYBRID:
+		if (!cpu_has_fre)
+			return SIGFPE;
+
+		/* set FRE */
+		write_c0_config5(read_c0_config5() | MIPS_CONF5_FRE);
+		goto fr_common;
+
 	case FPU_64BIT:
 #if !(defined(CONFIG_CPU_MIPS32_R2) || defined(CONFIG_64BIT))
 		/* we only have a 32-bit FPU */
@@ -64,8 +74,11 @@ static inline int __enable_fpu(enum fpu_mode mode)
 #endif
 		/* fall through */
 	case FPU_32BIT:
+		/* clear FRE */
+		write_c0_config5(read_c0_config5() & ~MIPS_CONF5_FRE);
+fr_common:
 		/* set CU1 & change FR appropriately */
-		fr = (int)mode;
+		fr = (int)mode & FPU_FR_MASK;
 		change_c0_status(ST0_CU1 | ST0_FR, ST0_CU1 | (fr ? ST0_FR : 0));
 		enable_fpu_hazard();
 
@@ -102,13 +115,17 @@ static inline int __own_fpu(void)
 	enum fpu_mode mode;
 	int ret;
 
-	mode = !test_thread_flag(TIF_32BIT_FPREGS);
+	if (test_thread_flag(TIF_HYBRID_FPREGS))
+		mode = FPU_HYBRID;
+	else
+		mode = !test_thread_flag(TIF_32BIT_FPREGS);
+
 	ret = __enable_fpu(mode);
 	if (ret)
 		return ret;
 
 	KSTK_STATUS(current) |= ST0_CU1;
-	if (mode == FPU_64BIT)
+	if (mode == FPU_64BIT || mode == FPU_HYBRID)
 		KSTK_STATUS(current) |= ST0_FR;
 	else /* mode == FPU_32BIT */
 		KSTK_STATUS(current) &= ~ST0_FR;
@@ -166,8 +183,24 @@ static inline int init_fpu(void)
 
 	if (cpu_has_fpu) {
 		ret = __own_fpu();
-		if (!ret)
+		if (!ret) {
+			unsigned int config5 = read_c0_config5();
+
+			/*
+			 * 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.
+			 */
+			write_c0_config5(config5 & ~MIPS_CONF5_FRE);
+			enable_fpu_hazard();
+
 			_init_fpu();
+
+			/* Restore FRE */
+			write_c0_config5(config5);
+			enable_fpu_hazard();
+		}
 	} else
 		fpu_emulator_init_fpu();