MIPS: Introduce isa-rev.h to define MIPS_ISA_REV

There are multiple instances in the kernel where we need to include or
exclude particular instructions based on the ISA revision of the target
processor. For MIPS32 / MIPS64, the compiler exports a __mips_isa_rev
define. However, when targeting MIPS I - V, this define is absent. This
leads to each use of __mips_isa_rev having to check that it is defined
first. To simplify this, introduce the isa-rev.h header which always
exports MIPS_ISA_REV. The name is changed so as to avoid confusion with
the compiler builtin and to avoid accidentally using the builtin.
MIPS_ISA_REV is defined to the compilers builtin if provided, or 0,
which satisfies all current usages.

Suggested-by: Paul Burton <[e-mail]>
Signed-off-by: Matt Redfearn <[e-mail]>
Reviewed-by: Maciej W. Rozycki <[e-mail]>
Cc: Ralf Baechle <[e-mail]>
Cc: [e-mail]
Patchwork: https://patchwork.linux-mips.org/patch/18676/
Signed-off-by: James Hogan <[e-mail]>

arch/mips/include/asm/isa-rev.h

···
+/* SPDX-License-Identifier: GPL-2.0 */
+/*
+ * Copyright (C) 2018 MIPS Tech, LLC
+ * Author: Matt Redfearn <[e-mail]>
+ */
+
+#ifndef __MIPS_ASM_ISA_REV_H__
+#define __MIPS_ASM_ISA_REV_H__
+
+/*
+ * The ISA revision level. This is 0 for MIPS I to V and N for
+ * MIPS{32,64}rN.
+ */
+
+/* If the compiler has defined __mips_isa_rev, believe it. */
+#ifdef __mips_isa_rev
+#define MIPS_ISA_REV __mips_isa_rev
+#else
+/* The compiler hasn't defined the isa rev so assume it's MIPS I - V (0) */
+#define MIPS_ISA_REV 0
+#endif
+
+
+#endif /* __MIPS_ASM_ISA_REV_H__ */