riscv: errata: Fix the PAUSE Opcode for MIPS P8700

Add ERRATA_MIPS and ERRATA_MIPS_P8700_PAUSE_OPCODE configs.
Handle errata for the MIPS PAUSE instruction.

Signed-off-by: Djordje Todorovic <djordje.todorovic@htecgroup.com>
Signed-off-by: Aleksandar Rikalo <arikalo@gmail.com>
Signed-off-by: Raj Vishwanathan4 <rvishwanathan@mips.com>
Signed-off-by: Aleksa Paunovic <aleksa.paunovic@htecgroup.com>
Reviewed-by: Alexandre Ghiti <alexghiti@rivosinc.com>
Link: https://lore.kernel.org/r/20250724-p8700-pause-v5-7-a6cbbe1c3412@htecgroup.com
[pjw@kernel.org: updated to apply and compile; fixed a checkpatch issue]
Signed-off-by: Paul Walmsley <pjw@kernel.org>

arch/riscv/Kconfig.errata

@@ -21,6 +21,29 @@ config ERRATA_ANDES_CMO
 
 	  If you don't know what to do here, say "Y".
 
+config ERRATA_MIPS
+	bool "MIPS errata"
+	depends on RISCV_ALTERNATIVE
+	help
+	  All MIPS errata Kconfig depend on this Kconfig. Disabling
+	  this Kconfig will disable all MIPS errata. Please say "Y"
+	  here if your platform uses MIPS CPU cores.
+
+	  Otherwise, please say "N" here to avoid unnecessary overhead.
+
+config ERRATA_MIPS_P8700_PAUSE_OPCODE
+	bool "Fix the PAUSE Opcode for MIPS P8700"
+	depends on ERRATA_MIPS && 64BIT
+	default n
+	help
+	   The RISCV MIPS P8700 uses a different opcode for PAUSE.
+	   It is a 'hint' encoding of the SLLI instruction,
+	   with rd=0, rs1=0 and imm=5. It will behave as a NOP
+	   instruction if no additional behavior beyond that of
+	   SLLI is implemented.
+
+	   If you are not using the P8700 processor, say n.
+
 config ERRATA_SIFIVE
 	bool "SiFive errata"
 	depends on RISCV_ALTERNATIVE

arch/riscv/errata/Makefile

@@ -13,5 +13,6 @@ endif
 endif
 
 obj-$(CONFIG_ERRATA_ANDES) += andes/
+obj-$(CONFIG_ERRATA_MIPS) += mips/
 obj-$(CONFIG_ERRATA_SIFIVE) += sifive/
 obj-$(CONFIG_ERRATA_THEAD) += thead/

arch/riscv/errata/mips/Makefile

@@ -0,0 +1,5 @@
+ifdef CONFIG_RISCV_ALTERNATIVE_EARLY
+CFLAGS_errata.o := -mcmodel=medany
+endif
+
+obj-y += errata.o

arch/riscv/errata/mips/errata.c

@@ -0,0 +1,67 @@
+// SPDX-License-Identifier: GPL-2.0-only
+/*
+ * Copyright (C) 2025 MIPS.
+ */
+
+#include <linux/memory.h>
+#include <linux/module.h>
+#include <asm/text-patching.h>
+#include <asm/alternative.h>
+#include <asm/errata_list.h>
+#include <asm/vendorid_list.h>
+#include <asm/vendor_extensions.h>
+#include <asm/vendor_extensions/mips.h>
+
+static inline bool errata_probe_pause(void)
+{
+	if (!IS_ENABLED(CONFIG_ERRATA_MIPS_P8700_PAUSE_OPCODE))
+		return false;
+
+	if (!riscv_isa_vendor_extension_available(MIPS_VENDOR_ID, XMIPSEXECTL))
+		return false;
+
+	return true;
+}
+
+static u32 mips_errata_probe(void)
+{
+	u32 cpu_req_errata = 0;
+
+	if (errata_probe_pause())
+		cpu_req_errata |= BIT(ERRATA_MIPS_P8700_PAUSE_OPCODE);
+
+	return cpu_req_errata;
+}
+
+void mips_errata_patch_func(struct alt_entry *begin, struct alt_entry *end,
+			    unsigned long archid, unsigned long impid,
+			    unsigned int stage)
+{
+	struct alt_entry *alt;
+	u32 cpu_req_errata = mips_errata_probe();
+	u32 tmp;
+
+	BUILD_BUG_ON(ERRATA_MIPS_NUMBER >= RISCV_VENDOR_EXT_ALTERNATIVES_BASE);
+
+	if (stage == RISCV_ALTERNATIVES_EARLY_BOOT)
+		return;
+
+	for (alt = begin; alt < end; alt++) {
+		if (alt->vendor_id != MIPS_VENDOR_ID)
+			continue;
+
+		if (alt->patch_id >= ERRATA_MIPS_NUMBER) {
+			WARN(1, "MIPS errata id:%d not in kernel errata list\n",
+			     alt->patch_id);
+			continue;
+		}
+
+		tmp = (1U << alt->patch_id);
+		if (cpu_req_errata && tmp) {
+			mutex_lock(&text_mutex);
+			patch_text_nosync(ALT_OLD_PTR(alt), ALT_ALT_PTR(alt),
+					  alt->alt_len);
+			mutex_unlock(&text_mutex);
+		}
+	}
+}

arch/riscv/include/asm/alternative.h

@@ -48,6 +48,9 @@ struct alt_entry {
 void andes_errata_patch_func(struct alt_entry *begin, struct alt_entry *end,
 			     unsigned long archid, unsigned long impid,
 			     unsigned int stage);
+void mips_errata_patch_func(struct alt_entry *begin, struct alt_entry *end,
+			    unsigned long archid, unsigned long impid,
+			    unsigned int stage);
 void sifive_errata_patch_func(struct alt_entry *begin, struct alt_entry *end,
 			      unsigned long archid, unsigned long impid,
 			      unsigned int stage);

arch/riscv/include/asm/cmpxchg.h

@@ -14,6 +14,7 @@
 #include <asm/insn-def.h>
 #include <asm/cpufeature-macros.h>
 #include <asm/processor.h>
+#include <asm/errata_list.h>
 
 #define __arch_xchg_masked(sc_sfx, swap_sfx, prepend, sc_append,		\
 			   swap_append, r, p, n)				\
@@ -438,7 +439,7 @@ static __always_inline void __cmpwait(volatile void *ptr,
 	return;
 
 no_zawrs:
-	asm volatile(RISCV_PAUSE : : : "memory");
+	ALT_RISCV_PAUSE();
 }
 
 #define __cmpwait_relaxed(ptr, val) \

arch/riscv/include/asm/errata_list.h

@@ -5,12 +5,12 @@
 #ifndef ASM_ERRATA_LIST_H
 #define ASM_ERRATA_LIST_H
 
-#include <asm/alternative.h>
 #include <asm/csr.h>
 #include <asm/insn-def.h>
 #include <asm/hwcap.h>
 #include <asm/vendorid_list.h>
 #include <asm/errata_list_vendors.h>
+#include <asm/vendor_extensions/mips.h>
 
 #ifdef __ASSEMBLER__
 
@@ -42,6 +42,17 @@ asm(ALTERNATIVE("sfence.vma %0, %1", "sfence.vma", SIFIVE_VENDOR_ID,	\
 		ERRATA_SIFIVE_CIP_1200, CONFIG_ERRATA_SIFIVE_CIP_1200)	\
 		: : "r" (addr), "r" (asid) : "memory")
 
+#define ALT_RISCV_PAUSE()					\
+asm(ALTERNATIVE(	\
+		RISCV_PAUSE, /* Original RISC‑V pause insn */	\
+		MIPS_PAUSE, /* Replacement for MIPS P8700 */	\
+		MIPS_VENDOR_ID, /* Vendor ID to match */	\
+		ERRATA_MIPS_P8700_PAUSE_OPCODE, /* patch_id */	\
+		CONFIG_ERRATA_MIPS_P8700_PAUSE_OPCODE)	\
+	: /* no outputs */	\
+	: /* no inputs */	\
+	: "memory")
+
 /*
  * _val is marked as "will be overwritten", so need to set it to 0
  * in the default case.

arch/riscv/include/asm/errata_list_vendors.h

@@ -21,4 +21,9 @@
 #define	ERRATA_THEAD_NUMBER 3
 #endif
 
+#ifdef CONFIG_ERRATA_MIPS
+#define	ERRATA_MIPS_P8700_PAUSE_OPCODE 0
+#define	ERRATA_MIPS_NUMBER 1
+#endif
+
 #endif /* ASM_ERRATA_LIST_VENDORS_H */

arch/riscv/include/asm/vdso/processor.h

@@ -5,6 +5,7 @@
 #ifndef __ASSEMBLER__
 
 #include <asm/barrier.h>
+#include <asm/errata_list.h>
 #include <asm/insn-def.h>
 
 static inline void cpu_relax(void)
@@ -19,7 +20,7 @@ static inline void cpu_relax(void)
 	 * Reduce instruction retirement.
 	 * This assumes the PC changes.
 	 */
-	__asm__ __volatile__ (RISCV_PAUSE);
+	ALT_RISCV_PAUSE();
 	barrier();
 }
 

arch/riscv/kernel/alternative.c

@@ -47,6 +47,11 @@ static void riscv_fill_cpu_mfr_info(struct cpu_manufacturer_info_t *cpu_mfr_info
 		cpu_mfr_info->patch_func = andes_errata_patch_func;
 		break;
 #endif
+#ifdef CONFIG_ERRATA_MIPS
+	case MIPS_VENDOR_ID:
+		cpu_mfr_info->patch_func = mips_errata_patch_func;
+		break;
+#endif
 #ifdef CONFIG_ERRATA_SIFIVE
 	case SIFIVE_VENDOR_ID:
 		cpu_mfr_info->patch_func = sifive_errata_patch_func;

arch/riscv/kernel/entry.S

@@ -7,6 +7,7 @@
 #include <linux/init.h>
 #include <linux/linkage.h>
 
+#include <asm/alternative-macros.h>
 #include <asm/asm.h>
 #include <asm/csr.h>
 #include <asm/scs.h>

arch/riscv/mm/init.c

@@ -23,6 +23,7 @@
 #include <linux/kfence.h>
 #include <linux/execmem.h>
 
+#include <asm/alternative.h>
 #include <asm/fixmap.h>
 #include <asm/io.h>
 #include <asm/kasan.h>