[PATCH v2 1/4] MIPS: Introduce WAR_4KC_LLSC config option

[Jiaxun Yang]

WAR_4KC_LLSC is used to control workaround of 4KC LLSC issue
that affects 4Kc up to version 0.9.

Early ath25 chips are known to be affected.

Signed-off-by: Jiaxun Yang <jiaxun.yang@xxxxxxxxxxx>
---
v2:
- Improve error message, taint kernel on error
- Don't override cpu_has_llsc if WAR_4KC_LLSC is not selected,
  cpu-probe logic can handle it, there is no need to mess around
  ifdef as suggested in previous review comments as WAR_4KC_LLSC
  is gated by SOC_AR5312.
---
 arch/mips/Kconfig                                        | 6 ++++++
 arch/mips/include/asm/cpu.h                              | 1 +
 arch/mips/include/asm/mach-ath25/cpu-feature-overrides.h | 6 ++----
 arch/mips/kernel/cpu-probe.c                             | 9 +++++++++
 4 files changed, 18 insertions(+), 4 deletions(-)

diff --git a/arch/mips/Kconfig b/arch/mips/Kconfig
index 1236ea122061..8ac467c1f9c8 100644
--- a/arch/mips/Kconfig
+++ b/arch/mips/Kconfig
@@ -221,6 +221,7 @@ config ATH25
 	select SYS_SUPPORTS_BIG_ENDIAN
 	select SYS_SUPPORTS_32BIT_KERNEL
 	select SYS_HAS_EARLY_PRINTK
+	select WAR_4KC_LLSC if SOC_AR5312
 	help
 	  Support for Atheros AR231x and Atheros AR531x based boards
 
@@ -2543,6 +2544,11 @@ config WAR_ICACHE_REFILLS
 config WAR_R10000_LLSC
 	bool
 
+# On 4Kc up to version 0.9 (PRID_REV < 1) there is a bug that may cause llsc
+# sequences to deadlock.
+config WAR_4KC_LLSC
+	bool
+
 # 34K core erratum: "Problems Executing the TLBR Instruction"
 config WAR_MIPS34K_MISSED_ITLB
 	bool
diff --git a/arch/mips/include/asm/cpu.h b/arch/mips/include/asm/cpu.h
index ecb9854cb432..84bb1931a8b4 100644
--- a/arch/mips/include/asm/cpu.h
+++ b/arch/mips/include/asm/cpu.h
@@ -247,6 +247,7 @@
 #define PRID_REV_VR4122			0x0070
 #define PRID_REV_VR4181A		0x0070	/* Same as VR4122 */
 #define PRID_REV_VR4130			0x0080
+#define PRID_REV_4KC_V1_0		0x0001
 #define PRID_REV_34K_V1_0_2		0x0022
 #define PRID_REV_LOONGSON1B		0x0020
 #define PRID_REV_LOONGSON1C		0x0020	/* Same as Loongson-1B */
diff --git a/arch/mips/include/asm/mach-ath25/cpu-feature-overrides.h b/arch/mips/include/asm/mach-ath25/cpu-feature-overrides.h
index ec3604c44ef2..4cf3d1ffba1a 100644
--- a/arch/mips/include/asm/mach-ath25/cpu-feature-overrides.h
+++ b/arch/mips/include/asm/mach-ath25/cpu-feature-overrides.h
@@ -24,14 +24,12 @@
 #define cpu_has_counter			1
 #define cpu_has_ejtag			1
 
-#if !defined(CONFIG_SOC_AR5312)
-#  define cpu_has_llsc			1
-#else
 /*
  * The MIPS 4Kc V0.9 core in the AR5312/AR2312 have problems with the
  * ll/sc instructions.
  */
-#  define cpu_has_llsc			0
+#if !defined(WAR_4KC_LLSC)
+#  define cpu_has_llsc			1
 #endif
 
 #define cpu_has_mips16			0
diff --git a/arch/mips/kernel/cpu-probe.c b/arch/mips/kernel/cpu-probe.c
index bda7f193baab..ff2905f59f2a 100644
--- a/arch/mips/kernel/cpu-probe.c
+++ b/arch/mips/kernel/cpu-probe.c
@@ -152,6 +152,15 @@ static inline void check_errata(void)
 	struct cpuinfo_mips *c = &current_cpu_data;
 
 	switch (current_cpu_type()) {
+	case CPU_4KC:
+		if ((c->processor_id & PRID_REV_MASK) < PRID_REV_4KC_V1_0) {
+			c->options &= ~MIPS_CPU_LLSC;
+			if (cpu_has_llsc) {
+				pr_crit("CPU has LLSC erratum, but cpu_has_llsc is force enabled!\n");
+				add_taint(TAINT_CPU_OUT_OF_SPEC, LOCKDEP_STILL_OK);
+			}
+		}
+		break;
 	case CPU_34K:
 		/*
 		 * Erratum "RPS May Cause Incorrect Instruction Execution"

-- 
2.43.0