Re: [PATCH] clocksource: Loongson constant timer support

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在 2021/9/10 15:18, zhengd 写道:

Loongson constant timer is found in Loongson-3A4000 processor.
It has a counter which is globally accessiable via rdhwr instruction,
also each core has a clock event generator connected to this clock
source sharing interrupt with MIPS cont & comapre cevt.

We add cevt, csrc, vdso, sched_clock support for this timer.

Signed-off-by: Jiaxun Yang<jiaxun.yang@xxxxxxxxxxx>
Signed-off-by: Huacai Chen<chenhuacai@xxxxxxxxxxxx>
Signed-off-by: zhengd<zhengd@xxxxxxxxxx>


Hi Zheng D

(I think you should provide your full name for sign-off purpose)
It seems like you are repasting my patch, it's appreciated as I don't have capacity to care it presently.
I noticed that you had squashed my patches together, it is discoraged as it will makes reviewing much more harder.
Also, I do have a personal request, could you please mark your modifications from my original set so I can ensure all concerns around the original issue are addressed? 

Thanks.

- Jiaxun



---
  arch/mips/include/asm/cpu-features.h          |   4 +
  arch/mips/include/asm/cpu.h                   |   1 +
  .../include/asm/mach-loongson64/loongson.h    |   9 +
  .../asm/mach-loongson64/loongson_regs.h       |   7 +
  arch/mips/include/asm/mipsregs.h              |   2 +
  arch/mips/include/asm/time.h                  |   3 +
  arch/mips/include/asm/vdso/clocksource.h      |   3 +-
  arch/mips/include/asm/vdso/gettimeofday.h     |  25 +-
  arch/mips/kernel/cevt-r4k.c                   |  25 ++
  arch/mips/kernel/cpu-probe.c                  |   6 +-
  arch/mips/kernel/csrc-r4k.c                   |   5 +-
  arch/mips/kernel/time.c                       |   3 +
  drivers/clocksource/Kconfig                   |  10 +
  drivers/clocksource/Makefile                  |   1 +
  drivers/clocksource/loongson_const_timer.c    | 249 ++++++++++++++++++
  include/linux/cpuhotplug.h                    |   1 +
  16 files changed, 348 insertions(+), 6 deletions(-)
  create mode 100644 drivers/clocksource/loongson_const_timer.c

diff --git a/arch/mips/include/asm/cpu-features.h b/arch/mips/include/asm/cpu-features.h
index 3d71081afc55..787bd53be900 100644
--- a/arch/mips/include/asm/cpu-features.h
+++ b/arch/mips/include/asm/cpu-features.h
@@ -573,6 +573,10 @@
  # define cpu_has_gsexcex	__opt(MIPS_CPU_GSEXCEX)
  #endif
+#ifndef cpu_has_extimer 
+# define cpu_has_extimer	__opt(MIPS_CPU_EXTIMER)
+#endif
+
  #ifdef CONFIG_SMP
  /*
   * Some systems share FTLB RAMs between threads within a core (siblings in
diff --git a/arch/mips/include/asm/cpu.h b/arch/mips/include/asm/cpu.h
index d45a52f65b7a..8d221b4c3fa3 100644
--- a/arch/mips/include/asm/cpu.h
+++ b/arch/mips/include/asm/cpu.h
@@ -428,6 +428,7 @@ enum cpu_type_enum {
  #define MIPS_CPU_MAC_2008_ONLY	BIT_ULL(60)	/* CPU Only support MAC2008 Fused multiply-add instruction */
  #define MIPS_CPU_FTLBPAREX	BIT_ULL(61)	/* CPU has FTLB parity exception */
  #define MIPS_CPU_GSEXCEX	BIT_ULL(62)	/* CPU has GSExc exception */
+#define MIPS_CPU_EXTIMER	BIT_ULL(63)	/* CPU has External Timer (Loongson) */
/* 
   * CPU ASE encodings
diff --git a/arch/mips/include/asm/mach-loongson64/loongson.h b/arch/mips/include/asm/mach-loongson64/loongson.h
index f7c3ab6d724e..8c7a85bce539 100644
--- a/arch/mips/include/asm/mach-loongson64/loongson.h
+++ b/arch/mips/include/asm/mach-loongson64/loongson.h
@@ -56,6 +56,15 @@ extern void *loongson_fdt_blob;
  extern void mach_irq_dispatch(unsigned int pending);
  extern int mach_i8259_irq(void);
+#ifdef CONFIG_LOONGSON_CONST_TIMER 
+extern int __init constant_timer_init(void);
+#else
+static inline int constant_timer_init(void)
+{
+	return -ENODEV;
+}
+#endif
+
  /* We need this in some places... */
  #define delay() ({		\
  	int x;				\
diff --git a/arch/mips/include/asm/mach-loongson64/loongson_regs.h b/arch/mips/include/asm/mach-loongson64/loongson_regs.h
index 165993514762..229465060be2 100644
--- a/arch/mips/include/asm/mach-loongson64/loongson_regs.h
+++ b/arch/mips/include/asm/mach-loongson64/loongson_regs.h
@@ -88,6 +88,7 @@ static inline u32 read_cpucfg(u32 reg)
  #define LOONGSON_CFG2_LGFTP	BIT(19)
  #define LOONGSON_CFG2_LGFTPREV	GENMASK(22, 20)
  #define LOONGSON_CFG2_LLFTP	BIT(23)
+#define LOONGSON_CFG2_LLFTPREVB	24
  #define LOONGSON_CFG2_LLFTPREV	GENMASK(26, 24)
  #define LOONGSON_CFG2_LCSRP	BIT(27)
  #define LOONGSON_CFG2_LDISBLIKELY	BIT(28)
@@ -237,6 +238,12 @@ static inline void csr_writeq(u64 val, u32 reg)
  #define CSR_MAIL_SEND_BUF_SHIFT		32
  #define CSR_MAIL_SEND_H32_MASK		0xFFFFFFFF00000000ULL
+#define LOONGSON_CSR_TIMER_CFG 0x1060 
+#define LOONGSON_CSR_TIMER_TICK		0x1070
+#define CONSTANT_TIMER_CFG_PERIODIC	(_ULCAST_(1) << 62)
+#define CONSTANT_TIMER_CFG_EN		(_ULCAST_(1) << 61)
+#define CONSTANT_TIMER_INITVAL_RESET	(_ULCAST_(0xffff) << 48)
+
  static inline u64 drdtime(void)
  {
  	int rID = 0;
diff --git a/arch/mips/include/asm/mipsregs.h b/arch/mips/include/asm/mipsregs.h
index acdf8c69220b..c16580a15336 100644
--- a/arch/mips/include/asm/mipsregs.h
+++ b/arch/mips/include/asm/mipsregs.h
@@ -699,6 +699,8 @@
/* Config6 feature bits for proAptiv/P5600 */ +#define LOONGSON_CONF6_INTIMER (_ULCAST_(1) << 6) 
+#define LOONGSON_CONF6_EXTIMER	(_ULCAST_(1) << 7)
  /* Jump register cache prediction disable */
  #define MTI_CONF6_JRCD		(_ULCAST_(1) << 0)
  /* MIPSr6 extensions enable */
diff --git a/arch/mips/include/asm/time.h b/arch/mips/include/asm/time.h
index e855a3611d92..e235b8a27768 100644
--- a/arch/mips/include/asm/time.h
+++ b/arch/mips/include/asm/time.h
@@ -28,6 +28,9 @@ extern void plat_time_init(void);
   */
  extern unsigned int mips_hpt_frequency;
+/* True if platform registered sched_clock by itself */ 
+extern bool plat_have_sched_clock __initdata;
+
  /*
   * The performance counter IRQ on MIPS is a close relative to the timer IRQ
   * so it lives here.
diff --git a/arch/mips/include/asm/vdso/clocksource.h b/arch/mips/include/asm/vdso/clocksource.h
index 510e1671d898..7fd43ca06eb1 100644
--- a/arch/mips/include/asm/vdso/clocksource.h
+++ b/arch/mips/include/asm/vdso/clocksource.h
@@ -4,6 +4,7 @@
#define VDSO_ARCH_CLOCKMODES \ 
  	VDSO_CLOCKMODE_R4K,	\
-	VDSO_CLOCKMODE_GIC
+	VDSO_CLOCKMODE_GIC,	\
+	VDSO_CLOCKMODE_CONST
#endif /* __ASM_VDSOCLOCKSOURCE_H */ 
diff --git a/arch/mips/include/asm/vdso/gettimeofday.h b/arch/mips/include/asm/vdso/gettimeofday.h
index 44a45f3fa4b0..2f860c1e3aa5 100644
--- a/arch/mips/include/asm/vdso/gettimeofday.h
+++ b/arch/mips/include/asm/vdso/gettimeofday.h
@@ -183,6 +183,24 @@ static __always_inline u64 read_gic_count(const struct vdso_data *data)
#endif +#ifdef CONFIG_LOONGSON_CONST_TIMER 
+
+static __always_inline u64 read_const_timer(void)
+{
+	u64 count;
+
+	__asm__ __volatile__(
+	"	.set push\n"
+	"	.set mips32r2\n"
+	"	rdhwr %0, $30\n"
+	"	.set pop\n"
+	: "=r" (count));
+
+	return count;
+}
+
+#endif
+
  static __always_inline u64 __arch_get_hw_counter(s32 clock_mode,
  						 const struct vdso_data *vd)
  {
@@ -193,6 +211,10 @@ static __always_inline u64 __arch_get_hw_counter(s32 clock_mode,
  #ifdef CONFIG_CLKSRC_MIPS_GIC
  	if (clock_mode == VDSO_CLOCKMODE_GIC)
  		return read_gic_count(vd);
+#endif
+#ifdef CONFIG_LOONGSON_CONST_TIMER
+	if (clock_mode == VDSO_CLOCKMODE_CONST)
+		return read_const_timer();
  #endif
  	/*
  	 * Core checks mode already. So this raced against a concurrent
@@ -205,7 +227,8 @@ static __always_inline u64 __arch_get_hw_counter(s32 clock_mode,
  static inline bool mips_vdso_hres_capable(void)
  {
  	return IS_ENABLED(CONFIG_CSRC_R4K) ||
-	       IS_ENABLED(CONFIG_CLKSRC_MIPS_GIC);
+	       IS_ENABLED(CONFIG_CLKSRC_MIPS_GIC) ||
+	       IS_ENABLED(CONFIG_LOONGSON_CONST_TIMER);
  }
  #define __arch_vdso_hres_capable mips_vdso_hres_capable
diff --git a/arch/mips/kernel/cevt-r4k.c b/arch/mips/kernel/cevt-r4k.c 
index 32ec67c9ab67..91fc62001149 100644
--- a/arch/mips/kernel/cevt-r4k.c
+++ b/arch/mips/kernel/cevt-r4k.c
@@ -15,6 +15,8 @@
#include <asm/time.h> 
  #include <asm/cevt-r4k.h>
+#include <asm/cpu-features.h>
+#include <asm/mipsregs.h>
static int mips_next_event(unsigned long delta, 
  			   struct clock_event_device *evt)
@@ -290,6 +292,24 @@ core_initcall(r4k_register_cpufreq_notifier);
#endif /* !CONFIG_CPU_FREQ */ +#ifdef CONFIG_CPU_LOONGSON64 
+static int c0_compare_int_enable(struct clock_event_device *cd)
+{
+	if (cpu_has_extimer)
+		set_c0_config6(LOONGSON_CONF6_INTIMER);
+
+	return 0;
+}
+
+static int c0_compare_int_disable(struct clock_event_device *cd)
+{
+	if (cpu_has_extimer)
+		clear_c0_config6(LOONGSON_CONF6_INTIMER);
+
+	return 0;
+}
+#endif
+
  int r4k_clockevent_init(void)
  {
  	unsigned long flags = IRQF_PERCPU | IRQF_TIMER | IRQF_SHARED;
@@ -325,6 +345,11 @@ int r4k_clockevent_init(void)
  	cd->set_next_event	= mips_next_event;
  	cd->event_handler	= mips_event_handler;
+#ifdef CONFIG_CPU_LOONGSON64 
+	cd->set_state_oneshot	= c0_compare_int_enable;
+	cd->set_state_shutdown	= c0_compare_int_disable;
+#endif
+
  	clockevents_config_and_register(cd, mips_hpt_frequency, min_delta, 0x7fffffff);
if (cp0_timer_irq_installed) 
diff --git a/arch/mips/kernel/cpu-probe.c b/arch/mips/kernel/cpu-probe.c
index 630fcb4cb30e..8a9b028a1df7 100644
--- a/arch/mips/kernel/cpu-probe.c
+++ b/arch/mips/kernel/cpu-probe.c
@@ -1723,6 +1723,9 @@ static inline void decode_cpucfg(struct cpuinfo_mips *c)
  	if (cfg2 & LOONGSON_CFG2_LEXT2)
  		c->ases |= MIPS_ASE_LOONGSON_EXT2;
+ if (cfg2 & LOONGSON_CFG2_LLFTP) 
+		c->options |= MIPS_CPU_EXTIMER;
+
  	if (cfg2 & LOONGSON_CFG2_LSPW) {
  		c->options |= MIPS_CPU_LDPTE;
  		c->guest.options |= MIPS_CPU_LDPTE;
@@ -1780,7 +1783,8 @@ static inline void cpu_probe_loongson(struct cpuinfo_mips *c, unsigned int cpu)
  		 * Also some early Loongson-3A2000 had wrong TLB type in Config
  		 * register, we correct it here.
  		 */
-		c->options |= MIPS_CPU_FTLB | MIPS_CPU_TLBINV | MIPS_CPU_LDPTE;
+		c->options |= MIPS_CPU_FTLB | MIPS_CPU_TLBINV | MIPS_CPU_LDPTE |
+			      MIPS_CPU_EXTIMER;
  		c->ases |= (MIPS_ASE_LOONGSON_MMI | MIPS_ASE_LOONGSON_CAM |
  			MIPS_ASE_LOONGSON_EXT | MIPS_ASE_LOONGSON_EXT2);
  		c->ases &= ~MIPS_ASE_VZ; /* VZ of Loongson-3A2000/3000 is incomplete */
diff --git a/arch/mips/kernel/csrc-r4k.c b/arch/mips/kernel/csrc-r4k.c
index edc4afc080fa..b50a24da6528 100644
--- a/arch/mips/kernel/csrc-r4k.c
+++ b/arch/mips/kernel/csrc-r4k.c
@@ -122,9 +122,8 @@ int __init init_r4k_clocksource(void)
clocksource_register_hz(&clocksource_mips, mips_hpt_frequency); -#ifndef CONFIG_CPU_FREQ 
-	sched_clock_register(r4k_read_sched_clock, 32, mips_hpt_frequency);
-#endif
+	if (!IS_ENABLED(CONFIG_CPU_FREQ) && !plat_have_sched_clock)
+		sched_clock_register(r4k_read_sched_clock, 32, mips_hpt_frequency);
return 0; 
  }
diff --git a/arch/mips/kernel/time.c b/arch/mips/kernel/time.c
index caa01457dce6..baf33da8c79d 100644
--- a/arch/mips/kernel/time.c
+++ b/arch/mips/kernel/time.c
@@ -123,6 +123,9 @@ EXPORT_SYMBOL(perf_irq);
  unsigned int mips_hpt_frequency;
  EXPORT_SYMBOL_GPL(mips_hpt_frequency);
+/* True if platform registered sched_clock by itself */ 
+bool plat_have_sched_clock __initdata;
+
  static __init int cpu_has_mfc0_count_bug(void)
  {
  	switch (current_cpu_type()) {
diff --git a/drivers/clocksource/Kconfig b/drivers/clocksource/Kconfig
index 0f5e3983951a..d12e3e4bedd1 100644
--- a/drivers/clocksource/Kconfig
+++ b/drivers/clocksource/Kconfig
@@ -708,4 +708,14 @@ config MICROCHIP_PIT64B
  	  modes and high resolution. It is used as a clocksource
  	  and a clockevent.
+config LOONGSON_CONST_TIMER 
+	bool "Loongson Constant Timer"
+	depends on MACH_LOONGSON64
+	default MACH_LOONGSON64
+	help
+	  This option enables Loongson constant timer for Loongson64
+	  systems. It supports the oneshot, the periodic modes and high
+	  resolution. It is used as a clocksource and a clockevent, it is
+	  also accessible in VDSO.
+
  endmenu
diff --git a/drivers/clocksource/Makefile b/drivers/clocksource/Makefile
index c17ee32a7151..f717a1e06900 100644
--- a/drivers/clocksource/Makefile
+++ b/drivers/clocksource/Makefile
@@ -88,3 +88,4 @@ obj-$(CONFIG_CSKY_MP_TIMER)		+= timer-mp-csky.o
  obj-$(CONFIG_GX6605S_TIMER)		+= timer-gx6605s.o
  obj-$(CONFIG_HYPERV_TIMER)		+= hyperv_timer.o
  obj-$(CONFIG_MICROCHIP_PIT64B)		+= timer-microchip-pit64b.o
+obj-$(CONFIG_LOONGSON_CONST_TIMER)	+= loongson_const_timer.o
diff --git a/drivers/clocksource/loongson_const_timer.c b/drivers/clocksource/loongson_const_timer.c
new file mode 100644
index 000000000000..3921b8c37092
--- /dev/null
+++ b/drivers/clocksource/loongson_const_timer.c
@@ -0,0 +1,249 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * Loongson64 constant timer driver
+ *
+ * Copyright (C) 2020 Huacai Chen<chenhuacai@xxxxxxxxxxxx>
+ * Copyright (C) 2020 Jiaxun Yang<jiaxun.yang@xxxxxxxxxxxx>
+ */
+
+#include <linux/init.h>
+#include <linux/interrupt.h>
+#include <linux/clockchips.h>
+#include <linux/cpuhotplug.h>
+#include <linux/sched_clock.h>
+#include <asm/time.h>
+#include <asm/mipsregs.h>
+#include <asm/cevt-r4k.h>
+#include <loongson.h>
+#include <loongson_regs.h>
+
+static unsigned int constant_timer_irq = -1;
+
+static DEFINE_PER_CPU(struct clock_event_device, constant_clockevent_device);
+
+static inline unsigned int calc_const_freq(void)
+{
+	unsigned int res;
+	unsigned int base_freq;
+	unsigned int cfm, cfd;
+
+	res = read_cpucfg(LOONGSON_CFG2);
+	if (!(res & LOONGSON_CFG2_LLFTP))
+		return 0;
+
+	res = read_cpucfg(LOONGSON_CFG5);
+	cfm = res & 0xffff;
+	cfd = (res >> 16) & 0xffff;
+	base_freq = read_cpucfg(LOONGSON_CFG4);
+
+	if (!base_freq || !cfm || !cfd)
+		return 0;
+	else
+		return (base_freq * cfm / cfd);
+}
+
+static irqreturn_t handle_constant_timer(int irq, void *data)
+{
+	int cpu = smp_processor_id();
+	struct clock_event_device *cd;
+
+	if ((cp0_perfcount_irq < 0) && perf_irq() == IRQ_HANDLED)
+		return IRQ_HANDLED;
+
+	cd = &per_cpu(mips_clockevent_device, cpu);
+
+
+	if (clockevent_state_detached(cd) || clockevent_state_shutdown(cd))
+		return IRQ_NONE;
+
+	if (read_c0_cause() & CAUSEF_TI) {
+		/* Clear Count/Compare Interrupt */
+		write_c0_compare(read_c0_compare());
+		cd = &per_cpu(constant_clockevent_device, cpu);
+		cd->event_handler(cd);
+
+		return IRQ_HANDLED;
+	}
+
+	return IRQ_NONE;
+}
+
+static int constant_set_state_oneshot(struct clock_event_device *evt)
+{
+	u64 cfg;
+
+	cfg = csr_readq(LOONGSON_CSR_TIMER_CFG);
+
+	cfg |= CONSTANT_TIMER_CFG_EN;
+	cfg &= ~CONSTANT_TIMER_CFG_PERIODIC;
+
+	csr_writeq(cfg, LOONGSON_CSR_TIMER_CFG);
+
+	set_c0_config6(LOONGSON_CONF6_EXTIMER);
+
+	return 0;
+}
+
+static int constant_set_state_periodic(struct clock_event_device *evt)
+{
+	unsigned int period;
+	u64 cfg;
+
+	cfg = csr_readq(LOONGSON_CSR_TIMER_CFG);
+
+	cfg &= CONSTANT_TIMER_INITVAL_RESET;
+	cfg |= (CONSTANT_TIMER_CFG_PERIODIC | CONSTANT_TIMER_CFG_EN);
+
+	period = calc_const_freq() / HZ;
+
+	csr_writeq(cfg | period, LOONGSON_CSR_TIMER_CFG);
+
+	set_c0_config6(LOONGSON_CONF6_EXTIMER);
+
+	return 0;
+}
+
+static int constant_set_state_shutdown(struct clock_event_device *evt)
+{
+	u64 cfg;
+
+	clear_c0_config6(LOONGSON_CONF6_EXTIMER);
+	cfg = csr_readq(LOONGSON_CSR_TIMER_CFG);
+	cfg &= ~CONSTANT_TIMER_CFG_EN;
+	csr_writeq(cfg, LOONGSON_CSR_TIMER_CFG);
+
+	return 0;
+}
+
+static int constant_next_event(unsigned long delta,
+				struct clock_event_device *evt)
+{
+	csr_writeq(delta | CONSTANT_TIMER_CFG_EN, LOONGSON_CSR_TIMER_CFG);
+	return 0;
+}
+
+static int constant_init_percpu(unsigned int cpu)
+{
+	unsigned int const_freq;
+	struct clock_event_device *cd;
+	unsigned long min_delta = 0x600;
+	unsigned long max_delta = (1UL << 48) - 1;
+
+	cd = &per_cpu(constant_clockevent_device, cpu);
+
+	const_freq = calc_const_freq();
+
+	cd->name = "constant";
+	cd->features = CLOCK_EVT_FEAT_ONESHOT | CLOCK_EVT_FEAT_PERIODIC |
+		       CLOCK_EVT_FEAT_PERCPU;
+
+	cd->rating = 500; /* Higher than cevt-r4k */
+	cd->irq = constant_timer_irq;
+	cd->cpumask = cpumask_of(cpu);
+	cd->set_state_oneshot = constant_set_state_oneshot;
+	cd->set_state_periodic = constant_set_state_periodic;
+	cd->set_state_shutdown = constant_set_state_shutdown;
+	cd->set_next_event = constant_next_event;
+
+	clockevents_config_and_register(cd, const_freq, min_delta, max_delta);
+
+	return 0;
+}
+
+static int __init constant_clockevent_init(void)
+{
+	unsigned long long flags = IRQF_PERCPU | IRQF_TIMER | IRQF_SHARED;
+
+	constant_timer_irq = get_c0_compare_int();
+	if (constant_timer_irq <= 0)
+		return -ENXIO;
+
+	/* Init on CPU0 firstly to keep away from IRQ storm */
+	constant_init_percpu(0);
+
+	if (request_irq(constant_timer_irq, handle_constant_timer, flags,
+			"constant_timer", handle_constant_timer)) {
+		pr_err("Failed to request irq %d (constant_timer)\n", constant_timer_irq);
+		return -ENXIO;
+	}
+
+	cpuhp_setup_state_nocalls(CPUHP_AP_MIPS_CONST_TIMER_STARTING,
+			  "clockevents/mips/constant/timer:starting",
+			   constant_init_percpu, NULL);
+
+	return 0;
+}
+
+static u64 read_const_counter(void)
+{
+	u64 count;
+
+	__asm__ __volatile__(
+		" .set push\n"
+		" .set mips64r2\n"
+		" rdhwr   %0, $30\n"
+		" .set pop\n"
+		: "=r" (count));
+
+	return count;
+}
+
+static u64 csrc_read_const_counter(struct clocksource *clk)
+{
+	return read_const_counter();
+}
+
+static struct clocksource clocksource_const = {
+	.name = "constant",
+	.rating = 8000,
+	.read = csrc_read_const_counter,
+	.mask = CLOCKSOURCE_MASK(64),
+	.flags = CLOCK_SOURCE_IS_CONTINUOUS,
+	.vdso_clock_mode = VDSO_CLOCKMODE_CONST,
+	.mult = 0,
+	.shift = 10,
+};
+
+static int __init constant_clocksource_init(unsigned long freq)
+{
+
+	clocksource_const.mult =
+		clocksource_hz2mult(freq, clocksource_const.shift);
+
+	return clocksource_register_hz(&clocksource_const, freq);
+}
+
+static void __init constant_sched_clock_init(unsigned long freq)
+{
+	plat_have_sched_clock = true;
+	sched_clock_register(read_const_counter, 64, freq);
+}
+
+int __init constant_timer_init(void)
+{
+	u32 ver;
+	unsigned long freq;
+
+	if (!cpu_has_extimer)
+		return -ENODEV;
+
+	if (!cpu_has_csr())
+		return -ENODEV;
+
+	ver = read_cpucfg(LOONGSON_CFG2);
+	ver &= LOONGSON_CFG2_LLFTPREV;
+	ver >>= LOONGSON_CFG2_LLFTPREVB;
+
+	if (ver < 2)
+		return -ENODEV;
+
+	freq = calc_const_freq();
+	if (!freq)
+		return -ENODEV;
+
+	constant_clockevent_init();
+	constant_clocksource_init(freq);
+	constant_sched_clock_init(freq);
+
+	return 0;
+}
diff --git a/include/linux/cpuhotplug.h b/include/linux/cpuhotplug.h
index ef3f48def8be..0230d13481c8 100644
--- a/include/linux/cpuhotplug.h
+++ b/include/linux/cpuhotplug.h
@@ -136,6 +136,7 @@ enum cpuhp_state {
  	CPUHP_AP_ARMADA_TIMER_STARTING,
  	CPUHP_AP_MARCO_TIMER_STARTING,
  	CPUHP_AP_MIPS_GIC_TIMER_STARTING,
+	CPUHP_AP_MIPS_CONST_TIMER_STARTING,
  	CPUHP_AP_ARC_TIMER_STARTING,
  	CPUHP_AP_RISCV_TIMER_STARTING,
  	CPUHP_AP_CLINT_TIMER_STARTING,
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