clocksource/drivers: Add Realtek system timer driver

Add a system timer driver for Realtek SoCs.

This driver registers the 1 MHz global hardware counter on Realtek
platforms as a clock event device. Since this hardware counter starts
counting automatically after SoC power-on, no clock initialization is
required. Because the counter does not stop or get affected by CPU power
down, and it supports oneshot mode, it is typically used as a tick
broadcast timer.

Signed-off-by: Hao-Wen Ting <haowen.ting@realtek.com>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Acked-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://patch.msgid.link/20251126060110.198330-3-haowen.ting@realtek.com

Author: Hao-Wen Ting

MAINTAINERS

@@ -21669,6 +21669,11 @@ S:	Maintained
 F:	Documentation/devicetree/bindings/spi/realtek,rtl9301-snand.yaml
 F:	drivers/spi/spi-realtek-rtl-snand.c
 
+REALTEK SYSTIMER DRIVER
+M:	Hao-Wen Ting <haowen.ting@realtek.com>
+S:	Maintained
+F:	drivers/clocksource/timer-realtek.c
+
 REALTEK WIRELESS DRIVER (rtlwifi family)
 M:	Ping-Ke Shih <pkshih@realtek.com>
 L:	linux-wireless@vger.kernel.org

drivers/clocksource/Kconfig

@@ -782,4 +782,15 @@ config NXP_STM_TIMER
           Enables the support for NXP System Timer Module found in the
           s32g NXP platform series.
 
+config RTK_SYSTIMER
+	bool "Realtek SYSTIMER support"
+	depends on ARM || ARM64
+	depends on ARCH_REALTEK || COMPILE_TEST
+	select TIMER_OF
+	help
+	  This option enables the driver that registers the global 1 MHz hardware
+	  counter as a clock event device on Realtek SoCs. Make sure to enable
+	  this option only when building for a Realtek platform or for compilation
+	  testing.
+
 endmenu

drivers/clocksource/Makefile

@@ -95,3 +95,4 @@ obj-$(CONFIG_CLKSRC_LOONGSON1_PWM)	+= timer-loongson1-pwm.o
 obj-$(CONFIG_EP93XX_TIMER)		+= timer-ep93xx.o
 obj-$(CONFIG_RALINK_TIMER)		+= timer-ralink.o
 obj-$(CONFIG_NXP_STM_TIMER)		+= timer-nxp-stm.o
+obj-$(CONFIG_RTK_SYSTIMER)		+= timer-realtek.o

drivers/clocksource/timer-realtek.c

@@ -0,0 +1,150 @@
+// SPDX-License-Identifier: GPL-2.0-only
+/*
+ * Copyright (c) 2025 Realtek Semiconductor Corp.
+ */
+
+#define pr_fmt(fmt)	KBUILD_MODNAME ": " fmt
+
+#include <linux/irqflags.h>
+#include <linux/interrupt.h>
+#include "timer-of.h"
+
+#define ENBL 1
+#define DSBL 0
+
+#define SYSTIMER_RATE 1000000
+#define SYSTIMER_MIN_DELTA 0x64
+#define SYSTIMER_MAX_DELTA ULONG_MAX
+
+/* SYSTIMER Register Offset (RTK Internal Use) */
+#define TS_LW_OFST 0x0
+#define TS_HW_OFST 0x4
+#define TS_CMP_VAL_LW_OFST 0x8
+#define TS_CMP_VAL_HW_OFST 0xC
+#define TS_CMP_CTRL_OFST 0x10
+#define TS_CMP_STAT_OFST 0x14
+
+/* SYSTIMER CMP CTRL REG Mask */
+#define TS_CMP_EN_MASK 0x1
+#define TS_WR_EN0_MASK 0x2
+
+static void __iomem *systimer_base;
+
+static u64 rtk_ts64_read(void)
+{
+	u32 low, high;
+	u64 ts;
+
+	/* Caution: Read LSB word (TS_LW_OFST) first then MSB (TS_HW_OFST) */
+	low = readl(systimer_base + TS_LW_OFST);
+	high = readl(systimer_base + TS_HW_OFST);
+	ts = ((u64)high << 32) | low;
+
+	return ts;
+}
+
+static void rtk_cmp_value_write(u64 value)
+{
+	u32 high, low;
+
+	low = value & 0xFFFFFFFF;
+	high = value >> 32;
+
+	writel(high, systimer_base + TS_CMP_VAL_HW_OFST);
+	writel(low, systimer_base + TS_CMP_VAL_LW_OFST);
+}
+
+static inline void rtk_cmp_en_write(bool cmp_en)
+{
+	u32 val;
+
+	val = TS_WR_EN0_MASK;
+	if (cmp_en == ENBL)
+		val |= TS_CMP_EN_MASK;
+
+	writel(val, systimer_base + TS_CMP_CTRL_OFST);
+}
+
+static int rtk_syst_clkevt_next_event(unsigned long cycles, struct clock_event_device *clkevt)
+{
+	u64 cmp_val;
+
+	rtk_cmp_en_write(DSBL);
+	cmp_val = rtk_ts64_read();
+
+	/* Set CMP value to current timestamp plus delta_us */
+	rtk_cmp_value_write(cmp_val + cycles);
+	rtk_cmp_en_write(ENBL);
+	return 0;
+}
+
+static irqreturn_t rtk_ts_match_intr_handler(int irq, void *dev_id)
+{
+	struct clock_event_device *clkevt = dev_id;
+	void __iomem *reg_base;
+	u32 val;
+
+	/* Disable TS CMP Match */
+	rtk_cmp_en_write(DSBL);
+
+	/* Clear TS CMP INTR */
+	reg_base = systimer_base + TS_CMP_STAT_OFST;
+	val = readl(reg_base) & TS_CMP_EN_MASK;
+	writel(val | TS_CMP_EN_MASK, reg_base);
+	clkevt->event_handler(clkevt);
+
+	return IRQ_HANDLED;
+}
+
+static int rtk_syst_shutdown(struct clock_event_device *clkevt)
+{
+	void __iomem *reg_base;
+	u64 cmp_val = 0;
+
+	/* Disable TS CMP Match */
+	rtk_cmp_en_write(DSBL);
+	/* Set compare value to 0 */
+	rtk_cmp_value_write(cmp_val);
+
+	/* Clear TS CMP INTR */
+	reg_base = systimer_base + TS_CMP_STAT_OFST;
+	writel(TS_CMP_EN_MASK, reg_base);
+	return 0;
+}
+
+static struct timer_of rtk_timer_to = {
+	.flags = TIMER_OF_IRQ | TIMER_OF_BASE,
+
+	.clkevt = {
+		.name			= "rtk-clkevt",
+		.rating			= 300,
+		.cpumask		= cpu_possible_mask,
+		.features		= CLOCK_EVT_FEAT_DYNIRQ |
+					  CLOCK_EVT_FEAT_ONESHOT,
+		.set_next_event		= rtk_syst_clkevt_next_event,
+		.set_state_oneshot	= rtk_syst_shutdown,
+		.set_state_shutdown	= rtk_syst_shutdown,
+	},
+
+	.of_irq = {
+		.flags			= IRQF_TIMER | IRQF_IRQPOLL,
+		.handler		= rtk_ts_match_intr_handler,
+	},
+};
+
+static int __init rtk_systimer_init(struct device_node *node)
+{
+	int ret;
+
+	ret = timer_of_init(node, &rtk_timer_to);
+	if (ret)
+		return ret;
+
+	systimer_base = timer_of_base(&rtk_timer_to);
+	clockevents_config_and_register(&rtk_timer_to.clkevt, SYSTIMER_RATE,
+					SYSTIMER_MIN_DELTA, SYSTIMER_MAX_DELTA);
+
+	return 0;
+}
+
+TIMER_OF_DECLARE(rtk_systimer, "realtek,rtd1625-systimer", rtk_systimer_init);