[PATCH v3 4/4] watchdog: Add support for VIA/WonderMedia SoC watchdog functionality

[Alexey Charkov <alchark@gmail.com>]

VIA/WonderMedia SoCs can use their system timer's first channel as a
watchdog device which will reset the system if the clocksource counter
matches the value given in its match register 0 and if the watchdog
function is enabled.

Since the watchdog function is tightly coupled to the timer itself, it
is implemented as a child platform device of the timer device, and just
reuses the MMIO registers already remapped by the timer driver. This is
similar to the approach taken by gxp-wdt.c

Signed-off-by: Alexey Charkov <alchark@gmail.com>
---
 MAINTAINERS                   |   1 +
 drivers/watchdog/Kconfig      |  14 +++++
 drivers/watchdog/Makefile     |   1 +
 drivers/watchdog/vt8500-wdt.c | 116 ++++++++++++++++++++++++++++++++++++++++++
 4 files changed, 132 insertions(+)

diff --git a/MAINTAINERS b/MAINTAINERS
index b8b2843491c47182a4fc6c76f5c29b6c411830a6..d809643b76ad299ca1c08804540b08cc4a7184a6 100644
--- a/MAINTAINERS
+++ b/MAINTAINERS
@@ -3446,6 +3446,7 @@ F:	drivers/tty/serial/vt8500_serial.c
 F:	drivers/video/fbdev/vt8500lcdfb.*
 F:	drivers/video/fbdev/wm8505fb*
 F:	drivers/video/fbdev/wmt_ge_rops.*
+F:	drivers/watchdog/vt8500-wdt.c
 
 ARM/ZYNQ ARCHITECTURE
 M:	Michal Simek <michal.simek@amd.com>
diff --git a/drivers/watchdog/Kconfig b/drivers/watchdog/Kconfig
index 0d8d37f712e8cfb4bf8156853baa13c23a57d6d9..429e4e775f1f458fd457067a3a039a5d544b4818 100644
--- a/drivers/watchdog/Kconfig
+++ b/drivers/watchdog/Kconfig
@@ -2003,6 +2003,20 @@ config PIC32_DMT
 	  To compile this driver as a loadable module, choose M here.
 	  The module will be called pic32-dmt.
 
+config VT8500_WATCHDOG
+	tristate "VIA/WonderMedia VT8500 watchdog support"
+	depends on ARCH_VT8500 || COMPILE_TEST
+	select WATCHDOG_CORE
+	help
+	  VIA/WonderMedia SoCs can use their system timer as a hardware
+	  watchdog, as long as the first timer channel is free from other
+	  uses and respective function is enabled in its registers. To
+	  make use of it, say Y here and ensure that the device tree
+	  lists at least two interrupts for the VT8500 timer device.
+
+	  To compile this driver as a module, choose M here.
+	  The module will be called vt8500-wdt.
+
 # PARISC Architecture
 
 # POWERPC Architecture
diff --git a/drivers/watchdog/Makefile b/drivers/watchdog/Makefile
index c9482904bf870a085c7fce2a439ac5089b6e6fee..3072786bf226c357102be3734fe6e701f753d45b 100644
--- a/drivers/watchdog/Makefile
+++ b/drivers/watchdog/Makefile
@@ -101,6 +101,7 @@ obj-$(CONFIG_MSC313E_WATCHDOG) += msc313e_wdt.o
 obj-$(CONFIG_APPLE_WATCHDOG) += apple_wdt.o
 obj-$(CONFIG_SUNPLUS_WATCHDOG) += sunplus_wdt.o
 obj-$(CONFIG_MARVELL_GTI_WDT) += marvell_gti_wdt.o
+obj-$(CONFIG_VT8500_WATCHDOG) += vt8500-wdt.o
 
 # X86 (i386 + ia64 + x86_64) Architecture
 obj-$(CONFIG_ACQUIRE_WDT) += acquirewdt.o
diff --git a/drivers/watchdog/vt8500-wdt.c b/drivers/watchdog/vt8500-wdt.c
new file mode 100644
index 0000000000000000000000000000000000000000..8e605c850dd9f42c90104f362b74adac63dd9fdb
--- /dev/null
+++ b/drivers/watchdog/vt8500-wdt.c
@@ -0,0 +1,116 @@
+// SPDX-License-Identifier: GPL-2.0
+/* Copyright (C) 2025 Alexey Charkov <alchark@gmail.com */
+
+#include <linux/delay.h>
+#include <linux/io.h>
+#include <linux/module.h>
+#include <linux/platform_device.h>
+#include <linux/types.h>
+#include <linux/watchdog.h>
+
+#define TIMER_MATCH_REG(x)	(4 * (x))
+#define TIMER_COUNT_REG		0x0010	 /* clocksource counter */
+
+#define TIMER_WATCHDOG_EN_REG	0x0018
+#define TIMER_WD_EN		BIT(0)
+
+#define TIMER_CTRL_REG		0x0020
+#define TIMER_CTRL_ENABLE	BIT(0)	 /* enable clocksource counter */
+#define TIMER_CTRL_RD_REQ	BIT(1)	 /* request counter read */
+
+#define TIMER_ACC_STS_REG	0x0024	 /* access status */
+#define TIMER_ACC_WR_MATCH(x)	BIT((x)) /* writing Match (x) value */
+#define TIMER_ACC_WR_COUNTER	BIT(4)	 /* writing clocksource counter */
+#define TIMER_ACC_RD_COUNTER	BIT(5)	 /* reading clocksource counter */
+
+#define VT8500_TIMER_HZ		3000000
+#define msecs_to_loops(t) (loops_per_jiffy / 1000 * HZ * t)
+
+struct vt8500_wdt {
+	void __iomem *regbase;
+	struct watchdog_device wdd;
+};
+
+static u64 vt8500_timer_read(void __iomem *regbase)
+{
+	int loops = msecs_to_loops(10);
+
+	writel(TIMER_CTRL_ENABLE | TIMER_CTRL_RD_REQ, regbase + TIMER_CTRL_REG);
+	while (readl(regbase + TIMER_ACC_STS_REG) & TIMER_ACC_RD_COUNTER
+	     && --loops)
+		cpu_relax();
+	return readl(regbase + TIMER_COUNT_REG);
+}
+
+static int vt8500_watchdog_start(struct watchdog_device *wdd)
+{
+	struct vt8500_wdt *drvdata = watchdog_get_drvdata(wdd);
+	u64 cycles = wdd->timeout * VT8500_TIMER_HZ;
+	u64 deadline = vt8500_timer_read(drvdata->regbase) + cycles;
+
+	writel((u32)deadline, drvdata->regbase + TIMER_MATCH_REG(0));
+	writel(TIMER_WD_EN, drvdata->regbase + TIMER_WATCHDOG_EN_REG);
+	return 0;
+}
+
+static int vt8500_watchdog_stop(struct watchdog_device *wdd)
+{
+	struct vt8500_wdt *drvdata = watchdog_get_drvdata(wdd);
+
+	writel(0, drvdata->regbase + TIMER_WATCHDOG_EN_REG);
+	return 0;
+}
+
+static const struct watchdog_ops vt8500_watchdog_ops = {
+	.start			= vt8500_watchdog_start,
+	.stop			= vt8500_watchdog_stop,
+};
+
+static const struct watchdog_info vt8500_watchdog_info = {
+	.identity		= "VIA VT8500 watchdog",
+	.options		= WDIOF_MAGICCLOSE |
+				  WDIOF_KEEPALIVEPING |
+				  WDIOF_SETTIMEOUT,
+};
+
+static int vt8500_wdt_probe(struct platform_device *pdev)
+{
+	struct device *dev = &pdev->dev;
+	struct vt8500_wdt *drvdata;
+
+	drvdata = devm_kzalloc(dev, sizeof(struct vt8500_wdt), GFP_KERNEL);
+	if (!drvdata)
+		return -ENOMEM;
+
+	/*
+	 * The register area where the timer and watchdog reside is disarranged.
+	 * Hence mapping individual register blocks for the timer and watchdog
+	 * is not recommended as they would have access to each others
+	 * registers. The timer driver creates the watchdog as a child device.
+	 * During the watchdogs creation, the timer driver passes the base
+	 * address to the watchdog over the private interface.
+	 */
+
+	drvdata->regbase = (void __iomem *)dev->platform_data;
+
+	drvdata->wdd.info = &vt8500_watchdog_info;
+	drvdata->wdd.ops = &vt8500_watchdog_ops;
+	drvdata->wdd.max_hw_heartbeat_ms = -1UL / (VT8500_TIMER_HZ / 1000);
+	drvdata->wdd.parent = dev;
+
+	watchdog_set_drvdata(&drvdata->wdd, drvdata);
+
+	return devm_watchdog_register_device(dev, &drvdata->wdd);
+}
+
+static struct platform_driver vt8500_wdt_driver = {
+	.probe = vt8500_wdt_probe,
+	.driver = {
+		.name =	"vt8500-wdt",
+	},
+};
+module_platform_driver(vt8500_wdt_driver);
+
+MODULE_AUTHOR("Alexey Charkov <alchark@gmail.com>");
+MODULE_DESCRIPTION("Driver for the VIA VT8500 watchdog timer");
+MODULE_LICENSE("GPL");

-- 
2.49.0