watchdog: Add a driver for the Apple watchdog

This driver supports the watchdog timer found on Apple's M1 SoC.
On systems that use these SoC, the watchdog timer is the primary
way to reboot the system.

Signed-off-by: Mark Kettenis <kettenis@openbsd.org>
Reviewed-by: Sven Peter <sven@svenpeter.dev>
Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Stefan Roese <sr@denx.de>
Tested-on: Apple M1 Macbook
Tested-by: Simon Glass <sjg@chromium.org>

MAINTAINERS

@@ -116,6 +116,7 @@ F:	arch/arm/mach-apple/
 F:	configs/apple_m1_defconfig
 F:	drivers/iommu/apple_dart.c
 F:	drivers/pinctrl/pinctrl-apple.c
+F:	drivers/watchdog/apple_wdt.c
 F:	include/configs/apple.h
 
 ARM

drivers/watchdog/Kconfig

@@ -81,6 +81,15 @@ config WDT
 	  What exactly happens when the timer expires is up to a particular
 	  device/driver.
 
+config WDT_APPLE
+	bool "Apple watchdog timer support"
+	depends on WDT
+	default y if ARCH_APPLE
+	help
+	  Enable support for the watchdog timer on Apple SoCs.
+	  The watchdog will perform a full SoC reset resulting in a
+	  reboot of the entire system.
+
 config WDT_ARMADA_37XX
 	bool "Marvell Armada 37xx watchdog timer support"
 	depends on WDT && ARMADA_3700

drivers/watchdog/Makefile

@@ -17,6 +17,7 @@ obj-$(CONFIG_DESIGNWARE_WATCHDOG) += designware_wdt.o
 obj-$(CONFIG_ULP_WATCHDOG) += ulp_wdog.o
 obj-$(CONFIG_$(SPL_TPL_)WDT) += wdt-uclass.o
 obj-$(CONFIG_WDT_SANDBOX) += sandbox_wdt.o
+obj-$(CONFIG_WDT_APPLE) += apple_wdt.o
 obj-$(CONFIG_WDT_ARMADA_37XX) += armada-37xx-wdt.o
 obj-$(CONFIG_WDT_ASPEED) += ast_wdt.o
 obj-$(CONFIG_WDT_AST2600) += ast2600_wdt.o

drivers/watchdog/apple_wdt.c

@@ -0,0 +1,115 @@
+// SPDX-License-Identifier: GPL-2.0+
+/*
+ * Copyright (C) 2021 Mark Kettenis <kettenis@openbsd.org>
+ */
+
+#include <clk.h>
+#include <dm.h>
+#include <wdt.h>
+#include <asm/io.h>
+#include <linux/delay.h>
+
+#define APPLE_WDT_CUR_TIME		0x10
+#define APPLE_WDT_BARK_TIME		0x14
+#define APPLE_WDT_CTRL			0x1c
+#define  APPLE_WDT_CTRL_RESET_EN	BIT(2)
+
+struct apple_wdt_priv {
+	void *base;
+	ulong clk_rate;
+};
+
+static int apple_wdt_reset(struct udevice *dev)
+{
+	struct apple_wdt_priv *priv = dev_get_priv(dev);
+
+	writel(0, priv->base + APPLE_WDT_CUR_TIME);
+
+	return 0;
+}
+
+static int apple_wdt_start(struct udevice *dev, u64 timeout_ms, ulong flags)
+{
+	struct apple_wdt_priv *priv = dev_get_priv(dev);
+	u64 timeout;
+
+	timeout = (timeout_ms * priv->clk_rate) / 1000;
+	if (timeout > U32_MAX)
+		return -EINVAL;
+
+	writel(0, priv->base + APPLE_WDT_CUR_TIME);
+	writel(timeout, priv->base + APPLE_WDT_BARK_TIME);
+	writel(APPLE_WDT_CTRL_RESET_EN, priv->base + APPLE_WDT_CTRL);
+
+	return 0;
+}
+
+static int apple_wdt_stop(struct udevice *dev)
+{
+	struct apple_wdt_priv *priv = dev_get_priv(dev);
+
+	writel(0, priv->base + APPLE_WDT_CTRL);
+
+	return 0;
+}
+
+static int apple_wdt_expire_now(struct udevice *dev, ulong flags)
+{
+	int ret;
+
+	ret = apple_wdt_start(dev, 0, flags);
+	if (ret)
+		return ret;
+
+	/*
+	 * It can take up to 25ms until the SoC actually resets, so
+	 * wait 50ms just to be sure.
+	 */
+	mdelay(50);
+
+	return 0;
+}
+
+static const struct wdt_ops apple_wdt_ops = {
+	.reset = apple_wdt_reset,
+	.start = apple_wdt_start,
+	.stop = apple_wdt_stop,
+	.expire_now = apple_wdt_expire_now,
+};
+
+static const struct udevice_id apple_wdt_ids[] = {
+	{ .compatible = "apple,wdt" },
+	{ /* sentinel */ }
+};
+
+static int apple_wdt_probe(struct udevice *dev)
+{
+	struct apple_wdt_priv *priv = dev_get_priv(dev);
+	struct clk clk;
+	int ret;
+
+	priv->base = dev_read_addr_ptr(dev);
+	if (!priv->base)
+		return -EINVAL;
+
+	ret = clk_get_by_index(dev, 0, &clk);
+	if (ret)
+		return ret;
+
+	ret = clk_enable(&clk);
+	if (ret)
+		return ret;
+
+	priv->clk_rate = clk_get_rate(&clk);
+
+	return 0;
+}
+
+U_BOOT_DRIVER(apple_wdt) = {
+	.name = "apple_wdt",
+	.id = UCLASS_WDT,
+	.of_match = apple_wdt_ids,
+	.priv_auto = sizeof(struct apple_wdt_priv),
+	.probe = apple_wdt_probe,
+	.ops = &apple_wdt_ops,
+};