power: reset: add MCF5441x RCM power-on reason driver

Add a driver that decodes the Reset Status Register (RSR) of the
Freescale ColdFire MCF5441x Reset Controller Module at probe time
and exposes the cause via the power_on_reason sysfs ABI.

The RSR can latch several cause bits simultaneously (Reference
Manual chapter 12.3.2); the driver picks one cause per a priority
that favours the most explanatory diagnostic.

The register layout is described in the driver so it can be built
with COMPILE_TEST on any architecture.

Signed-off-by: Jean-Michel Hautbois <jeanmichel.hautbois@yoseli.org>
Link: https://patch.msgid.link/20260617-coldfire-rcm-power-on-reason-v3-1-1a7c7b6b14e7@yoseli.org
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>

MAINTAINERS

@@ -10291,6 +10291,12 @@ S:	Maintained
 F:	drivers/mmc/host/sdhci-esdhc-mcf.c
 F:	include/linux/platform_data/mmc-esdhc-mcf.h
 
+FREESCALE COLDFIRE M5441X RCM POWER-ON REASON DRIVER
+M:	Jean-Michel Hautbois <jeanmichel.hautbois@yoseli.org>
+L:	linux-m68k@lists.linux-m68k.org
+S:	Maintained
+F:	drivers/power/reset/mcf-rcm-reset.c
+
 FREESCALE DIU FRAMEBUFFER DRIVER
 M:	Timur Tabi <timur@kernel.org>
 L:	linux-fbdev@vger.kernel.org

drivers/power/reset/Kconfig

@@ -128,6 +128,18 @@ config POWER_RESET_LINKSTATION
 
 	  Say Y here if you have a Buffalo LinkStation LS421D/E.
 
+config POWER_RESET_MCF_RCM
+	tristate "Freescale ColdFire RCM power-on reason driver"
+	depends on M5441x || COMPILE_TEST
+	depends on HAS_IOMEM
+	help
+	  This driver exposes the cause of the last reset on Freescale
+	  ColdFire 5441x SoCs through the standard power_on_reason sysfs
+	  ABI. It reads the Reset Status Register (RSR) of the Reset
+	  Controller Module once at probe time and reports a normalised
+	  string (regular power-up, reset button action, software reset
+	  or unknown reason).
+
 config POWER_RESET_MACSMC
 	tristate "Apple SMC reset/power-off driver"
 	depends on MFD_MACSMC

drivers/power/reset/Makefile

@@ -13,6 +13,7 @@ obj-$(CONFIG_POWER_RESET_GPIO) += gpio-poweroff.o
 obj-$(CONFIG_POWER_RESET_GPIO_RESTART) += gpio-restart.o
 obj-$(CONFIG_POWER_RESET_HISI) += hisi-reboot.o
 obj-$(CONFIG_POWER_RESET_LINKSTATION) += linkstation-poweroff.o
+obj-$(CONFIG_POWER_RESET_MCF_RCM) += mcf-rcm-reset.o
 obj-$(CONFIG_POWER_RESET_MACSMC) += macsmc-reboot.o
 obj-$(CONFIG_POWER_RESET_MSM) += msm-poweroff.o
 obj-$(CONFIG_POWER_RESET_MT6323) += mt6323-poweroff.o

drivers/power/reset/mcf-rcm-reset.c

@@ -0,0 +1,126 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * Freescale ColdFire MCF5441x RCM power-on reason driver
+ *
+ * Copyright (C) 2026 Jean-Michel Hautbois <jeanmichel.hautbois@yoseli.org>
+ */
+
+#include <linux/io.h>
+#include <linux/mod_devicetable.h>
+#include <linux/module.h>
+#include <linux/platform_device.h>
+#include <linux/power/power_on_reason.h>
+
+/*
+ * Reset Status Register (RSR) layout, MCF54418 Reference Manual chapter
+ * 12.3.2. The register is 8-bit read-only and latches the cause (or
+ * causes) of the most recent reset until the next one occurs.
+ */
+#define MCF_RSR_SOFT		0x20	/* Last reset caused by software */
+#define MCF_RSR_LOC		0x10	/* Last reset caused by PLL loss of clock */
+#define MCF_RSR_POR		0x08	/* Last reset caused by power-on */
+#define MCF_RSR_EXT		0x04	/* Last reset caused by external pin */
+#define MCF_RSR_WDRCORE		0x02	/* Last reset caused by core watchdog */
+#define MCF_RSR_LOL		0x01	/* Last reset caused by PLL loss of lock */
+
+#define MCF_RSR_KNOWN_CAUSES	(MCF_RSR_POR | MCF_RSR_EXT | MCF_RSR_WDRCORE | \
+				 MCF_RSR_LOC | MCF_RSR_LOL | MCF_RSR_SOFT)
+
+struct mcf_rcm {
+	const char *reason;
+};
+
+/*
+ * Decode RSR into a power_on_reason string.
+ *
+ * The MCF5441x Reset Status Register can latch several cause bits at the
+ * same time (Reference Manual chapter 12.3.2: "one or more status bits
+ * may be set at the same time"). A power-on, for example, also resets
+ * the PLL and may co-flag LOC and LOL during the boot sequence. The
+ * power_on_reason ABI carries a single string, so this routine picks
+ * one cause; the chosen priority surfaces the most explanatory one for
+ * diagnostics:
+ *
+ *   POR              cold boot dominates any spurious co-flagged cause
+ *   EXT              operator action via the RESET pin
+ *   WDRCORE          core watchdog timeout, a fault to investigate
+ *   LOC, LOL         PLL clock or lock failure, hardware fault
+ *   SOFT             explicit software-requested reset
+ */
+static const char *mcf_rcm_decode(u8 rsr)
+{
+	if (rsr & MCF_RSR_POR)
+		return POWER_ON_REASON_REGULAR;
+	if (rsr & MCF_RSR_EXT)
+		return POWER_ON_REASON_RST_BTN;
+	if (rsr & MCF_RSR_WDRCORE)
+		return POWER_ON_REASON_WATCHDOG;
+	if (rsr & (MCF_RSR_LOC | MCF_RSR_LOL))
+		return POWER_ON_REASON_CPU_CLK_FAIL;
+	if (rsr & MCF_RSR_SOFT)
+		return POWER_ON_REASON_SOFTWARE;
+	return POWER_ON_REASON_UNKNOWN;
+}
+
+static ssize_t power_on_reason_show(struct device *dev,
+				    struct device_attribute *attr, char *buf)
+{
+	struct mcf_rcm *rcm = platform_get_drvdata(to_platform_device(dev));
+
+	return sysfs_emit(buf, "%s\n", rcm->reason);
+}
+static DEVICE_ATTR_RO(power_on_reason);
+
+static struct attribute *mcf_rcm_attrs[] = {
+	&dev_attr_power_on_reason.attr,
+	NULL,
+};
+ATTRIBUTE_GROUPS(mcf_rcm);
+
+static int mcf_rcm_probe(struct platform_device *pdev)
+{
+	struct mcf_rcm *rcm;
+	void __iomem *base;
+	u8 rsr;
+
+	rcm = devm_kzalloc(&pdev->dev, sizeof(*rcm), GFP_KERNEL);
+	if (!rcm)
+		return -ENOMEM;
+
+	base = devm_platform_ioremap_resource(pdev, 0);
+	if (IS_ERR(base))
+		return PTR_ERR(base);
+
+	rsr = readb_relaxed(base);
+	rcm->reason = mcf_rcm_decode(rsr);
+
+	platform_set_drvdata(pdev, rcm);
+
+	if (!(rsr & MCF_RSR_KNOWN_CAUSES))
+		dev_warn(&pdev->dev, "Unknown reset cause (RSR=0x%02x)\n", rsr);
+	else
+		dev_info(&pdev->dev, "Starting after %s (RSR=0x%02x)\n",
+			 rcm->reason, rsr);
+
+	return 0;
+}
+
+static const struct platform_device_id mcf_rcm_id[] = {
+	{ "mcf-rcm-reset" },
+	{ }
+};
+MODULE_DEVICE_TABLE(platform, mcf_rcm_id);
+
+static struct platform_driver mcf_rcm_driver = {
+	.probe = mcf_rcm_probe,
+	.id_table = mcf_rcm_id,
+	.driver = {
+		.name = "mcf-rcm-reset",
+		.dev_groups = mcf_rcm_groups,
+	},
+};
+module_platform_driver(mcf_rcm_driver);
+
+MODULE_AUTHOR("Jean-Michel Hautbois <jeanmichel.hautbois@yoseli.org>");
+MODULE_DESCRIPTION("Freescale ColdFire RCM power-on reason driver");
+MODULE_LICENSE("GPL");