power: reset: at91-reset: add sysfs interface to the power on reason

Introduce a list of generic reset sources and use them to export the
power on reason through sysfs. Update the ABI documentation to describe
this new interface.

Signed-off-by: Kamel Bouhara <[e-mail]>
Acked-by: Nicolas Ferre <[e-mail]>
[Miquel Raynal: Follow-up on Kamel's work, 4 years later]
Signed-off-by: Miquel Raynal <[e-mail]>
Signed-off-by: Sebastian Reichel <[e-mail]>

Documentation/ABI/testing/sysfs-platform-power-on-reason

···
+What:		/sys/devices/platform/.../power_on_reason
+Date:		June 2023
+KernelVersion:	6.5
+Contact:	Kamel Bouhara <[e-mail]>
+Description:	Shows system power on reason. The following strings/reasons can
+		be read (the list can be extended):
+		"regular power-up", "RTC wakeup", "watchdog timeout",
+		"software reset", "reset button action", "CPU clock failure",
+		"crystal oscillator failure", "brown-out reset",
+		"unknown reason".
+
+		The file is read only.

drivers/power/reset/at91-reset.c

···
 #include <linux/platform_device.h>
 #include <linux/reboot.h>
 #include <linux/reset-controller.h>
+#include <linux/power/power_on_reason.h>
 
 #include <soc/at91/at91sam9_ddrsdr.h>
 #include <soc/at91/at91sam9_sdramc.h>
···
 	return NOTIFY_DONE;
 }
 
-static const char * __init at91_reset_reason(struct at91_reset *reset)
+static const char *at91_reset_reason(struct at91_reset *reset)
 {
 	u32 reg = readl(reset->rstc_base + AT91_RSTC_SR);
 	const char *reason;
 
 	switch ((reg & AT91_RSTC_RSTTYP) >> 8) {
 	case RESET_TYPE_GENERAL:
-		reason = "general reset";
+		reason = POWER_ON_REASON_REGULAR;
 		break;
 	case RESET_TYPE_WAKEUP:
-		reason = "wakeup";
+		reason = POWER_ON_REASON_RTC;
 		break;
 	case RESET_TYPE_WATCHDOG:
-		reason = "watchdog reset";
+		reason = POWER_ON_REASON_WATCHDOG;
 		break;
 	case RESET_TYPE_SOFTWARE:
-		reason = "software reset";
+		reason = POWER_ON_REASON_SOFTWARE;
 		break;
 	case RESET_TYPE_USER:
-		reason = "user reset";
+		reason = POWER_ON_REASON_RST_BTN;
 		break;
 	case RESET_TYPE_CPU_FAIL:
-		reason = "CPU clock failure detection";
+		reason = POWER_ON_REASON_CPU_CLK_FAIL;
 		break;
 	case RESET_TYPE_XTAL_FAIL:
-		reason = "32.768 kHz crystal failure detection";
+		reason = POWER_ON_REASON_XTAL_FAIL;
 		break;
 	case RESET_TYPE_ULP2:
-		reason = "ULP2 reset";
+		reason = POWER_ON_REASON_BROWN_OUT;
 		break;
 	default:
-		reason = "unknown reset";
+		reason = POWER_ON_REASON_UNKNOWN;
 		break;
 	}
 
 	return reason;
 }
+
+static ssize_t power_on_reason_show(struct device *dev,
+				    struct device_attribute *attr, char *buf)
+{
+	struct platform_device *pdev = to_platform_device(dev);
+	struct at91_reset *reset = platform_get_drvdata(pdev);
+
+	return sprintf(buf, "%s\n", at91_reset_reason(reset));
+}
+static DEVICE_ATTR_RO(power_on_reason);
 
 static const struct of_device_id at91_ramc_of_match[] = {
 	{
···
 	ret = register_restart_handler(&reset->nb);
 	if (ret)
 		goto disable_clk;
+
+	ret = device_create_file(&pdev->dev, &dev_attr_power_on_reason);
+	if (ret) {
+		dev_err(&pdev->dev, "Could not create sysfs entry\n");
+		return ret;
+	}
 
 	dev_info(&pdev->dev, "Starting after %s\n", at91_reset_reason(reset));
 

include/linux/power/power_on_reason.h

···
+/* SPDX-License-Identifier: GPL-2.0-only */
+/*
+ * Author: Kamel Bouhra <[e-mail]>
+ */
+
+#ifndef POWER_ON_REASON_H
+#define POWER_ON_REASON_H
+
+#define POWER_ON_REASON_REGULAR "regular power-up"
+#define POWER_ON_REASON_RTC "RTC wakeup"
+#define POWER_ON_REASON_WATCHDOG "watchdog timeout"
+#define POWER_ON_REASON_SOFTWARE "software reset"
+#define POWER_ON_REASON_RST_BTN "reset button action"
+#define POWER_ON_REASON_CPU_CLK_FAIL "CPU clock failure"
+#define POWER_ON_REASON_XTAL_FAIL "crystal oscillator failure"
+#define POWER_ON_REASON_BROWN_OUT "brown-out reset"
+#define POWER_ON_REASON_UNKNOWN "unknown reason"
+
+#endif /* POWER_ON_REASON_H */