hwmon: Add support for RPi voltage sensor

Currently there is no easy way to detect undervoltage conditions on a
remote Raspberry Pi. This hwmon driver retrieves the state of the
undervoltage sensor via mailbox interface. The handling based on
Noralf's modifications to the downstream firmware driver. In case of
an undervoltage condition only an entry is written to the kernel log.

CC: "Noralf Trønnes" <noralf@tronnes.org>
Signed-off-by: Stefan Wahren <stefan.wahren@i2se.com>
Signed-off-by: Eric Anholt <eric@anholt.net>
Acked-by: Guenter Roeck <linux@roeck-us.net>

Documentation/hwmon/raspberrypi-hwmon

@@ -0,0 +1,22 @@
+Kernel driver raspberrypi-hwmon
+===============================
+
+Supported boards:
+  * Raspberry Pi A+ (via GPIO on SoC)
+  * Raspberry Pi B+ (via GPIO on SoC)
+  * Raspberry Pi 2 B (via GPIO on SoC)
+  * Raspberry Pi 3 B (via GPIO on port expander)
+  * Raspberry Pi 3 B+ (via PMIC)
+
+Author: Stefan Wahren <stefan.wahren@i2se.com>
+
+Description
+-----------
+
+This driver periodically polls a mailbox property of the VC4 firmware to detect
+undervoltage conditions.
+
+Sysfs entries
+-------------
+
+in0_lcrit_alarm		Undervoltage alarm

drivers/hwmon/Kconfig

@@ -1298,6 +1298,16 @@ config SENSORS_PWM_FAN
 	  This driver can also be built as a module.  If so, the module
 	  will be called pwm-fan.
 
+config SENSORS_RASPBERRYPI_HWMON
+	tristate "Raspberry Pi voltage monitor"
+	depends on RASPBERRYPI_FIRMWARE || (COMPILE_TEST && !RASPBERRYPI_FIRMWARE)
+	help
+	  If you say yes here you get support for voltage sensor on the
+	  Raspberry Pi.
+
+	  This driver can also be built as a module. If so, the module
+	  will be called raspberrypi-hwmon.
+
 config SENSORS_SHT15
 	tristate "Sensiron humidity and temperature sensors. SHT15 and compat."
 	depends on GPIOLIB || COMPILE_TEST

drivers/hwmon/Makefile

@@ -141,6 +141,7 @@ obj-$(CONFIG_SENSORS_PC87427)	+= pc87427.o
 obj-$(CONFIG_SENSORS_PCF8591)	+= pcf8591.o
 obj-$(CONFIG_SENSORS_POWR1220)  += powr1220.o
 obj-$(CONFIG_SENSORS_PWM_FAN)	+= pwm-fan.o
+obj-$(CONFIG_SENSORS_RASPBERRYPI_HWMON)	+= raspberrypi-hwmon.o
 obj-$(CONFIG_SENSORS_S3C)	+= s3c-hwmon.o
 obj-$(CONFIG_SENSORS_SCH56XX_COMMON)+= sch56xx-common.o
 obj-$(CONFIG_SENSORS_SCH5627)	+= sch5627.o

drivers/hwmon/raspberrypi-hwmon.c

@@ -0,0 +1,166 @@
+// SPDX-License-Identifier: GPL-2.0+
+/*
+ * Raspberry Pi voltage sensor driver
+ *
+ * Based on firmware/raspberrypi.c by Noralf Trønnes
+ *
+ * Copyright (C) 2018 Stefan Wahren <stefan.wahren@i2se.com>
+ */
+#include <linux/device.h>
+#include <linux/err.h>
+#include <linux/hwmon.h>
+#include <linux/module.h>
+#include <linux/platform_device.h>
+#include <linux/slab.h>
+#include <linux/workqueue.h>
+#include <soc/bcm2835/raspberrypi-firmware.h>
+
+#define UNDERVOLTAGE_STICKY_BIT	BIT(16)
+
+struct rpi_hwmon_data {
+	struct device *hwmon_dev;
+	struct rpi_firmware *fw;
+	u32 last_throttled;
+	struct delayed_work get_values_poll_work;
+};
+
+static void rpi_firmware_get_throttled(struct rpi_hwmon_data *data)
+{
+	u32 new_uv, old_uv, value;
+	int ret;
+
+	/* Request firmware to clear sticky bits */
+	value = 0xffff;
+
+	ret = rpi_firmware_property(data->fw, RPI_FIRMWARE_GET_THROTTLED,
+				    &value, sizeof(value));
+	if (ret) {
+		dev_err_once(data->hwmon_dev, "Failed to get throttled (%d)\n",
+			     ret);
+		return;
+	}
+
+	new_uv = value & UNDERVOLTAGE_STICKY_BIT;
+	old_uv = data->last_throttled & UNDERVOLTAGE_STICKY_BIT;
+	data->last_throttled = value;
+
+	if (new_uv == old_uv)
+		return;
+
+	if (new_uv)
+		dev_crit(data->hwmon_dev, "Undervoltage detected!\n");
+	else
+		dev_info(data->hwmon_dev, "Voltage normalised\n");
+
+	sysfs_notify(&data->hwmon_dev->kobj, NULL, "in0_lcrit_alarm");
+}
+
+static void get_values_poll(struct work_struct *work)
+{
+	struct rpi_hwmon_data *data;
+
+	data = container_of(work, struct rpi_hwmon_data,
+			    get_values_poll_work.work);
+
+	rpi_firmware_get_throttled(data);
+
+	/*
+	 * We can't run faster than the sticky shift (100ms) since we get
+	 * flipping in the sticky bits that are cleared.
+	 */
+	schedule_delayed_work(&data->get_values_poll_work, 2 * HZ);
+}
+
+static int rpi_read(struct device *dev, enum hwmon_sensor_types type,
+		    u32 attr, int channel, long *val)
+{
+	struct rpi_hwmon_data *data = dev_get_drvdata(dev);
+
+	*val = !!(data->last_throttled & UNDERVOLTAGE_STICKY_BIT);
+	return 0;
+}
+
+static umode_t rpi_is_visible(const void *_data, enum hwmon_sensor_types type,
+			      u32 attr, int channel)
+{
+	return 0444;
+}
+
+static const u32 rpi_in_config[] = {
+	HWMON_I_LCRIT_ALARM,
+	0
+};
+
+static const struct hwmon_channel_info rpi_in = {
+	.type = hwmon_in,
+	.config = rpi_in_config,
+};
+
+static const struct hwmon_channel_info *rpi_info[] = {
+	&rpi_in,
+	NULL
+};
+
+static const struct hwmon_ops rpi_hwmon_ops = {
+	.is_visible = rpi_is_visible,
+	.read = rpi_read,
+};
+
+static const struct hwmon_chip_info rpi_chip_info = {
+	.ops = &rpi_hwmon_ops,
+	.info = rpi_info,
+};
+
+static int rpi_hwmon_probe(struct platform_device *pdev)
+{
+	struct device *dev = &pdev->dev;
+	struct rpi_hwmon_data *data;
+	int ret;
+
+	data = devm_kzalloc(dev, sizeof(*data), GFP_KERNEL);
+	if (!data)
+		return -ENOMEM;
+
+	/* Parent driver assure that firmware is correct */
+	data->fw = dev_get_drvdata(dev->parent);
+
+	/* Init throttled */
+	ret = rpi_firmware_property(data->fw, RPI_FIRMWARE_GET_THROTTLED,
+				    &data->last_throttled,
+				    sizeof(data->last_throttled));
+
+	data->hwmon_dev = devm_hwmon_device_register_with_info(dev, "rpi_volt",
+							       data,
+							       &rpi_chip_info,
+							       NULL);
+
+	INIT_DELAYED_WORK(&data->get_values_poll_work, get_values_poll);
+	platform_set_drvdata(pdev, data);
+
+	if (!PTR_ERR_OR_ZERO(data->hwmon_dev))
+		schedule_delayed_work(&data->get_values_poll_work, 2 * HZ);
+
+	return PTR_ERR_OR_ZERO(data->hwmon_dev);
+}
+
+static int rpi_hwmon_remove(struct platform_device *pdev)
+{
+	struct rpi_hwmon_data *data = platform_get_drvdata(pdev);
+
+	cancel_delayed_work_sync(&data->get_values_poll_work);
+
+	return 0;
+}
+
+static struct platform_driver rpi_hwmon_driver = {
+	.probe = rpi_hwmon_probe,
+	.remove = rpi_hwmon_remove,
+	.driver = {
+		.name = "raspberrypi-hwmon",
+	},
+};
+module_platform_driver(rpi_hwmon_driver);
+
+MODULE_AUTHOR("Stefan Wahren <stefan.wahren@i2se.com>");
+MODULE_DESCRIPTION("Raspberry Pi voltage sensor driver");
+MODULE_LICENSE("GPL v2");