hwmon: add fan speed monitoring driver for Surface devices

Adds a driver that provides read only access to the fan speed for Microsoft
Surface Pro devices. The fan speed is always regulated by the EC and cannot
be influenced directly.

Signed-off-by: Ivor Wanders <ivor@iwanders.net>
Link: linux-surface/kernel#144
Patchset: surface-sam

Documentation/hwmon/index.rst

@@ -208,6 +208,7 @@ Hardware Monitoring Kernel Drivers
    smsc47m1
    sparx5-temp
    stpddc60
+   surface_fan
    sy7636a-hwmon
    tc654
    tc74

Documentation/hwmon/surface_fan.rst

@@ -0,0 +1,25 @@
+.. SPDX-License-Identifier: GPL-2.0-or-later
+
+Kernel driver surface_fan
+=========================
+
+Supported Devices:
+
+  * Microsoft Surface Pro 9
+
+Author: Ivor Wanders <ivor@iwanders.net>
+
+Description
+-----------
+
+This provides monitoring of the fan found in some Microsoft Surface Pro devices,
+like the Surface Pro 9. The fan is always controlled by the onboard controller.
+
+Sysfs interface
+---------------
+
+======================= ======= =========================================
+Name                    Perm    Description
+======================= ======= =========================================
+``fan1_input``          RO      Current fan speed in RPM.
+======================= ======= =========================================

MAINTAINERS

@@ -14576,6 +14576,14 @@ F:	Documentation/driver-api/surface_aggregator/clients/dtx.rst
 F:	drivers/platform/surface/surface_dtx.c
 F:	include/uapi/linux/surface_aggregator/dtx.h
 
+MICROSOFT SURFACE SENSOR FAN DRIVER
+M:	Maximilian Luz <luzmaximilian@gmail.com>
+M:	Ivor Wanders <ivor@iwanders.net>
+L:	linux-hwmon@vger.kernel.org
+S:	Maintained
+F:	Documentation/hwmon/surface_fan.rst
+F:	drivers/hwmon/surface_fan.c
+
 MICROSOFT SURFACE GPE LID SUPPORT DRIVER
 M:	Maximilian Luz <luzmaximilian@gmail.com>
 L:	platform-driver-x86@vger.kernel.org

drivers/hwmon/Kconfig

@@ -1994,6 +1994,19 @@ config SENSORS_SFCTEMP
 	  This driver can also be built as a module.  If so, the module
 	  will be called sfctemp.
 
+config SENSORS_SURFACE_FAN
+	tristate "Surface Fan Driver"
+	depends on SURFACE_AGGREGATOR
+	help
+	  Driver that provides monitoring of the fan on Surface Pro devices that
+	  have a fan, like the Surface Pro 9.
+
+	  This makes the fan's current speed accessible through the hwmon
+	  system. It does not provide control over the fan, the firmware is
+	  responsible for that, this driver merely provides monitoring.
+
+	  Select M or Y here, if you want to be able to read the fan's speed.
+
 config SENSORS_ADC128D818
 	tristate "Texas Instruments ADC128D818"
 	depends on I2C

drivers/hwmon/Makefile

@@ -201,6 +201,7 @@ obj-$(CONFIG_SENSORS_SMSC47M1)	+= smsc47m1.o
 obj-$(CONFIG_SENSORS_SMSC47M192)+= smsc47m192.o
 obj-$(CONFIG_SENSORS_SPARX5)	+= sparx5-temp.o
 obj-$(CONFIG_SENSORS_STTS751)	+= stts751.o
+obj-$(CONFIG_SENSORS_SURFACE_FAN)+= surface_fan.o
 obj-$(CONFIG_SENSORS_SY7636A)	+= sy7636a-hwmon.o
 obj-$(CONFIG_SENSORS_AMC6821)	+= amc6821.o
 obj-$(CONFIG_SENSORS_TC74)	+= tc74.o

drivers/hwmon/surface_fan.c

@@ -0,0 +1,93 @@
+// SPDX-License-Identifier: GPL-2.0+
+/*
+ * Surface Fan driver for Surface System Aggregator Module. It provides access
+ * to the fan's rpm through the hwmon system.
+ *
+ * Copyright (C) 2023 Ivor Wanders <ivor@iwanders.net>
+ */
+
+#include <linux/hwmon.h>
+#include <linux/kernel.h>
+#include <linux/module.h>
+#include <linux/surface_aggregator/device.h>
+#include <linux/types.h>
+
+// SSAM
+SSAM_DEFINE_SYNC_REQUEST_CL_R(__ssam_fan_rpm_get, __le16, {
+	.target_category = SSAM_SSH_TC_FAN,
+	.command_id      = 0x01,
+});
+
+// hwmon
+umode_t surface_fan_hwmon_is_visible(const void *drvdata,
+				     enum hwmon_sensor_types type, u32 attr,
+				     int channel)
+{
+	return 0444;
+}
+
+static int surface_fan_hwmon_read(struct device *dev,
+				  enum hwmon_sensor_types type, u32 attr,
+				  int channel, long *val)
+{
+	struct ssam_device *sdev = dev_get_drvdata(dev);
+	int ret;
+	__le16 value;
+
+	ret = __ssam_fan_rpm_get(sdev, &value);
+	if (ret)
+		return ret;
+
+	*val = le16_to_cpu(value);
+
+	return ret;
+}
+
+static const struct hwmon_channel_info *const surface_fan_info[] = {
+	HWMON_CHANNEL_INFO(fan, HWMON_F_INPUT),
+	NULL
+};
+
+static const struct hwmon_ops surface_fan_hwmon_ops = {
+	.is_visible = surface_fan_hwmon_is_visible,
+	.read = surface_fan_hwmon_read,
+};
+
+static const struct hwmon_chip_info surface_fan_chip_info = {
+	.ops = &surface_fan_hwmon_ops,
+	.info = surface_fan_info,
+};
+
+static int surface_fan_probe(struct ssam_device *sdev)
+{
+	struct device *hdev;
+
+	hdev = devm_hwmon_device_register_with_info(&sdev->dev,
+						    "surface_fan", sdev,
+						    &surface_fan_chip_info,
+						    NULL);
+	if (IS_ERR(hdev))
+		return PTR_ERR(hdev);
+
+	return 0;
+}
+
+static const struct ssam_device_id ssam_fan_match[] = {
+	{ SSAM_SDEV(FAN, SAM, 0x01, 0x01) },
+	{},
+};
+MODULE_DEVICE_TABLE(ssam, ssam_fan_match);
+
+static struct ssam_device_driver surface_fan = {
+	.probe = surface_fan_probe,
+	.match_table = ssam_fan_match,
+	.driver = {
+		.name = "surface_fan",
+		.probe_type = PROBE_PREFER_ASYNCHRONOUS,
+	},
+};
+module_ssam_device_driver(surface_fan);
+
+MODULE_AUTHOR("Ivor Wanders <ivor@iwanders.net>");
+MODULE_DESCRIPTION("Fan Driver for Surface System Aggregator Module");
+MODULE_LICENSE("GPL");