[PATCH v2] hwmon: add driver for ARCTIC Fan Controller

[Aureo Serrano de Souza]

Add hwmon driver for the ARCTIC Fan Controller (USB HID VID 0x3904,
PID 0xF001) with 10 fan channels. Exposes fan RPM and PWM via sysfs.
Device pushes IN reports at ~1 Hz; PWM is set via interrupt OUT reports.

Fan speed control is manual-only: the device does not change PWM
autonomously. After applying an OUT report, the device sends back a
2-byte ACK (Report ID 0x02); the driver waits up to 1 s for this ACK
using a completion.

The report buffer is kmalloc'd per write for two reasons:

1. Stack placement (suggested by reviewers with __aligned()) was tested
   but hard-rejected by usb_hcd_map_urb_for_dma(), which calls
   object_is_on_stack() and returns -EAGAIN regardless of alignment
   attributes. Confirmed on AMD X670E with AMD-Vi (IOMMU) enabled.

2. Struct-embedded placement (suggested as an alternative) would create
   a shared resource between concurrent arctic_fan_write() calls, causing
   a race if two users write different PWM channels simultaneously.

kmalloc per write avoids both problems: the buffer is heap-allocated
(passes DMA checks) and private to each write call (no sharing).

Signed-off-by: Aureo Serrano de Souza <aureo.serrano@xxxxxxxxx>
---
Hi,

Thanks for the detailed review. Here is v2 addressing your feedback.
If I missed anything, please let me know.

Changes since v1:
- Remove module_param debug flag; use hid_dbg() / dyndbg instead
- Move hid_device_id table next to hid_driver struct
- Replace spinlock_irqsave with mutex for fan_rpm/pwm_duty; use a
  lightweight spinlock only to protect the completion signal shared with
  raw_event() which runs in BH/softirq context
- Use guard() from cleanup.h to eliminate manual unlock on error paths
- Use get_unaligned_le16() for RPM parsing instead of manual byte shift
- Remove all impossible bounds/NULL checks that the hwmon core never triggers
- Remove retry loop; use a single hid_hw_output_report() call
- Add hid_is_usb() guard in probe
- Remove hwmon_dev field from struct; use local variable only
- Remove VID/PID from MODULE_DESCRIPTION
- Fix: do not update pwm_duty from IN reports; device is manual-only so
  the host cache is the authoritative PWM source. This eliminates the race
  where an incoming report could overwrite a value just written.
- Add completion/ACK mechanism: wait for device ACK after OUT report,
  signalled from raw_event() via complete(); return device ACK status
- Report buffer is kmalloc'd per write; see commit message for full
  rationale. In summary: stack placement (suggested with __aligned()) is
  hard-rejected by usb_hcd_map_urb_for_dma() via object_is_on_stack();
  struct-embedded placement (suggested as alternative) creates a shared
  resource race between concurrent writes. kmalloc per write avoids both.
- Add Documentation/hwmon/arctic_fan_controller.rst to index.rst
- Add MAINTAINERS entry

 Documentation/hwmon/arctic_fan_controller.rst |  40 +++
 Documentation/hwmon/index.rst                 |   1 +
 MAINTAINERS                                   |   7 +
 drivers/hwmon/Kconfig                         |  12 +
 drivers/hwmon/Makefile                        |   1 +
 drivers/hwmon/arctic_fan_controller.c         | 278 ++++++++++++++++++
 6 files changed, 339 insertions(+)
 create mode 100644 Documentation/hwmon/arctic_fan_controller.rst
 create mode 100644 drivers/hwmon/arctic_fan_controller.c

diff --git a/Documentation/hwmon/arctic_fan_controller.rst b/Documentation/hwmon/arctic_fan_controller.rst
new file mode 100644
index 000000000..58366551c
--- /dev/null
+++ b/Documentation/hwmon/arctic_fan_controller.rst
@@ -0,0 +1,40 @@
+.. SPDX-License-Identifier: GPL-2.0-or-later
+
+Kernel driver arctic_fan_controller
+=====================================
+
+Supported devices:
+
+* ARCTIC Fan Controller (USB HID, VID 0x3904, PID 0xF001)
+
+Author: Aureo Serrano de Souza <aureo.serrano@xxxxxxxxx>
+
+Description
+-----------
+
+This driver provides hwmon support for the ARCTIC Fan Controller, a USB
+Custom HID device with 10 fan channels. The device sends IN reports about
+once per second containing current RPM values (bytes 11-30, 10 x uint16 LE).
+Fan speed control is manual-only: the device does not change PWM
+autonomously; it only applies a new duty cycle when it receives an OUT
+report from the host.
+
+After the device applies an OUT report, it sends back a 2-byte ACK IN
+report (Report ID 0x02, byte 1 = 0x00 on success) confirming the command
+was applied.
+
+Usage notes
+-----------
+
+Since it is a USB device, hotplug is supported. The device is autodetected.
+
+Sysfs entries
+-------------
+
+================ ==============================================================
+fan[1-10]_input  Fan speed in RPM (read-only). Updated from IN reports at ~1 Hz.
+pwm[1-10]        PWM duty cycle (0-255). Write: sends an OUT report setting the
+                 duty cycle (scaled from 0-255 to 0-100% for the device). Read:
+                 returns the last value written; the device is manual-only so
+                 the host cache is authoritative.
+================ ==============================================================
diff --git a/Documentation/hwmon/index.rst b/Documentation/hwmon/index.rst
index b2ca8513c..c34713040 100644
--- a/Documentation/hwmon/index.rst
+++ b/Documentation/hwmon/index.rst
@@ -42,6 +42,7 @@ Hardware Monitoring Kernel Drivers
    aht10
    amc6821
    aquacomputer_d5next
+   arctic_fan_controller
    asb100
    asc7621
    aspeed-g6-pwm-tach
diff --git a/MAINTAINERS b/MAINTAINERS
index 837db4f7b..8c32dddd6 100644
--- a/MAINTAINERS
+++ b/MAINTAINERS
@@ -2053,6 +2053,13 @@ S:	Maintained
 F:	drivers/net/arcnet/
 F:	include/uapi/linux/if_arcnet.h
 
+ARCTIC FAN CONTROLLER DRIVER
+M:	Aureo Serrano de Souza <aureo.serrano@xxxxxxxxx>
+L:	linux-hwmon@xxxxxxxxxxxxxxx
+S:	Maintained
+F:	Documentation/hwmon/arctic_fan_controller.rst
+F:	drivers/hwmon/arctic_fan_controller.c
+
 ARM AND ARM64 SoC SUB-ARCHITECTURES (COMMON PARTS)
 M:	Arnd Bergmann <arnd@xxxxxxxx>
 M:	Krzysztof Kozlowski <krzk@xxxxxxxxxx>
diff --git a/drivers/hwmon/Kconfig b/drivers/hwmon/Kconfig
index 486152a8e..b6ab72752 100644
--- a/drivers/hwmon/Kconfig
+++ b/drivers/hwmon/Kconfig
@@ -388,6 +388,18 @@ config SENSORS_APPLESMC
 	  Say Y here if you have an applicable laptop and want to experience
 	  the awesome power of applesmc.
 
+config SENSORS_ARCTIC_FAN_CONTROLLER
+	tristate "ARCTIC Fan Controller"
+	depends on HID
+	help
+	  If you say yes here you get support for the ARCTIC Fan Controller,
+	  a USB HID device (VID 0x3904, PID 0xF001) with 10 fan channels.
+	  The driver exposes fan speed (RPM) and PWM control via the hwmon
+	  sysfs interface.
+
+	  This driver can also be built as a module. If so, the module
+	  will be called arctic_fan_controller.
+
 config SENSORS_ARM_SCMI
 	tristate "ARM SCMI Sensors"
 	depends on ARM_SCMI_PROTOCOL
diff --git a/drivers/hwmon/Makefile b/drivers/hwmon/Makefile
index 5833c807c..ef831c337 100644
--- a/drivers/hwmon/Makefile
+++ b/drivers/hwmon/Makefile
@@ -49,6 +49,7 @@ obj-$(CONFIG_SENSORS_ADT7475)	+= adt7475.o
 obj-$(CONFIG_SENSORS_AHT10)	+= aht10.o
 obj-$(CONFIG_SENSORS_APPLESMC)	+= applesmc.o
 obj-$(CONFIG_SENSORS_AQUACOMPUTER_D5NEXT) += aquacomputer_d5next.o
+obj-$(CONFIG_SENSORS_ARCTIC_FAN_CONTROLLER)	+= arctic_fan_controller.o
 obj-$(CONFIG_SENSORS_ARM_SCMI)	+= scmi-hwmon.o
 obj-$(CONFIG_SENSORS_ARM_SCPI)	+= scpi-hwmon.o
 obj-$(CONFIG_SENSORS_AS370)	+= as370-hwmon.o
diff --git a/drivers/hwmon/arctic_fan_controller.c b/drivers/hwmon/arctic_fan_controller.c
new file mode 100644
index 000000000..24db8a7ee
--- /dev/null
+++ b/drivers/hwmon/arctic_fan_controller.c
@@ -0,0 +1,278 @@
+// SPDX-License-Identifier: GPL-2.0-or-later
+/*
+ * Linux hwmon driver for ARCTIC Fan Controller
+ *
+ * USB Custom HID device (VID 0x3904, PID 0xF001) with 10 fan channels.
+ * Exposes fan RPM (input) and PWM (0-255) via hwmon. Device pushes IN reports
+ * at ~1 Hz; no GET_REPORT. OUT reports set PWM duty (bytes 1-10, 0-100%).
+ * PWM is manual-only: the device does not change duty autonomously, only
+ * when it receives an OUT report from the host.
+ */
+
+#include <linux/cleanup.h>
+#include <linux/completion.h>
+#include <linux/err.h>
+#include <linux/hid.h>
+#include <linux/hwmon.h>
+#include <linux/hwmon-sysfs.h>
+#include <linux/jiffies.h>
+#include <linux/kernel.h>
+#include <linux/module.h>
+#include <linux/mutex.h>
+#include <linux/slab.h>
+#include <linux/spinlock.h>
+#include <linux/unaligned.h>
+
+#define ARCTIC_VID		0x3904
+#define ARCTIC_PID		0xF001
+#define ARCTIC_NUM_FANS		10
+#define ARCTIC_REPORT_ID	0x01
+#define ARCTIC_REPORT_LEN	32
+#define ARCTIC_RPM_OFFSET	11	/* bytes 11-30: 10 x uint16 LE */
+/* ACK report: device sends Report ID 0x02, 2 bytes (ID + status) after applying OUT report */
+#define ARCTIC_ACK_REPORT_ID	0x02
+#define ARCTIC_ACK_REPORT_LEN	2
+/* Time to wait for ACK report after send */
+#define ARCTIC_ACK_TIMEOUT_MS	1000
+
+struct arctic_fan_data {
+	struct hid_device *hdev;
+	struct mutex lock;		/* protects fan_rpm, pwm_duty */
+	spinlock_t in_report_lock;	/* protects ack_status and in_report_received */
+	struct completion in_report_received; /* ACK (ID 0x02) received in raw_event */
+	int ack_status;			/* 0 = OK, negative errno on device error */
+	u32 fan_rpm[ARCTIC_NUM_FANS];
+	u8 pwm_duty[ARCTIC_NUM_FANS];	/* 0-255 matching sysfs range; converted to 0-100 on send */
+};
+
+/*
+ * Parse RPM values from the periodic status report (10 x uint16 LE at rpm_off).
+ * pwm_duty is not updated from the report: the device is manual-only, so the
+ * host cache is the authoritative source for PWM.
+ */
+static void arctic_fan_parse_report(struct arctic_fan_data *priv, u8 *buf,
+				    int len, int rpm_off)
+{
+	int i;
+
+	if (len < rpm_off + 20)
+		return;
+	{
+		guard(mutex)(&priv->lock);
+		for (i = 0; i < ARCTIC_NUM_FANS; i++)
+			priv->fan_rpm[i] = get_unaligned_le16(&buf[rpm_off + i * 2]);
+	}
+}
+
+/*
+ * raw_event: IN reports.
+ *
+ * Status report: Report ID 0x01, 32 bytes:
+ *   byte 0 = report ID, bytes 1-10 = PWM 0-100%, bytes 11-30 = 10 x RPM uint16 LE.
+ *   Device pushes these at ~1 Hz; no GET_REPORT.
+ *
+ * ACK report: Report ID 0x02, 2 bytes:
+ *   byte 0 = 0x02, byte 1 = status (0x00 = OK, 0x01 = ERROR).
+ *   Sent once after accepting and applying an OUT report (ID 0x01).
+ */
+static int arctic_fan_raw_event(struct hid_device *hdev,
+				struct hid_report *report, u8 *data, int size)
+{
+	struct arctic_fan_data *priv = hid_get_drvdata(hdev);
+
+	hid_dbg(hdev, "arctic_fan: raw_event id=%u size=%d\n", report->id, size);
+
+	if (report->id == ARCTIC_ACK_REPORT_ID && size == ARCTIC_ACK_REPORT_LEN) {
+		spin_lock(&priv->in_report_lock);
+		priv->ack_status = data[1] == 0x00 ? 0 : -EIO;
+		complete(&priv->in_report_received);
+		spin_unlock(&priv->in_report_lock);
+		return 0;
+	}
+
+	if (report->id != ARCTIC_REPORT_ID || size != ARCTIC_REPORT_LEN) {
+		hid_dbg(hdev, "arctic_fan: raw_event id=%u size=%d ignored\n",
+			report->id, size);
+		return 0;
+	}
+
+	arctic_fan_parse_report(priv, data, size, ARCTIC_RPM_OFFSET);
+	return 0;
+}
+
+static umode_t arctic_fan_is_visible(const void *data,
+				     enum hwmon_sensor_types type,
+				     u32 attr, int channel)
+{
+	if (type == hwmon_fan && attr == hwmon_fan_input)
+		return 0444;
+	if (type == hwmon_pwm && attr == hwmon_pwm_input)
+		return 0644;
+	return 0;
+}
+
+static int arctic_fan_read(struct device *dev, enum hwmon_sensor_types type,
+			   u32 attr, int channel, long *val)
+{
+	struct arctic_fan_data *priv = dev_get_drvdata(dev);
+
+	{
+		guard(mutex)(&priv->lock);
+		if (type == hwmon_fan && attr == hwmon_fan_input) {
+			*val = priv->fan_rpm[channel];
+			return 0;
+		}
+		if (type == hwmon_pwm && attr == hwmon_pwm_input) {
+			*val = priv->pwm_duty[channel];
+			return 0;
+		}
+		return -EINVAL;
+	}
+}
+
+static int arctic_fan_write(struct device *dev, enum hwmon_sensor_types type,
+			    u32 attr, int channel, long val)
+{
+	struct arctic_fan_data *priv = dev_get_drvdata(dev);
+	u8 *buf;
+	long t;
+	int i, ret;
+
+	/*
+	 * Must use a heap-allocated buffer: usb_hcd_map_urb_for_dma() rejects
+	 * stack buffers with -EAGAIN, preventing USB transfers on DMA-capable
+	 * host controllers.
+	 */
+	buf = kmalloc(ARCTIC_REPORT_LEN, GFP_KERNEL);
+	if (!buf)
+		return -ENOMEM;
+
+	{
+		guard(mutex)(&priv->lock);
+		priv->pwm_duty[channel] = (u8)val;
+		buf[0] = ARCTIC_REPORT_ID;
+		for (i = 0; i < ARCTIC_NUM_FANS; i++)
+			buf[1 + i] = DIV_ROUND_CLOSEST((unsigned int)priv->pwm_duty[i] * 100, 255);
+	}
+	memset(buf + 11, 0, ARCTIC_REPORT_LEN - 11);
+
+	spin_lock_bh(&priv->in_report_lock);
+	priv->ack_status = -ETIMEDOUT;
+	reinit_completion(&priv->in_report_received);
+	spin_unlock_bh(&priv->in_report_lock);
+
+	ret = hid_hw_output_report(priv->hdev, buf, ARCTIC_REPORT_LEN);
+	kfree(buf);
+	if (ret < 0)
+		return ret;
+
+	t = wait_for_completion_interruptible_timeout(&priv->in_report_received,
+						      msecs_to_jiffies(ARCTIC_ACK_TIMEOUT_MS));
+	if (t == 0)
+		return -ETIMEDOUT;
+	if (t < 0)
+		return t; /* interrupted by signal */
+	return priv->ack_status; /* 0=OK, -EIO=device error */
+}
+
+static const struct hwmon_ops arctic_fan_ops = {
+	.is_visible = arctic_fan_is_visible,
+	.read = arctic_fan_read,
+	.write = arctic_fan_write,
+};
+
+static const struct hwmon_channel_info *arctic_fan_info[] = {
+	HWMON_CHANNEL_INFO(fan,
+			   HWMON_F_INPUT, HWMON_F_INPUT, HWMON_F_INPUT,
+			   HWMON_F_INPUT, HWMON_F_INPUT, HWMON_F_INPUT,
+			   HWMON_F_INPUT, HWMON_F_INPUT, HWMON_F_INPUT,
+			   HWMON_F_INPUT),
+	HWMON_CHANNEL_INFO(pwm,
+			   HWMON_PWM_INPUT, HWMON_PWM_INPUT, HWMON_PWM_INPUT,
+			   HWMON_PWM_INPUT, HWMON_PWM_INPUT, HWMON_PWM_INPUT,
+			   HWMON_PWM_INPUT, HWMON_PWM_INPUT, HWMON_PWM_INPUT,
+			   HWMON_PWM_INPUT),
+	NULL
+};
+
+static const struct hwmon_chip_info arctic_fan_chip_info = {
+	.ops = &arctic_fan_ops,
+	.info = arctic_fan_info,
+};
+
+static int arctic_fan_probe(struct hid_device *hdev,
+			    const struct hid_device_id *id)
+{
+	struct arctic_fan_data *priv;
+	struct device *hwmon_dev;
+	int ret;
+
+	if (!hid_is_usb(hdev))
+		return -ENODEV;
+
+	ret = hid_parse(hdev);
+	if (ret)
+		return ret;
+
+	priv = devm_kzalloc(&hdev->dev, sizeof(*priv), GFP_KERNEL);
+	if (!priv)
+		return -ENOMEM;
+
+	priv->hdev = hdev;
+	mutex_init(&priv->lock);
+	spin_lock_init(&priv->in_report_lock);
+	init_completion(&priv->in_report_received);
+	hid_set_drvdata(hdev, priv);
+
+	ret = hid_hw_start(hdev, HID_CONNECT_DRIVER);
+	if (ret)
+		return ret;
+
+	ret = hid_hw_open(hdev);
+	if (ret)
+		goto out_stop;
+
+	hid_device_io_start(hdev);
+
+	hwmon_dev = devm_hwmon_device_register_with_info(&hdev->dev, "arctic_fan",
+							 priv, &arctic_fan_chip_info,
+							 NULL);
+	if (IS_ERR(hwmon_dev)) {
+		ret = PTR_ERR(hwmon_dev);
+		goto out_close;
+	}
+	return 0;
+
+out_close:
+	hid_hw_close(hdev);
+out_stop:
+	hid_hw_stop(hdev);
+	return ret;
+}
+
+static void arctic_fan_remove(struct hid_device *hdev)
+{
+	hid_device_io_stop(hdev);
+	hid_hw_close(hdev);
+	hid_hw_stop(hdev);
+}
+
+static const struct hid_device_id arctic_fan_id_table[] = {
+	{ HID_USB_DEVICE(ARCTIC_VID, ARCTIC_PID) },
+	{ }
+};
+MODULE_DEVICE_TABLE(hid, arctic_fan_id_table);
+
+static struct hid_driver arctic_fan_driver = {
+	.name = "arctic_fan",
+	.id_table = arctic_fan_id_table,
+	.probe = arctic_fan_probe,
+	.remove = arctic_fan_remove,
+	.raw_event = arctic_fan_raw_event,
+};
+
+module_hid_driver(arctic_fan_driver);
+
+MODULE_AUTHOR("Aureo Serrano de Souza <aureo.serrano@xxxxxxxxx>");
+MODULE_DESCRIPTION("HID hwmon driver for ARCTIC Fan Controller");
+MODULE_LICENSE("GPL");
-- 
2.43.0