Add hwmon driver for the ARCTIC Fan Controller, a USB HID device
(VID 0x3904, PID 0xF001) with 10 fan channels. Exposes fan speed in
RPM (read-only) and PWM duty cycle (0-255, read/write) via sysfs.
The device pushes IN reports at ~1 Hz containing RPM readings. PWM is
set via OUT reports; the device applies the new duty cycle and sends
back a 2-byte ACK (Report ID 0x02). The driver waits up to 1 s for
the ACK using a completion. Measured device latency: max ~563 ms over
500 iterations. PWM control is manual-only: the device never changes
duty cycle autonomously.
raw_event() may run in hardirq context, so fan_rpm[] is protected by
a spinlock with irq-save. pwm_duty[] and the report buffer are
serialized by the hwmon core, which holds its lock for the duration of
the read/write callbacks.
Signed-off-by: Aureo Serrano de Souza <aureo.serrano@arctic.de>
---
Thanks to Guenter Roeck and Thomas Weißschuh for the reviews.
Changes since v4:
- arctic_fan_write(): switch to wait_for_completion_timeout() (non-
interruptible); eliminates the signal-interrupted write case of the
late-ACK race that write_pending could not fully prevent
- arctic_fan_write(): guard pwm_duty[channel] commit with
ack_status == 0 check; a device error ACK (status 0x01) no longer
silently poisons the cached duty used in future OUT reports
- arctic_fan_probe()/remove(): replace devm_add_action_or_reset() +
no-op remove() with explicit hid_device_io_stop/hid_hw_close/
hid_hw_stop in remove(); devm_add_action_or_reset() was called after
hdev->driver = NULL, causing a NULL deref in hid_hw_close() on unbind
- add reset_resume callback: device resets PWM to hardware defaults on
power loss during suspend; driver now clears cached pwm_duty[] on
reset-resume so stale pre-suspend values are not re-sent as if valid
- Documentation/hwmon/arctic_fan_controller.rst: document suspend/
resume behaviour and the updated pwm[1-10] read semantics
Changes since v3:
- buf[]: upgrade from __aligned(8) to ____cacheline_aligned so the
DMA buffer occupies its own cache line, preventing false sharing with
adjacent fan_rpm[]/pwm_duty[] fields on non-coherent architectures
- arctic_fan_write(): add write_pending flag (protected by
in_report_lock) so raw_event() delivers ACKs only while a write is
in flight
- arctic_fan_write(): commit pwm_duty[channel] only after the device
ACKs the command; a failed or timed-out write no longer leaves a
stale value in the cached duty state
- arctic_fan_probe(): start IO (hid_device_io_start) before registering
with hwmon; previously a sysfs write arriving between hwmon
registration and io_start could send an OUT report whose ACK would be
discarded by the HID core, causing a spurious timeout
- Documentation/hwmon/arctic_fan_controller.rst: document that cached
PWM values start at 0 (hardware state unknown at probe) and that each
OUT report carries all 10 channel values
Changes since v2:
- buf[]: add __aligned(8) for DMA safety
- ARCTIC_ACK_TIMEOUT_MS: restore 1000 ms; note observed max ~563 ms
- arctic_fan_parse_report(): replace hwmon_lock/hwmon_unlock with
spin_lock_irqsave; hwmon_lock() may sleep and is unsafe when
raw_event() runs in hardirq/softirq context
- arctic_fan_raw_event(): use spin_lock_irqsave for ACK path
- arctic_fan_write(): use spin_lock_irqsave for completion reinit
- arctic_fan_write(): clamp val to [0, 255] before u8 cast
- remove priv->hwmon_dev (no longer needed)
Changes since v1:
- Use hid_dbg() instead of module_param debug flag
- Move hid_device_id table adjacent to hid_driver struct
- Use get_unaligned_le16() for RPM parsing
- Remove impossible bounds/NULL checks; remove retry loop
- Add hid_is_usb() guard
- Do not update pwm_duty from IN reports (device is manual-only)
- Add completion/ACK mechanism for OUT report acknowledgement
- Add Documentation/hwmon/arctic_fan_controller.rst and MAINTAINERS
diff --git a/Documentation/hwmon/arctic_fan_controller.rst b/Documentation/hwmon/arctic_fan_controller.rst
new file mode 100644
index 0000000000..b5be88ae46
--- /dev/null
+++ b/Documentation/hwmon/arctic_fan_controller.rst
@@ -0,0 +1,56 @@
+.. 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@arctic.de>
+
+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.
+
+The device does not support GET_REPORT, so the driver cannot read back the
+current hardware PWM state at probe time. The cached PWM values (readable
+via pwm[1-10]) start at 0 and reflect only values that have been
+successfully written. Because each OUT report carries all 10 channel values,
+writing a single channel also sends the cached values for all other channels.
+Users should set all channels to the desired values before relying on the
+cached state.
+
+On system suspend, the device may lose power and reset its PWM channels to
+hardware defaults. The driver clears its cached duty values on resume so
+that reads reflect the unknown hardware state rather than stale pre-suspend
+values. Userspace is responsible for re-applying the desired duty cycles
+after resume.
+
+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);
+ the cached value is updated only after the device ACKs the
+ command with a success status. Read: returns the last
+ successfully written value; initialized to 0 at driver load
+ and after resume (hardware state unknown).
+================ ==============================================================
diff --git a/Documentation/hwmon/index.rst b/Documentation/hwmon/index.rst
index b2ca8513cf..c34713040e 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 96ea84948d..ec3112bd41 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@arctic.de>
+L: linux-hwmon@vger.kernel.org
+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@arndb.de>
M: Krzysztof Kozlowski <krzk@kernel.org>
diff --git a/drivers/hwmon/Kconfig b/drivers/hwmon/Kconfig
index 328867242c..6c90a8dd40 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 USB_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 5833c807c6..ef831c3375 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 0000000000..f50a9a6689
--- /dev/null
+++ b/drivers/hwmon/arctic_fan_controller.c
@@ -0,0 +1,338 @@
+// SPDX-License-Identifier: GPL-2.0-or-later
+/*
+ * Linux hwmon driver for ARCTIC Fan Controller
+ *
+ * USB Custom HID device 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/cache.h>
+#include <linux/completion.h>
+#include <linux/err.h>
+#include <linux/hid.h>
+#include <linux/hwmon.h>
+#include <linux/jiffies.h>
+#include <linux/minmax.h>
+#include <linux/module.h>
+#include <linux/spinlock.h>
+#include <linux/string.h>
+#include <linux/unaligned.h>
+
+#define ARCTIC_VID 0x3904
+#define ARCTIC_PID 0xF001
+#define ARCTIC_NUM_FANS 10
+#define ARCTIC_OUTPUT_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.
+ * Measured over 500 iterations: max ~563 ms. Keep 1 s as margin.
+ */
+#define ARCTIC_ACK_TIMEOUT_MS 1000
+
+struct arctic_fan_data {
+ struct hid_device *hdev;
+ spinlock_t in_report_lock; /* protects fan_rpm[], ack_status, write_pending */
+ struct completion in_report_received; /* ACK (ID 0x02) received in raw_event */
+ int ack_status; /* 0 = OK, negative errno on device error */
+ bool write_pending; /* true while an OUT report ACK is in flight */
+ u32 fan_rpm[ARCTIC_NUM_FANS];
+ u8 pwm_duty[ARCTIC_NUM_FANS]; /* 0-255 matching sysfs range; converted to 0-100 on send */
+ /*
+ * OUT report buffer. Cache-line aligned so it occupies its own cache
+ * line, preventing DMA cache-coherency issues with adjacent fields
+ * (fan_rpm[], pwm_duty[]) on non-coherent architectures.
+ * Embedded in the devm_kzalloc'd struct so it is heap-allocated and
+ * passes usb_hcd_map_urb_for_dma(). Serialized by the hwmon core.
+ */
+ u8 buf[ARCTIC_REPORT_LEN] ____cacheline_aligned;
+};
+
+/*
+ * 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.
+ * Called from raw_event which may run in IRQ context; must not sleep.
+ */
+static void arctic_fan_parse_report(struct arctic_fan_data *priv, u8 *buf,
+ int len, int rpm_off)
+{
+ unsigned long flags;
+ int i;
+
+ if (len < rpm_off + 20)
+ return;
+
+ spin_lock_irqsave(&priv->in_report_lock, flags);
+ for (i = 0; i < ARCTIC_NUM_FANS; i++)
+ priv->fan_rpm[i] = get_unaligned_le16(&buf[rpm_off + i * 2]);
+ spin_unlock_irqrestore(&priv->in_report_lock, flags);
+}
+
+/*
+ * 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);
+ unsigned long flags;
+
+ 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_irqsave(&priv->in_report_lock, flags);
+ /*
+ * Only deliver if a write is in flight. This prevents a
+ * late-arriving ACK from a timed-out write from erroneously
+ * satisfying a subsequent write's completion wait.
+ */
+ if (priv->write_pending) {
+ priv->ack_status = data[1] == 0x00 ? 0 : -EIO;
+ complete(&priv->in_report_received);
+ }
+ spin_unlock_irqrestore(&priv->in_report_lock, flags);
+ return 0;
+ }
+
+ if (report->id != ARCTIC_OUTPUT_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);
+ unsigned long flags;
+
+ if (type == hwmon_fan && attr == hwmon_fan_input) {
+ spin_lock_irqsave(&priv->in_report_lock, flags);
+ *val = priv->fan_rpm[channel];
+ spin_unlock_irqrestore(&priv->in_report_lock, flags);
+ return 0;
+ }
+ if (type == hwmon_pwm && attr == hwmon_pwm_input) {
+ /* pwm_duty is modified only in write(), which the hwmon core serializes */
+ *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 new_duty = (u8)clamp_val(val, 0, 255);
+ unsigned long flags;
+ unsigned long t;
+ int i, ret;
+
+ /*
+ * The hwmon core holds its lock for the duration of this callback,
+ * serializing concurrent writes. priv->buf is heap-allocated (embedded
+ * in the devm_kzalloc'd struct), satisfying usb_hcd_map_urb_for_dma().
+ *
+ * Use new_duty for the target channel and the cached value for all
+ * others. pwm_duty[channel] is committed only after a positive device
+ * ACK so a failed or timed-out write does not corrupt the cached state.
+ */
+ priv->buf[0] = ARCTIC_OUTPUT_REPORT_ID;
+ for (i = 0; i < ARCTIC_NUM_FANS; i++) {
+ u8 d = i == channel ? new_duty : priv->pwm_duty[i];
+
+ priv->buf[1 + i] = DIV_ROUND_CLOSEST((unsigned int)d * 100, 255);
+ }
+
+ /*
+ * Set write_pending before sending the report and clear it in every
+ * exit path. raw_event() calls complete() only when write_pending is
+ * true, so a stale ACK from a timed-out write cannot satisfy a
+ * subsequent write's completion wait. The wait is non-interruptible
+ * to prevent the same race in the signal-interrupted case.
+ * Serialized by the hwmon core: only one arctic_fan_write() at a time.
+ * Use irqsave to match the IRQ context in which raw_event may run.
+ */
+ spin_lock_irqsave(&priv->in_report_lock, flags);
+ priv->ack_status = -ETIMEDOUT;
+ priv->write_pending = true;
+ reinit_completion(&priv->in_report_received);
+ spin_unlock_irqrestore(&priv->in_report_lock, flags);
+
+ ret = hid_hw_output_report(priv->hdev, priv->buf, ARCTIC_REPORT_LEN);
+ if (ret < 0) {
+ spin_lock_irqsave(&priv->in_report_lock, flags);
+ priv->write_pending = false;
+ spin_unlock_irqrestore(&priv->in_report_lock, flags);
+ return ret;
+ }
+
+ t = wait_for_completion_timeout(&priv->in_report_received,
+ msecs_to_jiffies(ARCTIC_ACK_TIMEOUT_MS));
+ spin_lock_irqsave(&priv->in_report_lock, flags);
+ priv->write_pending = false;
+ spin_unlock_irqrestore(&priv->in_report_lock, flags);
+
+ if (!t)
+ return -ETIMEDOUT;
+
+ /* Commit the new duty only after hardware has positively acknowledged it */
+ if (priv->ack_status == 0)
+ priv->pwm_duty[channel] = new_duty;
+ 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_reset_resume(struct hid_device *hdev)
+{
+ struct arctic_fan_data *priv = hid_get_drvdata(hdev);
+
+ /*
+ * The device resets its PWM channels to hardware defaults on power
+ * loss during suspend. Clear the cached duty values so they reflect
+ * the unknown hardware state, consistent with probe-time behaviour
+ * (the device has no GET_REPORT support).
+ */
+ memset(priv->pwm_duty, 0, sizeof(priv->pwm_duty));
+ return 0;
+}
+
+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;
+ 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;
+
+ /*
+ * Start IO before registering with hwmon. If IO were started after
+ * hwmon registration, a sysfs write arriving in that narrow window
+ * would send an OUT report but the ACK could not be delivered (the HID
+ * core discards events until io_started), causing a spurious timeout.
+ */
+ 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_device_io_stop(hdev);
+ 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,
+ .reset_resume = arctic_fan_reset_resume,
+};
+
+module_hid_driver(arctic_fan_driver);
+
+MODULE_AUTHOR("Aureo Serrano de Souza <aureo.serrano@arctic.de>");
+MODULE_DESCRIPTION("HID hwmon driver for ARCTIC Fan Controller");
+MODULE_LICENSE("GPL");
On 3/20/26 11:00, Aureo Serrano de Souza wrote:
> Add hwmon driver for the ARCTIC Fan Controller, a USB HID device
> (VID 0x3904, PID 0xF001) with 10 fan channels. Exposes fan speed in
> RPM (read-only) and PWM duty cycle (0-255, read/write) via sysfs.
>
> The device pushes IN reports at ~1 Hz containing RPM readings. PWM is
> set via OUT reports; the device applies the new duty cycle and sends
> back a 2-byte ACK (Report ID 0x02). The driver waits up to 1 s for
> the ACK using a completion. Measured device latency: max ~563 ms over
> 500 iterations. PWM control is manual-only: the device never changes
> duty cycle autonomously.
>
> raw_event() may run in hardirq context, so fan_rpm[] is protected by
> a spinlock with irq-save. pwm_duty[] and the report buffer are
> serialized by the hwmon core, which holds its lock for the duration of
> the read/write callbacks.
>
> Signed-off-by: Aureo Serrano de Souza <aureo.serrano@arctic.de>
> ---
> Thanks to Guenter Roeck and Thomas Weißschuh for the reviews.
>
> Changes since v4:
> - arctic_fan_write(): switch to wait_for_completion_timeout() (non-
> interruptible); eliminates the signal-interrupted write case of the
> late-ACK race that write_pending could not fully prevent
> - arctic_fan_write(): guard pwm_duty[channel] commit with
> ack_status == 0 check; a device error ACK (status 0x01) no longer
> silently poisons the cached duty used in future OUT reports
> - arctic_fan_probe()/remove(): replace devm_add_action_or_reset() +
> no-op remove() with explicit hid_device_io_stop/hid_hw_close/
> hid_hw_stop in remove(); devm_add_action_or_reset() was called after
> hdev->driver = NULL, causing a NULL deref in hid_hw_close() on unbind
> - add reset_resume callback: device resets PWM to hardware defaults on
> power loss during suspend; driver now clears cached pwm_duty[] on
> reset-resume so stale pre-suspend values are not re-sent as if valid
> - Documentation/hwmon/arctic_fan_controller.rst: document suspend/
> resume behaviour and the updated pwm[1-10] read semantics
>
> Changes since v3:
> - buf[]: upgrade from __aligned(8) to ____cacheline_aligned so the
> DMA buffer occupies its own cache line, preventing false sharing with
> adjacent fan_rpm[]/pwm_duty[] fields on non-coherent architectures
> - arctic_fan_write(): add write_pending flag (protected by
> in_report_lock) so raw_event() delivers ACKs only while a write is
> in flight
> - arctic_fan_write(): commit pwm_duty[channel] only after the device
> ACKs the command; a failed or timed-out write no longer leaves a
> stale value in the cached duty state
> - arctic_fan_probe(): start IO (hid_device_io_start) before registering
> with hwmon; previously a sysfs write arriving between hwmon
> registration and io_start could send an OUT report whose ACK would be
> discarded by the HID core, causing a spurious timeout
> - Documentation/hwmon/arctic_fan_controller.rst: document that cached
> PWM values start at 0 (hardware state unknown at probe) and that each
> OUT report carries all 10 channel values
>
> Changes since v2:
> - buf[]: add __aligned(8) for DMA safety
> - ARCTIC_ACK_TIMEOUT_MS: restore 1000 ms; note observed max ~563 ms
> - arctic_fan_parse_report(): replace hwmon_lock/hwmon_unlock with
> spin_lock_irqsave; hwmon_lock() may sleep and is unsafe when
> raw_event() runs in hardirq/softirq context
> - arctic_fan_raw_event(): use spin_lock_irqsave for ACK path
> - arctic_fan_write(): use spin_lock_irqsave for completion reinit
> - arctic_fan_write(): clamp val to [0, 255] before u8 cast
> - remove priv->hwmon_dev (no longer needed)
>
> Changes since v1:
> - Use hid_dbg() instead of module_param debug flag
> - Move hid_device_id table adjacent to hid_driver struct
> - Use get_unaligned_le16() for RPM parsing
> - Remove impossible bounds/NULL checks; remove retry loop
> - Add hid_is_usb() guard
> - Do not update pwm_duty from IN reports (device is manual-only)
> - Add completion/ACK mechanism for OUT report acknowledgement
> - Add Documentation/hwmon/arctic_fan_controller.rst and MAINTAINERS
>
> diff --git a/Documentation/hwmon/arctic_fan_controller.rst b/Documentation/hwmon/arctic_fan_controller.rst
> new file mode 100644
> index 0000000000..b5be88ae46
> --- /dev/null
> +++ b/Documentation/hwmon/arctic_fan_controller.rst
> @@ -0,0 +1,56 @@
> +.. 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@arctic.de>
> +
> +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.
> +
> +The device does not support GET_REPORT, so the driver cannot read back the
> +current hardware PWM state at probe time. The cached PWM values (readable
> +via pwm[1-10]) start at 0 and reflect only values that have been
> +successfully written. Because each OUT report carries all 10 channel values,
> +writing a single channel also sends the cached values for all other channels.
> +Users should set all channels to the desired values before relying on the
> +cached state.
> +
> +On system suspend, the device may lose power and reset its PWM channels to
> +hardware defaults. The driver clears its cached duty values on resume so
> +that reads reflect the unknown hardware state rather than stale pre-suspend
> +values. Userspace is responsible for re-applying the desired duty cycles
> +after resume.
> +
> +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);
> + the cached value is updated only after the device ACKs the
> + command with a success status. Read: returns the last
> + successfully written value; initialized to 0 at driver load
> + and after resume (hardware state unknown).
> +================ ==============================================================
> diff --git a/Documentation/hwmon/index.rst b/Documentation/hwmon/index.rst
> index b2ca8513cf..c34713040e 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 96ea84948d..ec3112bd41 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@arctic.de>
> +L: linux-hwmon@vger.kernel.org
> +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@arndb.de>
> M: Krzysztof Kozlowski <krzk@kernel.org>
> diff --git a/drivers/hwmon/Kconfig b/drivers/hwmon/Kconfig
> index 328867242c..6c90a8dd40 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 USB_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 5833c807c6..ef831c3375 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 0000000000..f50a9a6689
> --- /dev/null
> +++ b/drivers/hwmon/arctic_fan_controller.c
> @@ -0,0 +1,338 @@
> +// SPDX-License-Identifier: GPL-2.0-or-later
> +/*
> + * Linux hwmon driver for ARCTIC Fan Controller
> + *
> + * USB Custom HID device 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/cache.h>
> +#include <linux/completion.h>
> +#include <linux/err.h>
> +#include <linux/hid.h>
> +#include <linux/hwmon.h>
> +#include <linux/jiffies.h>
> +#include <linux/minmax.h>
> +#include <linux/module.h>
> +#include <linux/spinlock.h>
> +#include <linux/string.h>
> +#include <linux/unaligned.h>
> +
> +#define ARCTIC_VID 0x3904
> +#define ARCTIC_PID 0xF001
> +#define ARCTIC_NUM_FANS 10
> +#define ARCTIC_OUTPUT_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.
> + * Measured over 500 iterations: max ~563 ms. Keep 1 s as margin.
> + */
> +#define ARCTIC_ACK_TIMEOUT_MS 1000
> +
> +struct arctic_fan_data {
> + struct hid_device *hdev;
> + spinlock_t in_report_lock; /* protects fan_rpm[], ack_status, write_pending */
> + struct completion in_report_received; /* ACK (ID 0x02) received in raw_event */
> + int ack_status; /* 0 = OK, negative errno on device error */
> + bool write_pending; /* true while an OUT report ACK is in flight */
> + u32 fan_rpm[ARCTIC_NUM_FANS];
> + u8 pwm_duty[ARCTIC_NUM_FANS]; /* 0-255 matching sysfs range; converted to 0-100 on send */
> + /*
> + * OUT report buffer. Cache-line aligned so it occupies its own cache
> + * line, preventing DMA cache-coherency issues with adjacent fields
> + * (fan_rpm[], pwm_duty[]) on non-coherent architectures.
> + * Embedded in the devm_kzalloc'd struct so it is heap-allocated and
> + * passes usb_hcd_map_urb_for_dma(). Serialized by the hwmon core.
> + */
> + u8 buf[ARCTIC_REPORT_LEN] ____cacheline_aligned;
> +};
> +
> +/*
> + * 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.
> + * Called from raw_event which may run in IRQ context; must not sleep.
> + */
> +static void arctic_fan_parse_report(struct arctic_fan_data *priv, u8 *buf,
> + int len, int rpm_off)
> +{
> + unsigned long flags;
> + int i;
> +
> + if (len < rpm_off + 20)
> + return;
> +
> + spin_lock_irqsave(&priv->in_report_lock, flags);
> + for (i = 0; i < ARCTIC_NUM_FANS; i++)
> + priv->fan_rpm[i] = get_unaligned_le16(&buf[rpm_off + i * 2]);
> + spin_unlock_irqrestore(&priv->in_report_lock, flags);
> +}
> +
> +/*
> + * 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);
> + unsigned long flags;
> +
> + 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_irqsave(&priv->in_report_lock, flags);
> + /*
> + * Only deliver if a write is in flight. This prevents a
> + * late-arriving ACK from a timed-out write from erroneously
> + * satisfying a subsequent write's completion wait.
> + */
... unless the next write is already pending at that time (see Sashiko reply).
No idea how to handle that situation cleanly. How do you know that this is not
a reply to a previously timed out request ?
> + if (priv->write_pending) {
> + priv->ack_status = data[1] == 0x00 ? 0 : -EIO;
> + complete(&priv->in_report_received);
> + }
> + spin_unlock_irqrestore(&priv->in_report_lock, flags);
> + return 0;
> + }
> +
> + if (report->id != ARCTIC_OUTPUT_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);
> + unsigned long flags;
> +
> + if (type == hwmon_fan && attr == hwmon_fan_input) {
> + spin_lock_irqsave(&priv->in_report_lock, flags);
> + *val = priv->fan_rpm[channel];
> + spin_unlock_irqrestore(&priv->in_report_lock, flags);
> + return 0;
> + }
> + if (type == hwmon_pwm && attr == hwmon_pwm_input) {
> + /* pwm_duty is modified only in write(), which the hwmon core serializes */
> + *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 new_duty = (u8)clamp_val(val, 0, 255);
> + unsigned long flags;
> + unsigned long t;
> + int i, ret;
> +
> + /*
> + * The hwmon core holds its lock for the duration of this callback,
> + * serializing concurrent writes. priv->buf is heap-allocated (embedded
> + * in the devm_kzalloc'd struct), satisfying usb_hcd_map_urb_for_dma().
> + *
> + * Use new_duty for the target channel and the cached value for all
> + * others. pwm_duty[channel] is committed only after a positive device
> + * ACK so a failed or timed-out write does not corrupt the cached state.
> + */
> + priv->buf[0] = ARCTIC_OUTPUT_REPORT_ID;
> + for (i = 0; i < ARCTIC_NUM_FANS; i++) {
> + u8 d = i == channel ? new_duty : priv->pwm_duty[i];
> +
> + priv->buf[1 + i] = DIV_ROUND_CLOSEST((unsigned int)d * 100, 255);
> + }
> +
> + /*
> + * Set write_pending before sending the report and clear it in every
> + * exit path. raw_event() calls complete() only when write_pending is
> + * true, so a stale ACK from a timed-out write cannot satisfy a
> + * subsequent write's completion wait. The wait is non-interruptible
> + * to prevent the same race in the signal-interrupted case.
> + * Serialized by the hwmon core: only one arctic_fan_write() at a time.
> + * Use irqsave to match the IRQ context in which raw_event may run.
> + */
> + spin_lock_irqsave(&priv->in_report_lock, flags);
> + priv->ack_status = -ETIMEDOUT;
> + priv->write_pending = true;
> + reinit_completion(&priv->in_report_received);
> + spin_unlock_irqrestore(&priv->in_report_lock, flags);
> +
> + ret = hid_hw_output_report(priv->hdev, priv->buf, ARCTIC_REPORT_LEN);
> + if (ret < 0) {
> + spin_lock_irqsave(&priv->in_report_lock, flags);
> + priv->write_pending = false;
> + spin_unlock_irqrestore(&priv->in_report_lock, flags);
> + return ret;
> + }
> +
> + t = wait_for_completion_timeout(&priv->in_report_received,
> + msecs_to_jiffies(ARCTIC_ACK_TIMEOUT_MS));
> + spin_lock_irqsave(&priv->in_report_lock, flags);
> + priv->write_pending = false;
> + spin_unlock_irqrestore(&priv->in_report_lock, flags);
> +
> + if (!t)
> + return -ETIMEDOUT;
> +
> + /* Commit the new duty only after hardware has positively acknowledged it */
> + if (priv->ack_status == 0)
> + priv->pwm_duty[channel] = new_duty;
> + 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_reset_resume(struct hid_device *hdev)
> +{
> + struct arctic_fan_data *priv = hid_get_drvdata(hdev);
> +
> + /*
> + * The device resets its PWM channels to hardware defaults on power
> + * loss during suspend. Clear the cached duty values so they reflect
> + * the unknown hardware state, consistent with probe-time behaviour
> + * (the device has no GET_REPORT support).
> + */
> + memset(priv->pwm_duty, 0, sizeof(priv->pwm_duty));
> + return 0;
> +}
> +
> +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;
> + 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;
> +
> + /*
> + * Start IO before registering with hwmon. If IO were started after
> + * hwmon registration, a sysfs write arriving in that narrow window
> + * would send an OUT report but the ACK could not be delivered (the HID
> + * core discards events until io_started), causing a spurious timeout.
> + */
> + 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_device_io_stop(hdev);
> + 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);
This doesn't work as intended. See
https://sashiko.dev/#/patchset/20260320180723.462765-1-aureo.serrano%40arctic.de
I don't really know how to handle this cleanly. Maybe other hid drivers
can give a hint. Either case, with the current code the hwmon device would
have to be unregistered first since it was registered last.
> +}
> +
> +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,
> + .reset_resume = arctic_fan_reset_resume,
> +};
> +
> +module_hid_driver(arctic_fan_driver);
> +
> +MODULE_AUTHOR("Aureo Serrano de Souza <aureo.serrano@arctic.de>");
> +MODULE_DESCRIPTION("HID hwmon driver for ARCTIC Fan Controller");
> +MODULE_LICENSE("GPL");
>
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