hwmon: document: add gpd-fan

Add GPD fan driver document

Signed-off-by: Cryolitia PukNgae <cryolitia@uniontech.com>
Link: https://lore.kernel.org/r/20250908-gpd_fan-v9-2-7b4506c03953@uniontech.com
Signed-off-by: Guenter Roeck <linux@roeck-us.net>

Documentation/hwmon/gpd-fan.rst

@@ -0,0 +1,78 @@
+.. SPDX-License-Identifier: GPL-2.0-or-later
+
+Kernel driver gpd-fan
+=========================
+
+Author:
+    - Cryolitia PukNgae <cryolitia@uniontech.com>
+
+Description
+------------
+
+Handheld devices from Shenzhen GPD Technology Co., Ltd. provide fan readings
+and fan control through their embedded controllers.
+
+Supported devices
+-----------------
+
+Currently the driver supports the following handhelds:
+
+ - GPD Win Mini (7840U)
+ - GPD Win Mini (8840U)
+ - GPD Win Mini (HX370)
+ - GPD Pocket 4
+ - GPD Duo
+ - GPD Win Max 2 (6800U)
+ - GPD Win Max 2 2023 (7840U)
+ - GPD Win Max 2 2024 (8840U)
+ - GPD Win Max 2 2025 (HX370)
+ - GPD Win 4 (6800U)
+ - GPD Win 4 (7840U)
+
+Module parameters
+-----------------
+
+gpd_fan_board
+  Force specific which module quirk should be used.
+  Use it like "gpd_fan_board=wm2".
+
+   - wm2
+       - GPD Win 4 (7840U)
+       - GPD Win Max 2 (6800U)
+       - GPD Win Max 2 2023 (7840U)
+       - GPD Win Max 2 2024 (8840U)
+       - GPD Win Max 2 2025 (HX370)
+   - win4
+       - GPD Win 4 (6800U)
+   - win_mini
+       - GPD Win Mini (7840U)
+       - GPD Win Mini (8840U)
+       - GPD Win Mini (HX370)
+       - GPD Pocket 4
+       - GPD Duo
+
+Sysfs entries
+-------------
+
+The following attributes are supported:
+
+fan1_input
+  Read Only. Reads current fan RPM.
+
+pwm1_enable
+  Read/Write. Enable manual fan control. Write "0" to disable control and run
+  at full speed. Write "1" to set to manual, write "2" to let the EC control
+  decide fan speed. Read this attribute to see current status.
+
+  NB：In consideration of the safety of the device, when setting to manual mode,
+  the pwm speed will be set to the maximum value (255) by default. You can set
+  a different value by writing pwm1 later.
+
+pwm1
+  Read/Write. Read this attribute to see current duty cycle in the range
+  [0-255]. When pwm1_enable is set to "1" (manual) write any value in the
+  range [0-255] to set fan speed.
+
+  NB: Many boards (except listed under wm2 above) don't support reading the
+  current pwm value in auto mode. That will just return EOPNOTSUPP. In manual
+  mode it will always return the real value.

Documentation/hwmon/index.rst

@@ -82,6 +82,7 @@ Hardware Monitoring Kernel Drivers
    gigabyte_waterforce
    gsc-hwmon
    gl518sm
+   gpd-fan
    gxp-fan-ctrl
    hih6130
    hp-wmi-sensors

MAINTAINERS

@@ -10425,6 +10425,7 @@ GPD FAN DRIVER
 M:	Cryolitia PukNgae <cryolitia@uniontech.com>
 L:	linux-hwmon@vger.kernel.org
 S:	Maintained
+F:	Documentation/hwmon/gpd-fan.rst
 F:	drivers/hwmon/gpd-fan.c
 
 GPD POCKET FAN DRIVER