linux-kernel - [PATCH v2 2/6] ACPI: fan: Workaround for 64-bit firmware bug

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Message-Id: <20251007234149.2769-3-W_Armin@gmx.de>
Date: Wed,  8 Oct 2025 01:41:45 +0200
From: Armin Wolf <W_Armin@....de>
To: rafael@...nel.org,
	lenb@...nel.org
Cc: linux-acpi@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: [PATCH v2 2/6] ACPI: fan: Workaround for 64-bit firmware bug

Some firmware implementations use the "Ones" ASL opcode to produce
an integer with all bits set in order to indicate missing speed or
power readings. This however only works when using 32-bit intgers,
as the ACPI spec requires a 32-bit integer (0xFFFFFFFF) to be
returned for missing speed/power readings. With 64-bit integers the
"Ones" opcode produces a 64-bit integer with all bits set, violating
the ACPI spec regarding the placeholder value for missing readings.

Work around such buggy firmware implementation by also checking for
64-bit integers with all bits set when reading _FST.

Signed-off-by: Armin Wolf <W_Armin@....de>
---
 drivers/acpi/fan.h       | 33 +++++++++++++++++++++++++++++++++
 drivers/acpi/fan_hwmon.c | 10 +++-------
 2 files changed, 36 insertions(+), 7 deletions(-)

diff --git a/drivers/acpi/fan.h b/drivers/acpi/fan.h
index d39bb6fd1326..022bc215cdbc 100644
--- a/drivers/acpi/fan.h
+++ b/drivers/acpi/fan.h
@@ -11,6 +11,7 @@
 #define _ACPI_FAN_H_
 
 #include <linux/kconfig.h>
+#include <linux/limits.h>
 
 #define ACPI_FAN_DEVICE_IDS	\
 	{"INT3404", }, /* Fan */ \
@@ -60,6 +61,38 @@ struct acpi_fan {
 	struct device_attribute fine_grain_control;
 };
 
+/**
+ * acpi_fan_speed_valid - Check if fan speed value is valid
+ * @speeed: Speed value returned by the ACPI firmware
+ *
+ * Check if the fan speed value returned by the ACPI firmware is valid. This function is
+ * necessary as ACPI firmware implementations can return 0xFFFFFFFF to signal that the
+ * ACPI fan does not support speed reporting. Additionally, some buggy ACPI firmware
+ * implementations return a value larger than the 32-bit integer value defined by
+ * the ACPI specification when using placeholder values. Such invalid values are also
+ * detected by this function.
+ *
+ * Returns: True if the fan speed value is valid, false otherwise.
+ */
+static inline bool acpi_fan_speed_valid(u64 speed)
+{
+	return speed < U32_MAX;
+}
+
+/**
+ * acpi_fan_power_valid - Check if fan power value is valid
+ * @power: Power value returned by the ACPI firmware
+ *
+ * Check if the fan power value returned by the ACPI firmware is valid.
+ * See acpi_fan_speed_valid() for details.
+ *
+ * Returns: True if the fan power value is valid, false otherwise.
+ */
+static inline bool acpi_fan_power_valid(u64 power)
+{
+	return power < U32_MAX;
+}
+
 int acpi_fan_get_fst(acpi_handle handle, struct acpi_fan_fst *fst);
 int acpi_fan_create_attributes(struct acpi_device *device);
 void acpi_fan_delete_attributes(struct acpi_device *device);
diff --git a/drivers/acpi/fan_hwmon.c b/drivers/acpi/fan_hwmon.c
index 4209a9923efc..5581aa6fdfa0 100644
--- a/drivers/acpi/fan_hwmon.c
+++ b/drivers/acpi/fan_hwmon.c
@@ -15,10 +15,6 @@
 
 #include "fan.h"
 
-/* Returned when the ACPI fan does not support speed reporting */
-#define FAN_SPEED_UNAVAILABLE	U32_MAX
-#define FAN_POWER_UNAVAILABLE	U32_MAX
-
 static struct acpi_fan_fps *acpi_fan_get_current_fps(struct acpi_fan *fan, u64 control)
 {
 	unsigned int i;
@@ -77,7 +73,7 @@ static umode_t acpi_fan_hwmon_is_visible(const void *drvdata, enum hwmon_sensor_
 			 * when the associated attribute should not be created.
 			 */
 			for (i = 0; i < fan->fps_count; i++) {
-				if (fan->fps[i].power != FAN_POWER_UNAVAILABLE)
+				if (acpi_fan_power_valid(fan->fps[i].power))
 					return 0444;
 			}
 
@@ -106,7 +102,7 @@ static int acpi_fan_hwmon_read(struct device *dev, enum hwmon_sensor_types type,
 	case hwmon_fan:
 		switch (attr) {
 		case hwmon_fan_input:
-			if (fst.speed == FAN_SPEED_UNAVAILABLE)
+			if (!acpi_fan_speed_valid(fst.speed))
 				return -ENODEV;
 
 			if (fst.speed > LONG_MAX)
@@ -134,7 +130,7 @@ static int acpi_fan_hwmon_read(struct device *dev, enum hwmon_sensor_types type,
 			if (!fps)
 				return -EIO;
 
-			if (fps->power == FAN_POWER_UNAVAILABLE)
+			if (!acpi_fan_power_valid(fps->power))
 				return -ENODEV;
 
 			if (fps->power > LONG_MAX / MICROWATT_PER_MILLIWATT)
-- 
2.39.5