hwmon: (aspeed-pwm-tacho) On read failure return -ETIMEDOUT

When the controller fails to provide an RPM reading within the alloted
time; the driver returns -ETIMEDOUT and no file contents.
Signed-off-by: Patrick Venture <venture@google.com>
Fixes: 2d7a548a ("drivers: hwmon: Support for ASPEED PWM/Fan tach")
Signed-off-by: Guenter Roeck <linux@roeck-us.net>

drivers/hwmon/aspeed-pwm-tacho.c

@@ -7,6 +7,7 @@
  */
 
 #include <linux/clk.h>
+#include <linux/errno.h>
 #include <linux/gpio/consumer.h>
 #include <linux/delay.h>
 #include <linux/hwmon.h>
@@ -494,7 +495,7 @@ static u32 aspeed_get_fan_tach_ch_measure_period(struct aspeed_pwm_tacho_data
 	return clk / (clk_unit * div_h * div_l * tacho_div * tacho_unit);
 }
 
-static u32 aspeed_get_fan_tach_ch_rpm(struct aspeed_pwm_tacho_data *priv,
+static int aspeed_get_fan_tach_ch_rpm(struct aspeed_pwm_tacho_data *priv,
 				      u8 fan_tach_ch)
 {
 	u32 raw_data, tach_div, clk_source, sec, val;
@@ -510,6 +511,9 @@ static u32 aspeed_get_fan_tach_ch_rpm(struct aspeed_pwm_tacho_data *priv,
 	msleep(sec);
 
 	regmap_read(priv->regmap, ASPEED_PTCR_RESULT, &val);
+	if (!(val & RESULT_STATUS_MASK))
+		return -ETIMEDOUT;
+
 	raw_data = val & RESULT_VALUE_MASK;
 	tach_div = priv->type_fan_tach_clock_division[type];
 	tach_div = 0x4 << (tach_div * 2);
@@ -561,12 +565,14 @@ static ssize_t show_rpm(struct device *dev, struct device_attribute *attr,
 {
 	struct sensor_device_attribute *sensor_attr = to_sensor_dev_attr(attr);
 	int index = sensor_attr->index;
-	u32 rpm;
+	int rpm;
 	struct aspeed_pwm_tacho_data *priv = dev_get_drvdata(dev);
 
 	rpm = aspeed_get_fan_tach_ch_rpm(priv, index);
+	if (rpm < 0)
+		return rpm;
 
-	return sprintf(buf, "%u\n", rpm);
+	return sprintf(buf, "%d\n", rpm);
 }
 
 static umode_t pwm_is_visible(struct kobject *kobj,