Add factor and offset for fan monitor

For fans controlled via pwm, the fan target and speed are different,
where the fan target is pwm and the speed is rpm.

Usually it is a linear mapping from pwm to rpm.
So this commit defines the optional configs, factor and offset for
calculating the expected fan speed from target, e.g.

  - name: fan0
    has_target: true
    factor: 21
    offset: 1600

The fan monitor service will calculate expected fan speed as:
 target * factor + offset

The default value is 1 for factor and 0 for offset if they are not
defined.

Tested: Use this config together with the following commit's changes,
        test on Romulus and ensures the fan monitor works OK;
        Without this config, fan monitor always mark fans as
        non-functional due to the fan speed does not match the pwm
        value.

Change-Id: If5e25368b4530df7a7face9377efb58804db21df
Signed-off-by: Lei YU <mine260309@gmail.com>

1 files changed, 30 insertions, 0 deletions

diff --git a/monitor/tach_sensor.hpp b/monitor/tach_sensor.hpp
index dd89bbf..50951cd 100644
--- a/monitor/tach_sensor.hpp
+++ b/monitor/tach_sensor.hpp
@@ -62,6 +62,8 @@ class TachSensor
          * @param[in] id - the id of the sensor
          * @param[in] hasTarget - if the sensor supports
          *                        setting the speed
+         * @param[in] factor - the factor of the sensor target
+         * @param[in] offset - the offset of the sensor target
          * @param[in] timeout - Normal timeout value to use
          * @param[in] events - sd_event pointer
          */
@@ -70,6 +72,8 @@ class TachSensor
                    Fan& fan,
                    const std::string& id,
                    bool hasTarget,
+                   size_t factor,
+                   size_t offset,
                    size_t timeout,
                    phosphor::fan::event::EventPtr& events);
 
@@ -95,6 +99,22 @@ class TachSensor
         }
 
         /**
+         * @brief Returns the factor of the sensor target
+         */
+        inline size_t getFactor() const
+        {
+            return _factor;
+        }
+
+        /**
+         * @brief Returns the offset of the sensor target
+         */
+        inline size_t getOffset() const
+        {
+            return _offset;
+        }
+
+        /**
          * Returns true if the hardware behind this
          * sensor is considered working OK/functional.
          */
@@ -217,6 +237,16 @@ class TachSensor
         const bool _hasTarget;
 
         /**
+         * @brief The factor of target to get fan rpm
+         */
+        const size_t _factor;
+
+        /**
+         * @brief The offset of target to get fan rpm
+         */
+        const size_t _offset;
+
+        /**
          * @brief The input speed, from the Value dbus property
          */
         int64_t _tachInput = 0;