Create a FanSpeedObject interface

Create the FanSpeedObject target interface when the fanx_target sysfs
file exists. The "xyz.openbmc_project.Control.Target" interface is on
the /xyz/openbmc_project/sensors/fan_tac/fan[#] object paths.

Change-Id: Ib4a099cff17cacea501b474969f292516db212b5
Signed-off-by: Matthew Barth <msbarth@us.ibm.com>
diff --git a/targets.hpp b/targets.hpp
new file mode 100644
index 0000000..2b0feb3
--- /dev/null
+++ b/targets.hpp
@@ -0,0 +1,64 @@
+#pragma once
+
+#include <experimental/filesystem>
+
+/** @class Targets
+ *  @brief Target type traits.
+ *
+ *  @tparam T - The target type.
+ */
+template <typename T>
+struct Targets
+{
+    static void fail()
+    {
+        static_assert(sizeof(Targets) == -1, "Unsupported Target type");
+    }
+};
+
+/**@brief Targets specialization for fan speed. */
+template <>
+struct Targets<FanSpeedObject>
+{
+    static constexpr InterfaceType type = InterfaceType::FAN_SPEED;
+    static uint64_t (FanSpeedObject::*const setTarget)(uint64_t);
+    static uint64_t (FanSpeedObject::*const getTarget)() const;
+};
+
+/** @brief addTarget
+ *
+ *  Creates the target type interface
+ *
+ *  @tparam T - The target type
+ *
+ *  @param[in] sensor - A sensor type and name
+ *  @param[in] hwmonRoot - The root hwmon path
+ *  @param[in] instance - The target instance name
+ *  @param[in] info - The sdbusplus server connection and interfaces
+ */
+template <typename T>
+void addTarget(const SensorSet::key_type& sensor,
+               const std::string& hwmonRoot,
+               const std::string& instance,
+               ObjectInfo& info)
+{
+    namespace fs = std::experimental::filesystem;
+    static constexpr bool deferSignals = true;
+
+    auto& bus = *std::get<sdbusplus::bus::bus*>(info);
+    auto& obj = std::get<Object>(info);
+    auto& objPath = std::get<std::string>(info);
+
+    // Check if target sysfs file exists
+    auto targetPath = hwmonRoot + '/' + instance;
+    auto sysfsFile = make_sysfs_path(targetPath,
+                                     sensor.first,
+                                     sensor.second,
+                                     hwmon::entry::target);
+    if (fs::exists(sysfsFile))
+    {
+        auto iface = std::make_shared<T>(bus, objPath.c_str(), deferSignals);
+        auto type = Targets<T>::type;
+        obj[type] = iface;
+    }
+}