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+# Control.ThermalMode dbus interface with Supported and Current properties
+
+Author:
+  Matthew Barth !msbarth
+Primary assignee:
+  Matthew Barth !msbarth
+Other contributors:
+  None
+Created:
+  2019-02-06
+
+## Problem Description
+An issue was discovered where the exhaust heat from the system GPUs causes
+overtemp warnings on optical cables on certain system configurations. The
+issue can be resolved by altering the fan control application's floor table,
+effectively raising the floor when these optical cables exist but an interface
+is needed to do so. Since the issue revolves around the optical cables
+themselves, where no current mechanism exists to detect the presence of the
+optical cables plugged into a card downwind from the GPUs' exhaust,
+an end-user must be presented with an ability to enable this raised floor
+speed table.
+
+## Background and References
+The witherspoon system supports pci cards that could have optical cables
+plugged in place of copper cables. These optical cables can report overtemp
+warnings to the OS when high GPU utilization workloads exist. When this occurs
+with low enough CPU utilization, the fans could be kept at a given floor speed
+that sufficiently cools the components within the chassis, but not the optical
+cables with the slow moving hot exhaust.
+
+Without an available exhaust temp sensor, there's no direct way to determine
+the exhaust temp and include that within the fan control algorithm. A similar
+issue exists on other system where mathematical calculations are done based on
+the overall power dissipation.
+
+Mathematical calculations to logically estimate exit air temps:
+https://github.com/openbmc/dbus-sensors/blob/master/src/ExitAirTempSensor.cpp
+
+## Requirements
+Create the ability for an end-user to enable the use of a thermal control mode
+other than the default. In this use-case, the mode is specific to an
+undetectable configuration that alters the fan floor speeds unrelated to
+standardized profile/modes such "Acoustic" and "Performance". Once the end-user
+selects a documented mode for the platform, the thermal control application
+alters its control algorithm according to the defined mode, which is
+implementation specific to that instance of the application on that platform.
+
+## Proposed Design
+Create a Control.ThermalMode dbus interface containing a supported list of
+available thermal control modes along with what current mode is in use.
+Initially the current mode would be set to "Default" and the implementation
+of the interface would populate the supported list of modes.
+
+As one implementation, phosphor-fan-presence/control would be updated to extend
+this dbus interface object which would fill in the list of supported modes
+from its fan control configuration for the platform. Once the fan control
+application starts, the interface would be added on the zone object and
+available to be queried for supported modes or update the current mode.
+An end-user may set the current mode to any of those supported modes and the
+current mode would be persisted each time it is updated. This is to ensure
+each time the fan control application zone objects are started, the last set
+control mode is used.
+
+## Alternatives Considered
+Mathematical calculation to create a virtual exhaust temp sensor value based
+on overall power dissipation. However, in the witherspoon situation, using
+this technique would not be reliable in adjusting the floor speeds for only
+configurations using optical cables. This would instead present the possibility
+of raising floor speeds for configurations where its unnecessary.
+
+## Impacts
+The thermal control application used must be configured to provide what thermal
+control modes are supported/available on the interface as well as perform the
+associated control changes when a mode is set.
+
+## Testing
+Trigger the use of an alternative fan floor table based on the thermal control
+mode selected on a witherspoon system.