Linux divides a System-on-Chip (SoC) into multiple thermal zones that correspond to an area in the silicon die where the temperature is deemed uniform. On SAMA7G54, one single thermal zone is available (thermal_zone0), which means that the temperature is uniform across the die. The thermal zone properties can be displayed with the following command:
# ls /sys/class/thermal/thermal_zone0/
available_policies...........k_pu.............................trip_point_0_type
cdev0........................mode.............................trip_point_1_hyst
cdev0_trip_point.............offset...........................trip_point_1_temp
cdev0_weight.................policy...........................trip_point_1_type
cdev1........................power............................trip_point_2_hyst
cdev1_trip_point.............slope............................trip_point_2_temp
cdev1_weight.................subsystem........................trip_point_2_type
integral_cutoff..............sustainable_power................type
k_d..........................temp.............................uevent
k_i..........................trip_point_0_hyst
k_po.........................trip_point_0_tempThe SAMA7G54 die temperature can be read from the “temp” file:
# cat /sys/class/thermal/thermal_zone0/temp
55201This result expressed in millidegrees Celsius provides the SAMAG54 die temperature (55.201°C).
Among the important attributes of a thermal zone are the temperature trip points. These points define when the Linux thermal governor makes decisions to start and stop cooling down the device (CPU frequency decrease, fan activation, etc.). These trip points may be of the following types:
The policies available to the thermal governor are listed below. By default, the policy is set to “step wise”.
As an example, the following settings can be used as a starting point for an industrial application. They may be adjusted depending on each system specificities:
These values account for the temperature sensor accuracy (±5°C).