thermal: Allow selecting the bang-bang governor as default

For many setups the bang-bang governor is exactly what we want. Many
ARM SoC-based devices use fans to cool down the entire SoC and that
works well only with the bang-bang governor because it uses the
hysteresis in order to let the fan run for a while to cool the SoC
down below the trip point before switching it off again.

The step-wise governor will behave strangely in these situations. It
doesn't use the hysteresis, so it can lead to situations where the fan
is turned on for only a very brief period and then is switched back off,
only to get switched back on again very quickly because the SoC hasn't
cooled down very much.

Signed-off-by: Thierry Reding <treding@nvidia.com>
Link: https://lore.kernel.org/r/20230609124408.3788680-1-thierry.reding@gmail.com
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>

drivers/thermal/Kconfig

@@ -130,6 +130,14 @@ config THERMAL_DEFAULT_GOV_POWER_ALLOCATOR
 	  system and device power allocation. This governor can only
 	  operate on cooling devices that implement the power API.
 
+config THERMAL_DEFAULT_GOV_BANG_BANG
+	bool "bang_bang"
+	depends on THERMAL_GOV_BANG_BANG
+	help
+	  Use the bang_bang governor as default. This throttles the
+	  devices one step at the time, taking into account the trip
+	  point hysteresis.
+
 endchoice
 
 config THERMAL_GOV_FAIR_SHARE

drivers/thermal/thermal_core.h

@@ -23,6 +23,8 @@
 #define DEFAULT_THERMAL_GOVERNOR       "user_space"
 #elif defined(CONFIG_THERMAL_DEFAULT_GOV_POWER_ALLOCATOR)
 #define DEFAULT_THERMAL_GOVERNOR       "power_allocator"
+#elif defined(CONFIG_THERMAL_DEFAULT_GOV_BANG_BANG)
+#define DEFAULT_THERMAL_GOVERNOR       "bang_bang"
 #endif
 
 /* Initial state of a cooling device during binding */