CONFIG_ETRAX_SERIAL_FAST_TIMER is not available for thedefaultarchitecture x86.
Result is shown for architecture cris
default or selected kernelversion does not have config value CONFIG_ETRAX_SERIAL_FAST_TIMER.
Result is shown for kernelversion 6.7-rc3

Use fast timers for serial DMA flush (experimental)


Linux Kernel Configuration
└─>Use fast timers for serial DMA flush (experimental)
In linux kernel since version 2.6.12  
Select this to have the serial DMAs flushed at a higher rate than
normally, possible by using the fast timer API, the timeout is
approx. 4 character times.
If unsure, say N.