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

Fast serial port DMA flush

configname: CONFIG_ETRAX_SERIAL_FLUSH_DMA_FAST

Linux Kernel Configuration
└─>Fast serial port DMA flush
In linux kernel since version 2.6.12  
Select this to have the serial DMAs flushed at a higher rate than
normally possible through a fast timer interrupt (currently at
15360 Hz).
If unsure, say N.