qmk_firmware/quantum/split_common/post_config.h

#if defined(USE_I2C)
// When using I2C, using rgblight implicitly involves split support.
#    if defined(RGBLIGHT_ENABLE) && !defined(RGBLIGHT_SPLIT)
#        define RGBLIGHT_SPLIT
#    endif

#    ifndef F_SCL
#        define F_SCL 100000UL  // SCL frequency
#    endif

#else  // use serial
// When using serial, the user must define RGBLIGHT_SPLIT explicitly
//  in config.h as needed.
//      see quantum/rgblight_post_config.h
#    if defined(RGBLIGHT_ENABLE) && defined(RGBLIGHT_SPLIT)
// When using serial and RGBLIGHT_SPLIT need separate transaction
#        define SERIAL_USE_MULTI_TRANSACTION
#    endif
#endif