init_uart_baud initial UART baud rate. Default 115200
init_uart_clock initial UART clock. Default 3000000 (3 MHz)
・/boot/cmdline.txtを編集(削除)
#dwc_otg.lpm_enable=0 console=ttyAMA0,115200 kgdboc=ttyAMA0,115200 console=tty1 root=/dev/mmcblk0p2 rootfstype=ext4 elevator=deadline rootwait
dwc_otg.lpm_enable=0 console=tty1 root=/dev/mmcblk0p2 rootfstype=ext4 elevator=deadline rootwait
・/etc/inittab ここをcomment out
T0:23:respawn:/sbin/getty -L ttyAMA0 115200 vt100
・ /boot/config.txtに追記
# Higher UART Speed
init_uart_baud=921600
init_uart_clock=14745600