Raspberry Pie: serial port login does not display print information

1. HDMI Start raspberry pie

2. open terminal A serial port

 Input ：
cat /proc/cmdline

If not serial=ttyAMA0,115200 That means the problem is here （ Not configured correctly ）.

3. see cmdline.txt The configuration file

cat /boot/cmdline.txt
ls -a /dev/ttyAMA0		# see ttyAMA0

4. Cover config.txt

# Additional overlays and parameters are documented /boot/overlays/README

# Enable audio (loads snd_bcm2835)
dtparam=audio=on

[pi4]
# Enable DRM VC4 V3D driver on top of the dispmanx display stack
dtoverlay=vc4-fkms-v3d
max_framebuffers=2

[all]
#dtoverlay=vc4-fkms-v3d
dtoverlay=pi3-disable-bt		// Stop Bluetooth , Remove the use of serial port 
start_x=1
gpu_mem=128

5. modify cmdline.txt file

dwc_otg.lpm_enable=0 console=serial0,115200 console=tty1 root=/dev/mmcblk0p2 rootfstype=ext4 elevator=deadline fsck.repair=yes rootwait

6. HDMI Start raspberry pie ->terminal

cat /proc/cmdline			# Keep looking at 

success ！！！

What if there is no extra monitor ？

1、 Take out the raspberry pie SD card , Plug it into the card reader , Open the above two files on the computer , Just modify it