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I am unable to write to or read from /dev/ttyS0

# cat /dev/ttyS0                
cat: /dev/ttyS0: Input/output error

the device exists, and root has permissions.

# ls -lAhF /dev/ttyS0
crw------- 1 root root 4, 64 Nov 18 12:09 /dev/ttyS0

# setserial -g /dev/ttyS0 
/dev/ttyS0, UART: 16550A, Port: 0x03f8, IRQ: 4

# dmesg | egrep -i 'serial|ttys'
[    1.243271] Serial: 8250/16550 driver, 1 ports, IRQ sharing disabled
[ 2703.151301] ttyS0: LSR safety check engaged!

# cat /proc/devices | grep tty  
  4 tty
  4 ttyS
  5 /dev/tty

I have serial enabled in my kernel:

# grep SERIAL config
CONFIG_SERIAL_8250=y
CONFIG_SERIAL_8250_NR_UARTS=1
CONFIG_SERIAL_8250_RUNTIME_UARTS=1
CONFIG_SERIAL_CORE=y

What do I have to do to make serial work ?

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