I'm running Armbian_5.20_Micro_Debian_jessie_3.4.112 on an Olimex A20 micro board.

I find that opening serial port device `/dev/ttyS2` causes the system to crash. The board heartbeat LED either stops or blinks rapidly. The board itself is not pingable.

I can reproduce this issue with a simple from the shell using: 

cat /dev/ttyS2

 or from c code using:

fd = open("/dev/ttyS2", O_RDWR | O_NOCTTY | O_NDELAY);

`/dev/ttyS2` is linked to UART6. In the fex file, I have configured it like so:

[uart_para6]
uart_used = 1
uart_port = 6
uart_type = 2
uart_tx = port:PI12<3><default><default><default>
uart_rx = port:PI13<3><default><default><default>

Note that the UART6 is on MUX3 as it should be according to sunxi's pio documentation for A20.

 

Any ideas?

 

`/dev/ttyS2` is linked to UART6.

Why do you think so? If I remember correctly uart_port = 6 should configure /dev/ttyS6, but you can check the source code if you want to confirm port numbering scheme

Why do you think so? If I remember correctly uart_port = 6 should configure /dev/ttyS6, but you can check the source code if you want to confirm port numbering scheme

 

I'm not sure why, but this is the behaviour I'm currently seeing. 

It may have to do with the fact that I've set uart_used = 0 for all UARTs but 0,3,6 & 7. So, although I do get ttyS0 -> ttyS7, UARTs 0,3,6 & 7 are configured as ttyS0 to ttyS3 & I can't access >ttyS3.

$ cat /dev/ttyS7
cat: /dev/ttyS7: Input/output error

In any case if you have a kernel crash, then you shouyld probably connect a serial adapter to any of the working consoles and try to capture the crash log.

Also you should check the dmesg to see what UART ports were registered (there should be at least some info)

Thanks for pointing me in the right direction @zador.blood.stained.
Here are the crash logs from serial console: 

root@micro:~# cat /dev/ttyS2
<4>i2c-0, xfer timeout

[   63.123577] i2c-0, xfer timeout
<3>axp_mfd 0-0034: failed reading at 0x23

[   63.130520] axp_mfd 0-0034: failed reading at 0x23
<3>[cpu_freq] ERR:try to set voltage failed!

[   63.139404] [cpu_freq] ERR:try to set voltage failed!

And here's the dmesg output for registered uarts:

root@micro:~# dmesg | grep uart
[    0.996902] [uart]: used uart info.: 0xc9
[    1.001620] [uart]: serial probe 0 irq 33 mapbase 0x01c28000
[    1.027944] sunxi-uart.0: ttyS0 at MMIO 0x1c28000 (irq = 33) is a U6_16550A
[    1.889704] [uart]: serial probe 3 irq 36 mapbase 0x01c28c00
[    1.921599] sunxi-uart.3: ttyS1 at MMIO 0x1c28c00 (irq = 36) is a U6_16550A
[    1.933439] [uart]: serial probe 6 irq 51 mapbase 0x01c29800
[    1.965318] sunxi-uart.6: ttyS2 at MMIO 0x1c29800 (irq = 51) is a U6_16550A
[    1.977174] [uart]: serial probe 7 irq 52 mapbase 0x01c29c00
[    2.009053] sunxi-uart.7: ttyS3 at MMIO 0x1c29c00 (irq = 52) is a U6_16550A

Have you seen something like this before?

I was able to solve the problem. It was related to UART settings in the Fex file.

Somehow, I hadn't set the pull-up on UART6 & UART7. ->

[uart_para6]

uart_used = 1

uart_port = 6

uart_type = 2

uart_tx = port:PI12<3><default><default><default>

uart_rx = port:PI13<3><default><default><default>

#                          ^

#                          |

#                         Here

It should've been ->

[uart_para6]

uart_used = 1

uart_port = 6

uart_type = 2

uart_tx = port:PI12<3><1><default><default>

uart_rx = port:PI13<3><1><default><default>

I guess I will be moving to the mainline kernel in the future.