What does "console [ttyS0] enabled mean?

linux serial-port uart
mikenycz picture mikenycz · May 9, 2014 · Viewed 12.2k times · Source

I am trying to communicate between a single board computer and a PC over COM ports. On the single board computer I am running Debian Linux and there are two UARTs that I can use. On one of the UARTs (ttyS1) I can communicate (send and receive) with no problems. When I try to communicate on the other UART (ttyS0) the send works perfectly however the receive loses the 1st byte in (sends an error message to the console on the PC saying that the character that is typed is not a command) and then all future characters are received correctly.

If I switch from receiving to sending and then back to receiving the same occurs. If I stay in receive mode I can receive characters for as long as I want with no data loss. It appears that the transitioning from send to receive is causing this to happen.

As I mentioned earlier I do not see this issue on the other UART (ttyS1). I started looking for reasons why the two ports are different. I used the same program to set up send and receive for both UARTS so the problem is not in the program setup. One thing that I did find when I typed the command dmesg |grep tty I get the following:

[    0.000000] Kernel command line: console=ttyS0,115200 root=/dev/mmcblk0p2 roo twait loglevel=8 panic=10
[    0.446780] sunxi-uart.1: ttyS0 at MMIO 0x1c28400 (irq = 2) is a U6_16550A
[    1.114996] console [ttyS0] enabled
[    1.154643] sunxi-uart.3: ttyS1 at MMIO 0x1c28c00 (irq = 4) is a U6_16550A

I see that ttyS0 has the "console [ttyS0] enabled" associated with it where the ttyS1 does not. I was wondering what the "console [ttyS0] enabled means?

Also, is there a way to disable it to see if this is what is causing my 'first byte of data loss'? I see in my setup for the single board computer that ttyS0 is designated the "debug port'.

I was also wondering if that is analogous to console?

Can someone please explain what having the console enabled implies?

Answer

David Aparicio picture David Aparicio · Jan 23, 2018

The console is the tty where the kernel logs go to.

You select a specific one through kernel parameters when booting, it is seen in the log you provided, in the line "Kernel command line: console=ttyS0,115200 root=/dev/mmcblk0p2 roo twait loglevel=8 panic=10"

You can select another console (if available in your platform) so ttyS0 will not get any bytes on kernel boot.

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