Re: PPP Modem Hangup

I have resolved the issue now:

I have commented out the following in my init.rc file
#service console /system/bin/sh
#    class core
#    console
#    disabled
#    user shell
#    group shell log readproc
#    seclabel u:r:shell:s0
#    setenv HOSTNAME console
#    shutdown critical

The console service was configured to use the same UART serial port
that I was using for the PPP connection.
It would interfere with pppd by resetting or controlling the serial
port, causing the "Modem hangup" issue.

I have a stable connection now running for the past 60 minutes. There
is a ppp connection established between Ubuntu Machine and my AOSP
Target Device.
I can ping from AOSP target device into the Ubuntu Machine the
following (it works):
ping -I ppp0 10.0.0.1
(I have to use -I and provide the specific interface to ping even
though the ip route is up to date)
And I cannot ping from Ubuntu to AOSP target.

Strangely, tcpdump shows that packets are received/sent back and forth
 # tcpdump -i ppp0
tcpdump: verbose output suppressed, use -v[v]... for full protocol decode
listening on ppp0, link-type LINUX_SLL (Linux cooked v1), snapshot
length 262144 bytes
14:49:17.949046 IP 10.0.0.1 > 10.0.0.2: ICMP echo request, id 552, seq
618, length 64
14:49:18.320546 IP 10.0.0.2 > 10.0.0.1: ICMP echo request, id 15, seq
34, length 64
14:49:18.351262 IP 10.0.0.1 > 10.0.0.2: ICMP echo reply, id 15, seq
34, length 64
14:49:18.972947 IP 10.0.0.1 > 10.0.0.2: ICMP echo request, id 552, seq
619, length 64
14:49:19.321893 IP 10.0.0.2 > 10.0.0.1: ICMP echo request, id 15, seq
35, length 64
14:49:19.353363 IP 10.0.0.1 > 10.0.0.2: ICMP echo reply, id 15, seq
35, length 64
14:49:19.997088 IP 10.0.0.1 > 10.0.0.2: ICMP echo request, id 552, seq
620, length 64
14:49:20.323381 IP 10.0.0.2 > 10.0.0.1: ICMP echo request, id 15, seq
36, length 64
14:49:20.356685 IP 10.0.0.1 > 10.0.0.2: ICMP echo reply, id 15, seq
36, length 64

I am not blocked any more, but it would be good to know if there is a way for:
1. I would be able to ping from Ubuntu machine as well into the AOSP target
2. Ideally, ping should work without -I and having to explicitly
mention the interface
(something like just ping 10.0.0.1 should work instead of ping -I ppp0 10.0.0.1

Thank you.

On Tue, Sep 10, 2024 at 4:53 PM Paul Mackerras <[email protected]> wrote:
>
> On Tue, Sep 10, 2024 at 11:30:48AM -0700, Devesh Chipade wrote:
> > In my options file, I have debug and kdebug 7.
> > This is all the logs I could get:
>
> The pppd logs look fine.  There is nothing in the kernel (dmesg) log
> that looks relevant (in fact kdebug 7 is mostly useful if you are
> actually debugging the kernel driver; it doesn't really help with
> problems like this).
>
> Do you have logs from pppd on the ubuntu/linux side?  Is something
> causing it to terminate?
>
> Alternatively, do you have another service running on either side
> which also thinks it is using the same serial port?  Something like a
> getty, or whatever the systemd equivalent is for providing a login
> prompt?  You could do "fuser /dev/ttyMSM0" or "fuser /dev/ttyUSB0" as
> root during the 6 seconds when the link is up to check if something
> other than pppd also has the serial port open.
>
> Paul.