A box fell on my USB Cisco console cable, so I used /dev/ttyS0 with the old style cable. Using a command like screen /dev/ttyS0 9600, produces no output until a char is sent, at which point the prompt scrolls off the screen and the router misinterprets input (nothing sent by the user). Tried with different bauds, but the routers default is 9600.

So, I left the cable attached and got this twice:

[294226.522846] BUG: unable to handle kernel NULL pointer dereference at 0000000000000099
[294226.523218] IP: [<ffffffff816b94f2>] uart_put_char+0x72/0x90
[294226.523418] PGD 2551ba067 PUD 251c51067 PMD 0
[294226.523615] Oops: 0002 [#1] SMP
[294226.523785] Modules linked in: ftdi_sio usbserial pci_stub vboxpci(O) vboxnetadp(O) vboxnetflt(O) vboxdrv(O) ipv6 video lp ppdev parport_pc parport fuse radeon ttm fbcon bitblit softcursor font tileblit drm_kms_helper snd_hda_codec_realtek coretemp drm snd_hda_codec_generic processor agpgart snd_hda_intel snd_hda_codec kvm_intel igb backlight cfbfillrect cfbimgblt cfbcopyarea fb_sys_fops syscopyarea snd_hda_core sysfillrect snd_hwdep sysimgblt ptp fb firewire_ohci gpio_ich firewire_core lpc_ich kvm snd_pcm pps_core ehci_pci uhci_hcd ehci_hcd fbdev dca i2c_dev aacraid i2c_i801 i2c_algo_bit evdev i2c_core i5k_amb hwmon snd_timer irqbypass shpchp snd soundcore loop
[294226.523785] CPU: 3 PID: 8850 Comm: kworker/u16:0 Tainted: G O 4.5.2slug #2
[294226.523785] Hardware name: Supermicro X7DWA/X7DWA, BIOS 1.2 11/04/2008
[294226.523785] Workqueue: events_unbound flush_to_ldisc
[294226.523785] task: ffff880230a37300 ti: ffff88002ec28000 task.ti: ffff88002ec28000
[294226.523785] RIP: 0010:[<ffffffff816b94f2>] [<ffffffff816b94f2>] uart_put_char+0x72/0x90
[294226.523785] RSP: 0018:ffff88002ec2bc50 EFLAGS: 00010002
[294226.523785] RAX: 0000000000000246 RBX: ffff88025627d800 RCX: 0000000000000099
[294226.523785] RDX: 0000000000000000 RSI: 000000000000006e RDI: ffffffff823a64c0
[294226.523785] RBP: ffff88002ec2bc70 R08: ffffc90006d4a000 R09: 0000000000000018
[294226.523785] R10: 0000000000000000 R11: 000000000000006e R12: ffffffff823a64c0
[294226.523785] R13: 000000000000006e R14: 0000000000000001 R15: ffffc90006d4a000
[294226.523785] FS: 0000000000000000(0000) GS:ffff88025fcc0000(0000) knlGS:0000000000000000
[294226.523785] CS: 0010 DS: 0000 ES: 0000 CR0: 000000008005003b
[294226.523785] CR2: 0000000000000099 CR3: 00000002557cc000 CR4: 00000000000006e0
[294226.523785] Stack:
[294226.523785] 0000000000000f76 ffff8802309ba000 0000000000000081 0000000000000097
[294226.523785] ffff88002ec2bc88 ffffffff8169bd04 ffffc96e06d4a000 ffff88002ec2bca0
[294226.523785] ffffffff816a04b5 0000000000000f76 ffff88002ec2bce0 ffffffff816a075d
[294226.523785] Call Trace:
[294226.523785] [<ffffffff8169bd04>] tty_put_char+0x24/0x40
[294226.523785] [<ffffffff816a04b5>] do_output_char+0xa5/0x200
[294226.523785] [<ffffffff816a075d>] __process_echoes+0x14d/0x280
[294226.523785] [<ffffffff816a13e6>] commit_echoes+0x66/0xa0
[294226.523785] [<ffffffff816a3b4f>] n_tty_receive_buf_common+0x73f/0xae0
[294226.523785] [<ffffffff816a3f04>] n_tty_receive_buf2+0x14/0x20
[294226.523785] [<ffffffff816a6668>] flush_to_ldisc+0xd8/0x180
[294226.523785] [<ffffffff810a353e>] process_one_work+0x14e/0x3f0
[294226.523785] [<ffffffff810a390b>] worker_thread+0x12b/0x490
[294226.523785] [<ffffffff81be75f4>] ? __schedule+0x2c4/0x8a0
[294226.523785] [<ffffffff810a37e0>] ? process_one_work+0x3f0/0x3f0
[294226.523785] [<ffffffff810a89b2>] kthread+0xd2/0xf0
[294226.523785] [<ffffffff810a88e0>] ? kthread_park+0x50/0x50
[294226.523785] [<ffffffff81beb56f>] ret_from_fork+0x3f/0x70
[294226.523785] [<ffffffff810a88e0>] ? kthread_park+0x50/0x50
[294226.523785] Code: 00 00 81 e2 ff 0f 00 00 75 17 48 89 c6 4c 89 e7 e8 84 17 53 00 44 89 f0 5b 41 5c 41 5d 41 5e 5d c3 48 8b 93 68 01 00 00 41 b6 01 <44> 88 2c 0a 8b b3 70 01 00 00 8d 56 01 81 e2 ff 0f 00 00 89 93
[294226.523785] RIP [<ffffffff816b94f2>] uart_put_char+0x72/0x90
[294226.523785] RSP <ffff88002ec2bc50>
[294226.523785] CR2: 0000000000000099
[294226.539882] ---[ end trace 89072a058dde5b68 ]---


Any ideas whats going on? Its Slackware 14.1 kernel 4.5.2 Supermicro X7DWA. Thanks

I'll go way out on a limb and say you need a new cable.

Make sure agetty or some other program isn't running on ttyS0. Besides baud rate, the other typical problems on serial ports are connector pinout and flow control settings.

Bingo! Thanks.

Bit surprised this brought the system down, totally forgot about getty.