serial port turnoff

[  240.497035] INFO: task kworker/0:1:15 blocked for more than 60 seconds.
[  240.504191] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
[  240.515579] Kernel panic - not syncing: hung_task: blocked tasks
[  240.521990] [<c0013250>] (unwind_backtrace+0x0/0xe0) from [<c05e1118>] (panic+0x84/0x1e0)
[  240.530677] [<c05e1118>] (panic+0x84/0x1e0) from [<c0092308>] (watchdog+0x1d0/0x224)
[  240.538894] [<c0092308>] (watchdog+0x1d0/0x224) from [<c0058ad0>] (kthread+0xa0/0xac)
[  240.547209] [<c0058ad0>] (kthread+0xa0/0xac) from [<c000d618>] (ret_from_fork+0x14/0x3c)