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device disconnection problem

Our application runs on Linux platform. Mobiles are connected to system USB ports  using hubs. The application communicates with these mobiles. We modified the usb-serial.c driver under ....linux/drivers/usb/serial/ to suit our application. One functionality of the mobile which needs to be supported by the application is Hardware reset( swith off the mobile and switch it on again).
The kernel version being used is Automated Hardware reset tests on 2 mobiles for 200 iterations were successful. But when tried the same tests with 4 mobiles, sometimes it was successful and sometimes the mobiles are getting disconnected after 30-40 iterations of tests. In case of mobile disconnection, physically mobiles are in proper state, but the core is not able to detect the mobiles (the mobiles are not listed in " /proc/bus/usb/devices"), but the hubs are still detected.
An observation is, when the disconnection problem is observed, unplugging and plugging back the cable from system USB port is helping to get back the devices. 
Error messages observed in  /var/log/messages are:

usb 1-1.1.1: khubd timed out on ep0in len=0/64


   usb 1-1.1.1: khubd timed out on ep0out len=0/0

usb 1-1.1.1: device descriptor read/64, error -110

usb 1-1.1.2: device not accepting address 127, error -110

hub 1-1.1:1.0: port 1 not enabled, trying reset again...

hub 1-1.1:1.0: Cannot enable port 1. Maybe the USB cable is bad?


Please share your ideas to solve this issue.


Thanks & Regards,


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