On Wed, Aug 31, 2016 at 02:30:15PM +0530, Malith Yapa wrote: > On Mon, Aug 29, 2016 at 11:29 PM, Alan Stern <stern@xxxxxxxxxxxxxxxxxxx> wrote: > > On Mon, 29 Aug 2016, Greg KH wrote: > > > >> > >> I have since compiled 4.8.0_rc1. And just as you guys suggested a part > >> > >> of problem is solved. The minor number now definitely goes up to 512 > >> > >> before giving no more free serial devices. But it still doesn't reuse > >> > >> the minors after disconnecting. > >> > >> > >> > >> In my code I'm calling modbus_close and modbus_free which in turn call > >> > >> close() and free() on the file descriptor. Shouldn't this make the > >> > >> minor reusable? > > > > Yes, it should. > > > >> > >> >From what i understand minor numbers are allocated and freed by the > >> > >> driver. So how does the driver know to release the minor number after > >> > >> the file descriptor is closed? > >> > > > >> > > The USB serial driver core code handles this for you automatically after > >> > > the last reference goes away. Are you sure that userspace is properly > >> > > releasing the device properly? > >> > > > >> > > thanks, > >> > > > >> > > greg k-h > >> > > >> > >From my understanding it is, but let me double check. If i compile the > >> > kernel with some print statements in usb-serial.c, will i get the > >> > output in stdout? > >> > >> You can use dynamic debugging on the usb_serial.ko kernel module to see > >> the open/close messages in the kernel log, along with when minors are > >> allocated. Read about how to turn that on in the Documentation/ > >> directory (search for dynamic debugging). > > > > The command is: > > > > echo module usbserial =p >/sys/kernel/debug/dynamic_debug/control > > > > (note that you need to mount a debugfs filesystem on /sys/kernel/debug > > first). > > > >> > In the userspace is it sufficient to just call close() on the file > >> > descriptor in /dev ? > >> > >> Yes. > >> > >> > Is it possible that udev has already replaced /dev/ttyUSBx with > >> > /dev/ttyUSBx+1 by the time the program calls close() on it? > >> > >> Yes, if it is open. And udev does not create the device node, the > >> kernel is doing so. > >> > >> If your program/device does the following: > >> - userspace open ttyUSB0 > >> - device disconnect > >> - device connect (ttyUSB1) > >> - userspace close ttyUSB0 > >> - kernel removes ttyUSB0 > >> > >> that could be what is happening here, you are racing and loosing :( > > > > But even in this case, the kernel would re-use the old minor numbers > > once the ttyUSB files had been closed. > > > > Alan Stern > > > > Turns out it was in fact a problem with libmodbus. Someone might even > say it was a problem with how i used libmodbus :/ Yeah, glad you figured it out! > Thanks a bunch for all the support. Never expected kernel threads to > be this supportive :) Hey now, we don't bite :) > I would very much like to contribute, if there is something trivial > that i can work on while learning. Take a look at: http://www.labbott.name/blog/2016/08/15/ideas-for-getting-started-in-the-linux-kernel/ for some suggestions. good luck! greg k-h -- To unsubscribe from this list: send the line "unsubscribe linux-usb" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
