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 :/ Thanks a bunch for all the support. Never expected kernel threads to be this supportive :) I would very much like to contribute, if there is something trivial that i can work on while learning. Thanks! Malith -- 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
