Hi, dear list, I want to be clear about the USB hotplug procedure. I read "Linux Device Driver" and "Documentation/usb/hotplug.txt", and google much. Per my understanding, the USB hotplug works as following: 1) Insertion of physical U-disk will trigger a hardware interrupt. And interrupt handler (installed by USB bus driver?) will create kobj at /sys. 2) According to Linux device model, the creation of kobj will raise hotplug event (implemented in USB bus driver). 3) user space hotplug helper is invoked to handle hotplug event. Hotplug helper will read "modules.usbmaps" and decide to load which USB driver. If there are many drivers matching the device, they will all be loaded? 4) udev will create approparite dev under /dev according to rules defined. Please correct me if I am wrong in the above. I am using Ubuntu 11, and with my own build kernel version 3.15.0. Currently a new plugged USB disk can be recognized by my Ubuntu. But I didn't see "modules.usbmaps" file under /lib/modules/`uname -r` root@thunderCat:/mnt/data# ls /lib/modules/`uname -r` build modules.alias modules.builtin modules.dep modules.devname modules.softdep modules.symbols.bin kernel modules.alias.bin modules.builtin.bin modules.dep.bin modules.order modules.symbols source And there is no /proc/sys/kernel/hotplug root@thunderCat:/mnt/data# cat /proc/sys/kernel/hotplug root@thunderCat:/mnt/data# file /sbin/hotplug /sbin/hotplug: ERROR: cannot open `/sbin/hotplug' (No such file or directory) My Question is: 1) How can my Ubuntu hotplug work without "modules.usbmaps" and "/sbin/hotplug"? 2) Is there any tech I can use to "trace" the kernel/driver behavior of the USB hotplug procedure? (for example, kernel functions traversed) Thanks, Simon The config should have enabled HOTPLUG: root@thunderCat:/mnt/data# grep CONFIG_HOTPLUG /boot/config-`uname -r` CONFIG_HOTPLUG_CPU=y CONFIG_HOTPLUG_PCI_PCIE=y CONFIG_HOTPLUG_PCI=y # CONFIG_HOTPLUG_PCI_COMPAQ is not set # CONFIG_HOTPLUG_PCI_IBM is not set # CONFIG_HOTPLUG_PCI_ACPI is not set CONFIG_HOTPLUG_PCI_CPCI=y # CONFIG_HOTPLUG_PCI_CPCI_ZT5550 is not set # CONFIG_HOTPLUG_PCI_CPCI_GENERIC is not set # CONFIG_HOTPLUG_PCI_SHPC is not set