The error#

I was having trouble with a USB joystick adapter (an EMS Playstation controller adapter, to be specific). When I plugged it in, it wouldn't work and checking dmesg showed the same error getting generated over and over again (at least once per second):

$ dmesg
...
[81700.968873] usbhid 6-1:1.0: can't add hid device: -71
[81700.968885] usbhid: probe of 6-1:1.0 failed with error -71
[81700.968986] usb usb6-port1: disabled by hub (EMI?), re-enabling...
[81700.968991] usb 6-1: USB disconnect, device number 53
[81701.208025] usb 6-1: new low-speed USB device number 54 using uhci_hcd
[81701.384866] usb 6-1: string descriptor 0 read error: -32
...

The wrong fix#

I decided the sensible thing to do was to reload the driver:

$ sudo modprobe -r usbhid  # Bad idea, don't run this

An aside on kernel modules#

Drivers in Linux are called kernel modules and are managed by the modprobe command whose -r or --remove option unloads a module. The lsmod command lists the currently loaded modules. The actual modules are the .ko files stored under /lib/modules/$(uname -r)/kernel/.

When compiling a kernel, most features that can be compiled as modules can also be compiled into the kernel. That means that just because a system supports a feature doesn't mean that feature is loaded as a module. But the kernels that come with most distributions tend to have everything as separate modules to best support as many different hardware configurations as possible.

The usbhid kernel module#

USB joysticks are handled by the usbhid module which handles all USB human interface devices including joysticks, mice, keyboards… See the reason for the title of this blog post now? After running that command, my computer's mouse and keyboard stopped working because I had just unloaded the driver for them.

Fixing my mistake#

The right thing to do would have been to run a command that included reloading the driver:

$ sudo modprobe -r usbhid; sudo modprobe usbhid

I was about to ssh in from another computer when I remembered a simpler workaround: unplugging the joystick adapter and plugging it back in. Then my keyboard and mouse worked again because whenever a device is plugged in, udev loads the appropriate kernel modules for it, in this case, reloading the usbhid module.

A happy ending#

The good news is, the joystick adapter worked, so unloading and reloading usbhid was the right fix.