Every now and again my xhci_hcd goes wonky and I can't get USB devices to pop up on it. I hate rebooting for usb problems, so I came up with this.

reset-usb-hub

#!/usr/bin/env bash

# old http://billauer.co.il/blog/2013/02/usb-reset-ehci-uhci-linux/
# old http://enc.com.au/2014/02/14/resetting-usb-devices/
# 
# http://www.linux.org/threads/resetting-the-usb-subsystem.6256/
# (page was dead, used google cache)
#
# XXX This definitely worked (2016-08-21), the trouble is finding the serial
# number and the bind/unbind kernel interfaces
# 
# echo 0000:00:14.0 | sudo tee /sys/bus/pci/drivers/xhci_hcd/unbind
# echo 0000:00:14.0 | sudo tee /sys/bus/pci/drivers/xhci_hcd/bind
# 
# XXX This program produces the right output, but will it work in 6 months or
# whenver the hell I need this reset again??
#

## dmesg notes
# [5809402.670110] usb 3-4: new full-speed USB device number 35 using xhci_hcd
# [5809407.786315] usb 3-4: device descriptor read/8, error -110
# [5809412.999259] usb 3-4: device descriptor read/8, error -110
# [5809413.212180] usb 3-4: new full-speed USB device number 36 using xhci_hcd
# [5809418.324116] usb 3-4: device descriptor read/8, error -110
# [5809423.537067] usb 3-4: device descriptor read/8, error -110

## serial notes
# the article uses lsusb, which is slow and meant for human parsing, not
# machine parsing. this file seems to have the serial I need though, and
# matches the bus number that was malfunctioning (3)
#
# echo $(< /sys/bus/usb/devices/usb3/serial)
# 0000:00:14.0
#

## find notes
# it's not always going to be xhci_hcd (there's also uhci and ehci, etc)
# this usually produces exactly one match.
# application of s/$serial/unbind/ aught to do the trick
#
# find /sys/bus/pci/drivers -name 0000:00:14.0
# /sys/bus/pci/drivers/xhci_hcd/0000:00:14.0

BUSNUMBER="$1"; shift

function perform-the-reset() {
    SERIAL="$1"; shift
    DRV="$(sed -e "s,/$SERIAL\$,," <<< "$1")"; shift

    echo "sending unbind: echo $SERIAL | sudo tee $DRV/unbind"
    [ -z "$DRY_RUN" ] && (echo "$SERIAL" | sudo tee "$DRV/unbind")
    [ -z "$DRY_RUN" ] && sleep 2 # doubt this is ever necessary … meh

    echo "sending   bind: echo $SERIAL | sudo tee $DRV/bind"
    [ -z "$DRY_RUN" ] && (echo "$SERIAL" | sudo tee "$DRV/bind")
}

if [ -z "$BUSNUMBER" ]; then
    echo
    echo BUSNUMBER unspecified, trying to guess
    dmesg | grep 'usb [0-9]-[0-9].*error' | tail -n 10 | sed -e 's/^/  /'
    lasterr="$(dmesg | grep 'usb [0-9]-[0-9].*error' | tail -n 1)"
    if [[ "$lasterr" =~ ([0-9])-([0-9]): ]]; then
        GUESS="${BASH_REMATCH[1]}"
        echo "guessing bus=$GUESS hit enter if this is right"
    else
        echo
    fi

    read -ep "usb bus number: " -i "$GUESS" BUSNUMBER
fi

if [ -d "/sys/bus/usb/devices/usb$BUSNUMBER" ]; then
    echo "found the bus device in sysfs /sys/bus/usb/devices/usb$BUSNUMBER"
    SERIAL="$(< /sys/bus/usb/devices/usb$BUSNUMBER/serial)"

    if [ -n "$SERIAL" ]; then
        echo "found the serial number: $SERIAL"
        echo "trying to find the driver bind/unbind files"
        drvsno=( $(find /sys/bus/pci/drivers -name "$SERIAL") )
        drvsnoc=${#drvsno[*]}
        if [ $drvsnoc == 1 ]; then
            echo "found this serial in exactly one driver $drvsno"
            perform-the-reset $SERIAL $drvsno

        elif [ $drvsnoc == 0 ]; then
            echo "unable to find any drivers with this serial, strange"
            echo "this tool was only ever tested on xhci_hcd … patches welcome"

        else
            echo "found more than one driver, please specify which you wish to try"
            for d in "${drvsno[@]}"; do
                echo $d
            done
            read -ep which: -i "${drvsno[0]}" driver

            perform-the-reset $SERIAL $driver
        fi
    fi
fi