Suspend USB devices (turn the power off)

昨天朋友問我把 USB 裝置停電的實際命令，Google 了一下找到了方法，筆記一下。

首先要知道你的 USB 裝置代號，可以在插入後使用 dmesg 查看^[?]。例：

[ 8944.330557] usb 1-8: USB disconnect, address 8
[ 9023.580525] usb 1-8: new high speed USB device using ehci_hcd and address 9
[ 9023.714733] usb 1-8: New USB device found, idVendor=0b27, idProduct=0163
[ 9023.714737] usb 1-8: New USB device strings: Mfr=1, Product=2, SerialNumber=3
[ 9023.714739] usb 1-8: Product: USB Mass Storage Device
[ 9023.714741] usb 1-8: Manufacturer: USBest Technology
[ 9023.714743] usb 1-8: SerialNumber: 000000000016F3
[ 9023.714828] usb 1-8: configuration #1 chosen from 1 choice
[ 9023.715549] scsi7 : SCSI emulation for USB Mass Storage devices
[ 9023.715694] usb-storage: device found at 9
[ 9023.715696] usb-storage: waiting for device to settle before scanning
[ 9028.712208] usb-storage: device scan complete
[ 9028.713173] scsi 7:0:0:0: Direct-Access     Ut163    USB2FlashStorage 0.00 PQ: 0 ANSI: 2
[ 9028.717275] sd 7:0:0:0: [sdd] 1974271 512-byte logical blocks: (1.01 GB/963 MiB)
[ 9028.718294] sd 7:0:0:0: [sdd] Write Protect is off
[ 9028.718299] sd 7:0:0:0: [sdd] Mode Sense: 00 00 00 00
[ 9028.718301] sd 7:0:0:0: [sdd] Assuming drive cache: write through
[ 9028.725637] sd 7:0:0:0: [sdd] Assuming drive cache: write through
[ 9028.725645]  sdd: sdd1
[ 9028.837666] sd 7:0:0:0: [sdd] Assuming drive cache: write through
[ 9028.837672] sd 7:0:0:0: [sdd] Attached SCSI removable disk
[ 9149.868714] Shorewall:net2fw:DROP:IN=eth0 OUT= MAC=00:1f:d0:37:37:0a:00:30:88:12:54:d6:08:00 SRC=189.1.169.123 DST=123.205.248.119 LEN=40 TOS=0x00 PREC=0x00 TTL=92 ID=256 PROTO=TCP SPT=6000 DPT=1433 WINDOW=16384 RES=0x00 SYN URGP=0
[ 9231.369074] sd 7:0:0:0: [sdd] 1974271 512-byte logical blocks: (1.01 GB/963 MiB)
[ 9231.369566] sd 7:0:0:0: [sdd] Assuming drive cache: write through
[ 9231.370687] sd 7:0:0:0: [sdd] Assuming drive cache: write through
[ 9231.370690]  sdd: sdd1

因為它寫 usb 1-8，所以要控制的 USB 接口代號就是 1-8，很簡單吧？反正找有 usb 字樣的就對了。底下的 sd 是表示它被分配到哪個磁區代號，因為這支是隨身碟才會是這樣，其他類型的裝置會有不同的訊息，不過這不是重點，所以沒差啦。

然後請到 /sys/bus/usb/devices 裡找剛剛的接口代號的資料夾，在那裡面找 power/level，該檔是一個文字檔，或者說看起來是文字檔，內容應該是 on。只要把 on 改成 suspend 就可以把電源切斷。注意不是 off，如果輸入錯誤的字串它會無法存檔，不用擔心會改錯。
以本例來說是 1-8 所以是：

# echo suspend > /sys/bus/usb/devices/1-8/power/level

電源燈就會熄了。
噢，當然你要有相當於 root 的權限。

Reference