本文轉自：http://blog.chinaunix.net/uid-9688646-id-5602722.html
USB接口上有4個管腳，OTG功能有5個。原來4個分別是電 D+ D- 地。 現在增加了一個ID。這個ID線就決定了自己做主設備還是從設備。如果ID線是高則自己是從設備，反之是主設備。
USB_OTG_ID定義爲W23，即GPIO1_IO24
/test/gpio_get.sh24用來讀出gpio value。讀出來總是0:低電平。所以也不知道前面這句描述是對是錯了。
imx6 kernel configas:
-> USB GadgetDrivers ( [=m])
->File-backedStorage Gadget(USB_FILE_STORAGE [=m]) 。記得：不是選massstorage. 而且選模塊方式，以方便安裝模塊時輸入參數。
內置驅動時，測試打印：
g_file_storagegadget: controller 'fsl-usb2-udc' not recognized
g_file_storagegadget: No serial-number string provided!
g_file_storagegadget: no file given for LUN0
顯然，這就是模塊參數未給定導致的。
編譯模塊：
makeM=drivers/usb/gadget/ modules
得到
root@ubuntu:/home/imx6s/linux-3.0.101#make M=drivers/usb/gadget/ modules
Building modules, stage 2.
MODPOST 1 modules
LD [M] drivers/usb/gadget/g_file_storage.ko
root@imx6s ~$insmod /lib/modules/g_file_storage.ko file=/dev/mmcblk0
g_file_storagegadget: controller 'fsl-usb2-udc' not recognized
g_file_storagegadget: No serial-number string provided!
g_file_storagegadget: File-backed Storage Gadget, version: 1 September 2010
g_file_storagegadget: Number of LUNs=1
g_file_storagegadget-lun0: ro=0, nofua=0, file: /dev/mmcblk0
Suspend udc forOTG auto detect
fsl-usb2-udc: bindto driver g_file_storage
root@imx6s ~$g_file_storage gadget: high speed config #1
win7也同時顯示說找到設備驅動，磁盤驅動器：linux File-Stor Gadget USB Device
下面反覆插拔USB-PC線幾次，然後換成USB-SD卡，都是可以的。但再換回USB-PC線就不行了，找不見設備。
root@imx6s ~$g_file_storage gadget: high speed config #1
root@imx6s ~$g_file_storage gadget: high speed config #1
root@imx6s ~$
root@imx6s ~$g_file_storage gadget: high speed config #1
root@imx6s ~$
root@imx6s ~$ usb1-1: new high speed USB device number 2 using fsl-ehci
usb 1-1: devicev0930 p6545 is not supported
scsi0 :usb-storage 1-1:1.0
scsi 0:0:0:0:Direct-Access TOSHIBA TransMemory PMAP PQ: 0 ANSI: 4
sd 0:0:0:0: [sda]30489408 512-byte logical blocks: (15.6 GB/14.5 GiB)
sd 0:0:0:0: [sda]Write Protect is off
sd 0:0:0:0: [sda]No Caching mode page found
sd 0:0:0:0: [sda]Assuming drive cache: write through
sd 0:0:0:0: [sda]No Caching mode page found
sd 0:0:0:0: [sda]Assuming drive cache: write through
sda: sda1
sd 0:0:0:0: [sda]No Caching mode page found
sd 0:0:0:0: [sda]Assuming drive cache: write through
sd 0:0:0:0: [sda]Attached SCSI removable disk
root@imx6s ~$ usb1-1: USB disconnect, device number 2
wait otg vbuschange timeout!
root@imx6s ~$ usb1-1: new high speed USB device number 3 using fsl-ehci
usb 1-1: devicev0930 p6545 is not supported
scsi1 :usb-storage 1-1:1.0
scsi 1:0:0:0:Direct-Access TOSHIBA TransMemory PMAP PQ: 0 ANSI:4
sd 1:0:0:0: [sda]30489408 512-byte logical blocks: (15.6 GB/14.5 GiB)
sd 1:0:0:0: [sda]Write Protect is off
sd 1:0:0:0: [sda]No Caching mode page found
sd 1:0:0:0: [sda]Assuming drive cache: write through
sd 1:0:0:0: [sda]No Caching mode page found
sd 1:0:0:0: [sda]Assuming drive cache: write through
sda: sda1
sd 1:0:0:0: [sda]No Caching mode page found
sd 1:0:0:0: [sda]Assuming drive cache: write through
sd 1:0:0:0: [sda]Attached SCSI removable disk
root@imx6s ~$
root@imx6s ~$ usb1-1: USB disconnect, device number 3
wait otgvbus change timeout!
root@imx6s ~$
root@imx6s ~$ usb1-1: new high speed USB device number 4 using fsl-ehci
usb 1-1: devicev0930 p6545 is not supported
scsi2 :usb-storage 1-1:1.0
scsi 2:0:0:0:Direct-Access TOSHIBA TransMemory PMAP PQ: 0 ANSI: 4
sd 2:0:0:0: [sda]30489408 512-byte logical blocks: (15.6 GB/14.5 GiB)
sd 2:0:0:0: [sda]Write Protect is off
sd 2:0:0:0: [sda]No Caching mode page found
sd 2:0:0:0: [sda]Assuming drive cache: write through
sd 2:0:0:0: [sda]No Caching mode page found
sd 2:0:0:0: [sda]Assuming drive cache: write through
sda: sda1
sd 2:0:0:0: [sda]No Caching mode page found
sd 2:0:0:0: [sda]Assuming drive cache: write through
sd 2:0:0:0: [sda]Attached SCSI removable disk
usb 1-1: USBdisconnect, device number 4
wait otg vbuschange timeout!
USB在host模式時，才需要對外輸出電源。slave mode時，不需要輸出電源，但也不影響工作。
電源管腳：
IMX_GPIO_NR(4, 15),111
/test/gpio_set.sh111 0 //關pwr
實測不影響工作。其實，我覺得USB_ID和USB_VBUS就沒什麼關係。
