USB OTG on the LG L3 (E400): host mode with a software switch

On the LG Optimus L3 (E400) running CyanogenMod 7, a flash drive plugged in through USB OTG works: it mounts by itself and can be read from the phone. This is what it took, compared with the C660, where there was no way.

What the phone already had

LG’s 2.6.38 kernel already includes the host controller (msm_hsusb_host.0), usb-storage and a debugfs control. The kernel did not need changes. What was missing:

1. Mounting the disk. A usbdisk entry in vold.fstab pointing at /devices/platform/msm_hsusb_host.0, mounting the drive at /mnt/usbdisk.
2. A switch. The micro-USB ID pin does not reach the processor (the OTGSC register always reads 1), so the phone never enters host mode by itself. It has to be forced.

The switch

The kernel accepts the mode in /sys/kernel/debug/otg/mode. I tied it to a system property (sys.otg.mode) through init.e0.rc, and wrote a small app, OtgSwitch, with a button that flips it between host and peripheral. It runs as the system user because only that user may write the property.

How to use it

1. Open OtgSwitch and turn host mode on before plugging the cable.
2. Connect the OTG adapter and the flash drive.
3. The disk shows up at /mnt/usbdisk.
4. To go back, turn host mode off: adb and USB charging return.

Which adapter I used

A 90° micro-USB OTG adapter (Mercado Libre listing).

Limits

• The phone does not supply 5 V. The connected device needs external power.
• Host mode disables VBUS and ID detection and is lost on reboot.
• With host mode on, the USB port is no good for adb: use adb over Wi-Fi (setprop service.adb.tcp.port 5555).
• A mouse and a keyboard show up as ordinary input devices. I tested the mouse on a CM10.1 build.

The code is in the device tree and the app in OtgSwitch.