[Nexus 4 Original Android Development] ziddey:
Patch Overview:

Kernel with modified msm_otg.c -- This will REPLACE whatever kernel you currently have installed. If you flash a different kernel on top, you will obviously lose OTG capability. This contains the necessary workaround to enable usb host mode ("OTG").

Modified init.mako.rc/init.mako.usbdisk.rc -- Required for creating usb drive directories.

Precompiled modified storage_list.xml -- Allows unmounting usb drive in Settings->Storage. Hex offsets for storageDescription patched during flash.

Addition to build.prop -- Enables downloading apps from play store that require usb host mode support.

Addition to platform.xml -- Workaround to allow apps write access to usb drives

Addition to handheld_core_hardware.xml -- Activate android.hardware.usb.host.xml

android.hardware.usb.host.xml -- Enables Android API support for usb host mode.

fstab.mako (4.3) / vold.fstab (4.2) -- Required for automounting usb drive

Modules cifs.ko, ff-memless.ko, hid-dr.ko, hid-logitech.ko, and xpad.ko (/system/lib/modules). Manually insmod as needed or create an appropriate init.d script to load on boot. These are only required for certain gamepads. Refer here for more information.

Patch Details:

There seems to be an issue with detecting the state of the ID pin on the OTG cable, so we need to come up with an alternate way of determining when to switch to host mode. drivers/usb/otg/msm_otg.c (kernel) is responsible for detecting the charger type and setting host mode, among other tasks. I noticed that when connected to a powered OTG cable, the charge type becomes USB_PROPRIETARY_CHARGER (vs USB_DCP_CHARGER when connected to the wall, and USB_SDP_CHARGER to a computer). This will be the condition that we use to trigger host mode.

Standard OTG cables will have the ID pin shorted to ground. There are also usb accessory charger adapters (ACA) that provide different resistances between these pins to signal functionality (see http://en.wikipedia.org/wiki/USB_On-The-Go#OTG_Micro_Plugs). Support for accessory charger detection isn't enabled in the kernel originally, and doesn't seem to work properly anyway. However, one of the modes is essentially what we're trying to achieve (ID_A): "A charger and a B-device are attached. The OTG device is allowed to charge and enter host mode." So I've added code when USB_PROPRIETARY_CHARGER is detected to simulate the case of ID_A being detected. Following through the code for host mode, certain events are handled differently when ACA support is enabled (specifically, suspension of host mode). In these instances, we need to simulate ACA support since ID_A is technically dependent on it (run into issues with the usb controller getting stuck in a suspended state otherwise). Now we have host mode with charging working properly.

Finally, we need a method of detecting when the OTG cable is unplugged so the device can switch out of host mode. Fortunately, since power (vbus) detection does work, we can use that. Normally, changes in vbus state are ignored while in host mode, so we need to address that. From there, we simulate ACA detection for the case of no ID_A, which is just clearing the ID_A bit and charger. Afterwards, it'll automatically reset the usb state, ready to start all over again.

The dirty hacks to msm_otg.c are complete, and externally-powered OTG is functional.

Refer here for actual changes: https://github.com/ziddey/mako/commits/nightlies-4.3-JSS

No changes are needed to the kernel's .config. Do not enable Drivers->USB->OTG support (we get our support through "OTG support for Qualcomm on-chip USB controller" which is already enabled) or Support for ACA (does not work and most users don't have the proper adapter anyway).

Now we run into a problem with usb storage. Since there is no /system/etc/vold.fstab, usb drives get automatically mounted to /mnt/shell/emulated/0 (at least in CM10.1), which overloads the emulated sdcard, and causes major problems. So we create /system/etc/vold.fstab:

dev_mount usbdisk /storage/usbdisk0 auto /devices/platform/msm_hsusb_host/usb2

Update:

Update:

But /storage/usbdisk0 does not exist, so it will fail to mount. We will be using /init.mako.rc to create this directory and symlink associated legacy ones. This file resides in a ramdisk (which combines with the kernel to form boot.img), so we need to modify that instead of /init.mako.rc on the device itself (since it wouldn't be able to persist through a reboot). As well, we define the environmental variable SECONDARY_STORAGE. Below the analogous /storage/sdcard0 lines, add:

export SECONDARY_STORAGE /storage/usbdisk0

mkdir /storage/usbdisk0 0666 system system

symlink /storage/usbdisk0 /usbdisk
symlink /storage/usbdisk0 /mnt/usbdisk

Update:

In order to enable Settings->Storage->USB Storage, res/xml/storage_list.xml in /system/framework/framework-res.apk needs to be modified. We should be able to simply inject an encoded version of our modified storage_list.xml. I'm not sure if it's possible to simply encode a single file, so I decompiled framework-res.apk in order to make the following addition to res/xml/storage_list.xml (inside StorageList):

<storage android:mountPoint="/storage/usbdisk0"
android:storageDescription="@string/storage_usb"
android:primary="false"
android:removable="true" />

After recompiling, we should now be able to extract our newly encoded storage_list.xml for use with any ROM's framework-res.apk.

To allow downloading apps from the market that require usb host support, we need to add the following to /system/build.prop:

ro.usb.host=1

To enable android api support for usb host, we need to create /system/etc/permissions/android.hardware.usb.host.xml with the following:

<?xml version="1.0" encoding="utf-8"?>
<permissions>
<feature name="android.hardware.usb.host" />
</permissions>

Now to "activate" this file, we add to /system/etc/permissions/handheld_core_hardware.xml:

<feature name="android.hardware.usb.host" />

Google assigned a different permission group for secondary storage devices (e.g. usb drives), media_rw, for which user apps cannot have write access (Chainfire has a good article on the issue here). We need to modify /system/etc/permissions/platform.xml to allow write access. Add the line in red ("officially" used by Samsung):

<permission name="android.permission.WRITE_EXTERNAL_STORAGE" >
[color=red]<group gid="media_rw" />[/color]
<group gid="sdcard_rw" />
</permission>

That's it! Externally powered usb host mode should be fully functional.

For full disclosure, these are the changes to the kernel config vs stock (really just NTFS and modules):

echo "CONFIG_INPUT_JOYSTICK=y" >> .config
echo "CONFIG_JOYSTICK_XPAD=m" >> .config
echo "CONFIG_JOYSTICK_XPAD_FF=y" >> .config
echo "CONFIG_JOYSTICK_XPAD_LEDS=y" >> .config
echo "CONFIG_INPUT_FF_MEMLESS=m" >> .config
echo "CONFIG_HID_LOGITECH=m" >> .config
echo "CONFIG_LOGITECH_FF=y" >> .config
echo "CONFIG_LOGIRUMBLEPAD2_FF=y" >> .config
echo "CONFIG_LOGIG940_FF=y" >> .config
echo "CONFIG_LOGIWHEELS_FF=y" >> .config
echo "CONFIG_HID_DRAGONRISE=m" >> .config
echo "CONFIG_DRAGONRISE_FF=y" >> .config
echo "CONFIG_CIFS=m" >> .config

echo "CONFIG_NTFS_FS=y" >> .config
# echo "CONFIG_USB_DEBUG=y" >> .config
sed 's/\(CONFIG_USB_STORAGE_DEBUG\)=y/# \1 is not set/' -i .config

Changelog:

4.4: 2013.12.01 0349ET: [fk r196] [fk r196-CM] [aosp r5] aosp includes gamepad kernel modules.

4.4: 2013.11.29 0219ET: [fk r195] [fk r195-CM] CM build attempts to patch Franco ramdisk mods on the fly, so be prepared if things go south.

4.4: 2013.11.21 1922ET: [fk r194] [aosp r2] Update to 4.4 configuration for mounting usbdisk.

4.3: 2013.10.22 2201ET r191: [JWR] [JSS/JLS] Allow potentially faster charging in host mode, re-add manual host mode

4.3: 2013.10.22 2204ET r191-CM: [JWR] [JSS/JLS] ^ + 2 "CAF" commits for CM compatibility

4.3: 2013.10.09 2148ET r190: [JWR] [JSS/JLS] Re-enable USB debug messages

4.3: 2013.10.01 1954ET r188: [JWR] [JSS/JLS] Zipalign framework-res.apk

ziddey-otg-r183-09141713.zip ziddey-otg-r183-JSS-09141713.zip Add SECONDARY_STORAGE env., do ramdisk patching during flash, include CIFS module -- rebase to r183.

ziddey-otg-r182-09041823.zip ziddey-otg-r182-JSS-09041823.zip Remove module unloading support, patch handheld_core_hardware.xml -- rebase to Franco r182.

ziddey-otg-r178-08240234.zip ziddey-otg-r178-JSS-08240238.zipDisable modversions, enable kernel wakelock stats-- rebase to Franco r178.

JSS15J 4.3.0 2013.08.13 1533ET: ziddey-otg-r174-08131533.zip First release for JSS15J. Updated to 4.3's new unified fstab (native mounting support). Using an "anyramdisk" method for compatibility with different ROMs (specifically, different su implementations). Based off Franco r174.

2013.07.29 1101ET: ziddey-otg-r165-07291101.zip Maintenance build-- rebase to Franco r165.

2013.07.14 2015ET: ziddey-otg-r163-07142015.zip Allow automatic host mode without charging-- rebase to Franco r163.

2013.07.08 1420ET: ziddey-otg-r162-07081420.zip Update storage_list.xml for compatibility with new CM nightlies-- rebase to Franco r162.

2013.06.28 1551ET: ziddey-otg-M3-06281551.zip Maintenance build-- rebase to Franco M3.

2013.06.27 0427ET: ziddey-otg-r156-06270427.zip Re-enable read-only NTFS support in kernel.

2013.06.06 1736ET: ziddey-otg-r151-06061736.zip Releases will now include modules ff-memless.ko, hid-dr.ko, hid-logitech.ko, and xpad.ko (/system/lib/modules). Manually insmod as needed or create an appropriate init.d script to load on boot. Rebase to Franco r151.

2013.05.25 0749ET: ziddey-otg-05250749.zip Fix compatibility issue with CWM (MTP crashes).

2013.05.23 2119ET: ziddey-otg-05232119.zip Start charging immediately when entering host mode. This resolves issues with proprietary chargers.

2013.05.22 2305ET: ziddey-otg-05222305.zip Rebase to Franco's r140. Revert checks for actual proprietary chargers in favor of manually disabling automatic host mode (temporary). Issue "# echo disable > /sys/kernel/debug/msm_otg/aca" to disable automatic host mode (enable to re-enable).

2013.05.17 0107ET: ziddey-otg-05170107.zip Added check for other proprietary charger case. Rebase to Franco's r137.

2013.05.15 0124ET: ziddey-otg-05150124.zip Attempt to detect actual proprietary chargers (Apple-compatible) and charge properly. Rebase to Franco's r136.

2013.05.09 1729ET: ziddey-otg-05091729.zip Should now patch precompiled storage_list.xml to address incorrect strings (Internal Storage/USB Storage). Re-enabled verbose usb debugging messages (dmesg).

2013.05.06 1846ET: ziddey-otg-05061846.zip Maintenance build. Based off Franco's current nightlies branch (r134?). Updated storage_list.xml to match current CM nightlites. Removed freshen flag to hopefully address issues with framework-res.apk not being patched (thanks sga999). Verbose usb debugging messages not enabled (unaltered r134 config)

2013.04.07 0355ET: ziddey-otg-04070355.zip Now modifies /system/etc/permissions/platform.xml to allow app write access to usb storage (thanks garyd9 for script code). Kernel build number reverted to 105 to match Franco's M1 build number.

2013.03.27.2338ET: ziddey-otg-03272334.zip Allow host mode when slimport connected (must manually enable for now. not sure if it actually works, so please report results.) Prevent forced host mode from entering suspended state so it won't get stuck (perhaps worth reinvestigating if we ever get internal power working). Allow forcing mode via /sys/kernel/debug/msm_otg/mode regardless of current state (original code had conditions that were unreasonable. it is designed to be used for debugging after all..). Forgot to reset the build number; should be 105. Would only potentially matter if using Franco's app.

2013.03.23 2000ET: ziddey-otg-03231951.zip All-in-one update now enables android api for usb host mode. Should automatically rescan media library when usb storage is connected. Kernel updated to Franco M1 (milestone), and will probably stay here for a while.

2013.03.17 1548ET: otg-aio-20130317.zip All-in-one flashable zip that includes modified kernel/ramdisk, vold.fstab, and precompiled storage_list.xml for framework-res.apk (thanks arpruss for precompiled zip-for-android). Kernel unchanged from last release (Franco r102 base), but removed unrelated line previously added to default.prop in ramdisk

2013.03.14 1648ET: otg-franco-boot-03141621.img Should have fixed all issues involving unpopulated hubs, unplugged devices, host mode timeout, and charging. Changed main mount point to /storage/usbdisk0 since that seems to be the new standard (manually update vold.fstab accordingly). Based on Franco's git as of 3/12 (after r102)

2013.03.11 2244ET: otg_boot_r3.img Interim build to address wall charging issues (do not attach/detach devices while otg cable connected to phone)

2013.03.09 0739ET: otg_boot_r2.img Should charge (faster) in host mode

2013.03.08 1128ET: otg_franco.zip Initial release in this thread.

2013.03.07 1350ET: franco-otg-201303071328.img "Pre-alpha" build posted in CaptainMuon's thread. Forces disabling of host mode on device unplug.

2013.03.07 1102ET: franco-otg-201303071032.img "Pre-alpha" build posted in CaptainMuon's thread. Forces host mode on detection of "proprietary" type charger (vs usb = sdp, wall = dcp).

References:

Typical Configuration Examples | Android Developers: http://source.android.com/devices/tech/storage/config-example.html

External Storage Technical Information | Android Open Source: http://source.android.com/tech/storage/

chainfire[dev~blog] - Is Google blocking apps writing to SD cards?: http://www.chainfire.eu/articles/113/Is_Google_blocking_apps_writing_to_SD_cards_/