1 files changed, 91 insertions, 0 deletions

diff --git a/README.md b/README.md
index 8e20b5a62..0aeadaeb4 100644
--- a/README.md
+++ b/README.md
@@ -47,3 +47,94 @@ image under recovery.
     1. `adb sync data` to make sure the test-dir has the images to test.
     2. The test will automatically pickup and verify all `_text.png` files in
        the test dir.
+
+Using `adb` under recovery
+--------------------------
+
+When running recovery image from debuggable builds (i.e. `-eng` or `-userdebug` build variants, or
+`ro.debuggable=1` in `/prop.default`), `adbd` service is enabled and started by default, which
+allows `adb` communication. A device should be listed under `adb devices`, either in `recovery` or
+`sideload` state.
+
+    $ adb devices
+    List of devices attached
+    1234567890abcdef    recovery
+
+Although `/sbin/adbd` shares the same binary between normal boot and recovery images, only a subset
+of `adb` commands are meaningful under recovery, such as `adb root`, `adb shell`, `adb push`, `adb
+pull` etc. `adb shell` works only after manually mounting `/system` from recovery menu (assuming a
+valid system image on device).
+
+## Troubleshooting
+
+### `adb devices` doesn't show the device.
+
+    $ adb devices
+    List of devices attached
+
+ * Ensure `adbd` is built and running.
+
+By default, `adbd` is always included into recovery image, as `/sbin/adbd`. `init` starts `adbd`
+service automatically only in debuggable builds. This behavior is controlled by the recovery
+specific `/init.rc`, whose source code is at `bootable/recovery/etc/init.rc`.
+
+The best way to confirm a running `adbd` is by checking the serial output, which shows a service
+start log as below.
+
+    [   18.961986] c1      1 init: starting service 'adbd'...
+
+ * Ensure USB gadget has been enabled.
+
+If `adbd` service has been started but device not shown under `adb devices`, use `lsusb(8)` (on
+host) to check if the device is visible to the host.
+
+`bootable/recovery/etc/init.rc` disables Android USB gadget (via sysfs) as part of the `fs` action
+trigger, and will only re-enable it in debuggable builds (the `on property` rule will always run
+_after_ `on fs`).
+
+    on fs
+        write /sys/class/android_usb/android0/enable 0
+
+    # Always start adbd on userdebug and eng builds
+    on property:ro.debuggable=1
+        write /sys/class/android_usb/android0/enable 1
+        start adbd
+
+If device is using [configfs](https://www.kernel.org/doc/Documentation/usb/gadget_configfs.txt),
+check if configfs has been properly set up in init rc scripts. See the [example
+configuration](https://android.googlesource.com/device/google/wahoo/+/master/init.recovery.hardware.rc)
+for Pixel 2 devices. Note that the flag set via sysfs (i.e. the one above) is no-op when using
+configfs.
+
+### `adb devices` shows the device, but in `unauthorized` state.
+
+    $ adb devices
+    List of devices attached
+    1234567890abcdef    unauthorized
+
+recovery image doesn't honor the USB debugging toggle and the authorizations added under normal boot
+(because such authorization data stays in /data, which recovery doesn't mount), nor does it support
+authorizing a host device under recovery. We can use one of the following options instead.
+
+ * **Option 1 (Recommended):** Authorize a host device with adb vendor keys.
+
+For debuggable builds, an RSA keypair can be used to authorize a host device that has the private
+key. The public key, defined via `PRODUCT_ADB_KEYS`, will be copied to `/adb_keys`. When starting
+the host-side `adbd`, make sure the filename (or the directory) of the matching private key has been
+added to `$ADB_VENDOR_KEYS`.
+
+    $ export ADB_VENDOR_KEYS=/path/to/adb/private/key
+    $ adb kill-server
+    $ adb devices
+
+`-user` builds filter out `PRODUCT_ADB_KEYS`, so no `/adb_keys` will be included there.
+
+Note that this mechanism applies to both of normal boot and recovery modes.
+
+ * **Option 2:** Allow `adbd` to connect without authentication.
+   * `adbd` is compiled with `ALLOW_ADBD_NO_AUTH` (only on debuggable builds).
+   * `ro.adb.secure` has a value of `0`.
+
+Both of the two conditions need to be satisfied. Although `ro.adb.secure` is a runtime property, its
+value is set at build time (written into `/prop.default`). It defaults to `1` on `-user` builds, and
+`0` for other build variants. The value is overridable via `PRODUCT_DEFAULT_PROPERTY_OVERRIDES`.