i.MX8 adb bring up

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The default BSP provided by NXP doesn’t support adb, we have to port it by ourselves. Our goal is making adbd running on SoC meanwhile we can use adb through USB port.
Let’s have a quick summary before I forget it(I already done this 3 months ago…)

Source code of adb & adbd

You can find adb & adbd source codes easily, but I found that there is a recipe in openembedded already provided adb & adbd:
meta-openembedded/meta-oe/recipes-devtools/android-tools
But the default recipes can’t work on imx8mm through USB. We need do some modifications later.

Adding support for login adb as root

There is a function called should_drop_privileges in adbd source code, modify this function that makes it always return 0 to disallow drop privileges. It has been added into newer openembedded but not the i.mx BSP.

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From 3a788e9168c9b9eac66c4fa479413f4a95c61be4 Mon Sep 17 00:00:00 2001
From: Florent Revest <revestflo@gmail.com>
Date: Mon, 30 Oct 2017 21:05:46 +0100
Subject: [PATCH] adb: Allow adbd to be ran as root

---
 adb/adb.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/adb/adb.c b/adb/adb.c
index 027edd9359..e0f7ecde45 100644
--- a/adb/adb.c
+++ b/adb/adb.c
@@ -1271,6 +1271,7 @@ static int should_drop_privileges() {
     int secure = 0;
     char value[PROPERTY_VALUE_MAX];
 
+    return 0;
    /* run adbd in secure mode if ro.secure is set and
     ** we are not in the emulator
     */

You need to add this patch into recipe sources as well:

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SRC_URI += " \
    file://0008-adb-Allow-adbd-to-be-ran-as-root.patch;patchdir=system/core \
"

Adding support for USB adb

To connect adbd through USB, we need support from usb function filesystem. To enable it in kernel:

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│ Symbol: USB_FUNCTIONFS [=m]                                                                      │  
│ Type  : tristate                                                                                 │  
│ Prompt: Function Filesystem                                                                      │  
│   Location:                                                                                      │  
│     -> Device Drivers                                                                            │  
│       -> USB support (USB_SUPPORT [=y])                                                          │  
│         -> USB Gadget Support (USB_GADGET [=y])                                                  │  
│ (4)       -> USB Gadget precomposed configurations (<choice> [=m])                               │  
│   Defined at drivers/usb/gadget/legacy/Kconfig:196                                               │  
│   Depends on: <choice>                                                                           │  
│   Selects: USB_LIBCOMPOSITE [=y] && USB_F_FS [=y] && USB_FUNCTIONFS_GENERIC [=y]

After installation of USB_FUNCTIONFS, we need to modprobe and mount functionfs before adbd start(initialization):

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#!/bin/sh

modprobe g_ffs idVendor=0x18d1 idProduct=0x4e42 iSerialNumber="codingspirit"
mkdir -p /dev/usb-ffs/adb
mount -t functionfs adb /dev/usb-ffs/adb -o uid=0,gid=0

Then we can start adbd now.

In my case I’m using systemd to do this initialization(as a ExecStartPre), then set /usr/bin/adbd as ExecStart.

After all of those steps, we can do quick test by adb devices! If everything goes well, you can see your device has been recognized. Then use adb shell to login, a # symbol should be shown which means you are in root.