Chapter 92: USB WiFi

Driver choice: Use the in-tree, maintained driver first. Use out-of-tree, spidev, or custom-driver paths only after you accept the kernel-version maintenance cost and document who owns updates.

What: USB WiFi dongles, the plug-in alternative to soldered SDIO WiFi (Ch 91). Three chips compared: Realtek RTL8188EUS (the ubiquitous cheap dongle), MediaTek MT7601 (common in $3 dongles), Ralink RT5370 (older, very mainline-friendly). The big theme here is in-tree versus out-of-tree drivers. Some dongles just work. Others need a constantly-rebuilt DKMS module. Plus bandwidth contention with other USB devices on the i.MX6ULL’s USB-2.0 bus.

Why: USB WiFi is the fastest way to add WiFi to a board that has a spare USB port, no SDIO bring-up, no NVRAM, no 32 kHz clock. The catch is driver support: some chips have excellent in-tree drivers, others need out-of-tree modules that break on every kernel upgrade. Chip choice is most of the work.

Focus: the chip you buy determines whether bringing up WiFi takes five minutes or five days. A rtw88- or rt2800usb-supported chip is plug-and-play. An RTL8188EUS needs the out-of-tree rtl8188eus driver, which you must rebuild for every kernel. This chapter is a buyer’s guide as much as a driver guide.

Tooling. This chapter uses wpa_supplicant, iw, chip firmware blob (Realtek/MediaTek/Ralink).

• Ubuntu-base (target): apt install wpasupplicant iw firmware-realtek firmware-misc-nonfree

• Buildroot: BR2_PACKAGE_WPA_SUPPLICANT=y BR2_PACKAGE_IW=y Buildroot: a configuration-driven build system that produces a complete root filesystem and related images.

• Full per-tool reference: Userspace tooling appendix. MCU bridge: Think of the rootfs as the firmware image’s file-backed runtime environment. On an MCU you link everything into flash. On Linux, programs and config live in this mounted tree. rootfs: root filesystem, the directory tree mounted at / that contains /bin, /etc, /dev, and libraries.

92.1 Chip comparison & driver status

	RTL8188EUS	MT7601	RT5370
Vendor	Realtek	MediaTek	Ralink (now MediaTek)
Standard	802.11n (2.4 GHz, 1×1)	802.11n (2.4 GHz)	802.11n (2.4 GHz)
Throughput	~50 Mbps	~50 Mbps	~40 Mbps
In-tree driver?	partial (r8188eu since 5.18, in staging)	yes (mt7601u)	yes (rt2800usb) ✓
Out-of-tree	rtl8188eus (aircrack-ng repo)	usually not needed	not needed
AP mode / monitor	yes (out-of-tree)	limited	yes
Volume price	$2–4	$3–5	$3–5

The crucial column is “in-tree driver?”:

• RT5370 (rt2800usb): the gold standard. Mainline since forever, rock-solid, supports AP + monitor mode. For the lowest-hassle path, buy an RT5370 dongle.

• MT7601 (mt7601u): in-tree, works well. Good second choice.

• RTL8188EUS: the r8188eu staging driver works for basic STA mode (recent kernels), but AP/monitor mode + best performance needs the out-of-tree rtl8188eus. The most common dongle, but the most painful driver.

92.2 The plug-and-play case (RT5370)

[root@pa-mini:~]# lsusb
Bus 001 Device 004: ID 148f:5370 Ralink Technology RT5370 Wireless Adapter

[root@pa-mini:~]# dmesg | grep -iE 'rt2800|usb'
usb 1-1.3: new high-speed USB device number 4
usb 1-1.3: RT chipset 5390, rev 0502 detected
ieee80211 phy0: rt2x00_set_rt: Info - RT chipset 5390, rev 0502 detected
ieee80211 phy0: Selected rate control algorithm 'minstrel_ht'
usbcore: registered new interface driver rt2800usb

[root@pa-mini:~]# ip link
3: wlan0: <BROADCAST,MULTICAST> ...

Plug in → wlan0 appears. Firmware (rt2870.bin) is loaded automatically from /lib/firmware/ (ships with linux-firmware). Then scan/connect exactly as Ch 55E:

[root@pa-mini:~]# ip link set wlan0 up
[root@pa-mini:~]# wpa_supplicant -B -i wlan0 -c /etc/wpa_supplicant.conf -D nl80211
[root@pa-mini:~]# udhcpc -i wlan0

Total bring-up is three steps. Insert the dongle. Copy the firmware if it is not present. Connect. About five minutes.

RT5370 and MT7601 are soft-MAC chips. They use mac80211: the kernel does the 802.11 MAC, the chip is just a radio. This is why integration is clean. Mac80211 and cfg80211 handle the protocol work, and the chip driver is a thin USB-to-radio shim.

MAC: Media Access Control in networking and radio chapters. It is the layer that owns framing and medium access.

92.3 The painful case (RTL8188EUS)

Most cheap “USB WiFi” dongles sold as “for Raspberry Pi” are RTL8188EUS. They’re full-MAC-ish (Realtek’s own MAC in a half-baked driver). Two driver options:

Option A: in-tree r8188eu (staging)

Since kernel 5.18, there’s a staging driver drivers/staging/r8188eu/. Enable CONFIG_R8188EU=m. It handles station mode (connect to an AP) adequately. For a product that just needs to join a WiFi network, this is enough and it’s in-tree (builds with the kernel, tracks API changes).

[root@pa-mini:~]# dmesg | grep 8188
r8188eu 1-1.3:1.0: Firmware Version 11, SubVersion 1, Signature 0x88e0
r8188eu 1-1.3:1.0 wlan0: renamed from wlan0

Option B: out-of-tree rtl8188eus

The aircrack-ng rtl8188eus driver supports AP mode, monitor mode, packet injection, and better performance. But it’s out-of-tree:

# On the build host or target:
git clone https://github.com/aircrack-ng/rtl8188eus
cd rtl8188eus
make ARCH=arm CROSS_COMPILE=arm-none-linux-gnueabihf- KSRC=/path/to/kernel
# Install the resulting 8188eu.ko

The pain:

• Breaks on kernel upgrade. The driver uses internal kernel APIs that change between versions. Each kernel bump may require patching the driver.

• DKMS automates the rebuild but only if a compatible driver version exists for the new kernel.

• Quality: power management is poor (the dongle runs hot, drains battery), and it occasionally wedges requiring a re-plug.

The recommendation

For a product: don’t use RTL8188EUS unless you need its AP/monitor features. Use RT5370 or MT7601 (in-tree, clean). The RTL8188EUS is fine for a hobby project or a dev board where you’ll manage the driver manually, but for an 8-year-life product, the out-of-tree maintenance cost is real.

92.4 How USB WiFi differs from SDIO WiFi

	SDIO WiFi (Ch 91)	USB WiFi
Bring-up effort	high (transport, pwrseq, NVRAM, clock)	low (plug in)
Per-board config	NVRAM file required	none (firmware is generic)
Bus	dedicated SDIO	shared USB
Bandwidth contention	none	competes with other USB devices
Power	soldered, low	dongle, often higher (no PM tuning out-of-tree)
Antenna	on-board (you design it)	on the dongle (vendor’s)
Form factor	soldered chip	external dongle (or soldered USB module)

USB WiFi trades bring-up ease for bus contention and (often) worse power management. For a quick bring-up or a field-swappable WiFi, it’s great. For a polished low-power product, soldered SDIO with an in-tree driver is better.

92.5 USB bandwidth contention

The i.MX6ULL has USB-2.0 (480 Mbps theoretical, ~320 practical). A USB WiFi dongle + another bandwidth-heavy USB device (a UVC camera from Ch 88, USB Ethernet) on the same USB controller compete:

[root@pa-mini:~]# # Camera + WiFi on the same hub:
[root@pa-mini:~]# # iperf3 over WiFi while streaming the camera → both degrade

Mitigation: the i.MX6ULL has two USB OTG controllers. Put the camera on one, WiFi on the other. Each gets its own 480 Mbps.

[root@pa-mini:~]# lsusb -t      # shows the bus topology
/:  Bus 01.Port 1: Dev 1, Class=root_hub, Driver=ci_hdrc/1p, 480M
    |__ Port 1: Dev 4, ... rt2800usb         ← WiFi on controller 1
/:  Bus 02.Port 1: Dev 1, Class=root_hub, Driver=ci_hdrc/1p, 480M
    |__ Port 1: Dev 2, ... uvcvideo          ← camera on controller 2

92.6 AP mode (hostapd)

USB dongles with mac80211 (RT5370, MT7601) and AP-capable firmware can be an access point, not just a client:

# /etc/hostapd.conf
interface=wlan0
driver=nl80211
ssid=MyEmbeddedAP
hw_mode=g
channel=6
wpa=2
wpa_passphrase=secretpassword
wpa_key_mgmt=WPA-PSK
rsn_pairwise=CCMP

[root@pa-mini:~]# hostapd /etc/hostapd.conf &
[root@pa-mini:~]# # Now devices can connect to "MyEmbeddedAP"
# Add a DHCP server (dnsmasq) + IP forwarding for a full hotspot.

This turns your i.MX6ULL into a WiFi hotspot, useful for “configure-the-device-from-your-phone” setup flows. RT5370 and MT7601 support this in-tree. RTL8188EUS needs the out-of-tree driver for AP mode.

92.7 Lab

1. RT5370 plug-and-play. Insert an RT5370 dongle. Verify wlan0 appears with zero config (firmware from linux-firmware). Connect.

2. Compare an RTL8188EUS. Insert one. With the in-tree r8188eu, verify station mode works. Note any AP-mode limitation.

3. Out-of-tree build (RTL8188EUS). Build the aircrack-ng rtl8188eus driver against your kernel. Load it. Test AP mode.

4. Kernel-upgrade pain demo. Note the out-of-tree driver’s kernel-version assumptions. (Conceptually: a kernel bump may break it.)

5. Bandwidth contention. Run iperf3 over USB WiFi while streaming a UVC camera on the same USB controller. Measure degradation. Move to the second controller. Measure improvement.

6. AP mode. With RT5370, run hostapd + dnsmasq. Connect a phone to your i.MX6ULL hotspot.

7. Power. Measure idle current with the USB dongle vs an SDIO module. USB dongles (especially RTL out-of-tree) often run hotter / draw more.

92.8 Pitfalls

• Buying RTL8188EUS expecting it to “just work.” The most common dongle, the most painful driver. For a product, prefer RT5370/MT7601.

• Out-of-tree driver + kernel upgrade. Breaks. Pin the kernel, use DKMS with a compatible version, or, better, switch to an in-tree chip.

• Missing firmware. rt2800usb needs rt2870.bin. mt7601u needs mt7601u.bin. Copy from linux-firmware. Symptom: chip detected, no wlan0.

• Bandwidth contention. Camera + WiFi on one USB controller → both degrade. Spread across the two controllers.

• Bus-power limits. Some dongles draw 400+ mA on TX. A weak VBUS rail browns out → disconnects. Ensure adequate USB power.

• Counterfeit chips. A dongle sold as “RT5370” may contain an RTL8188 (or vice versa). Always lsusb to confirm the actual chip before committing a design.

• Monitor/AP mode on the wrong chip. Not all chips/drivers support AP or monitor mode. Check before designing a feature around it.

• regdb / country code. Same as SDIO, install wireless-regdb or you’re limited to restrictive channels.

• Soldered USB-WiFi modules. Some “USB WiFi” is a soldered-on module, not a removable dongle. The driver situation is the same, but now you cannot swap the chip if the driver turns out to be bad. Choose the chip even more carefully.

92.9 Going deeper

• drivers/net/wireless/ralink/rt2x00/: the rt2800usb driver (RT5370). A clean mac80211 USB driver.

• drivers/net/wireless/mediatek/mt7601u/: MT7601.

• drivers/staging/r8188eu/: the in-tree RTL8188EU staging driver.

• drivers/net/wireless/realtek/rtw88/: newer Realtek parts coming in-tree.

• linux-firmware.git: rt2870.bin, mt7601u.bin.

• hostapd documentation: for AP mode.

• aircrack-ng rtl8188eus repo: the out-of-tree driver (and its README’s kernel-compatibility notes).

• Documentation/networking/mac80211-injection.rst: monitor/injection mode.

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Note on Group K so far: SDIO WiFi (Ch 91) for soldered, low-power, in-tree (AP6212). USB WiFi (Ch 92) for quick/swappable but watch the driver story (prefer RT5370/MT7601 in-tree over RTL8188EUS out-of-tree). The next two chapters cover the hosted approach (Ch 93: WiFi via an ESP32 co-processor) and WiFi+BT combo modules (Ch 94).

Next chapter: Chapter 93: Hosted WiFi via ESP32 / ESP8266. When the SoC has no SDIO and no spare USB, offload WiFi to an ESP co-processor over UART or SPI.