hi all! first of all, consider that we are low level LM user mostly for job, but the way could be useful to somebody if we share our experience with the rtl8821cu (Realtek 802.11ac NIC type: Network driver: rtl8821cu rev: 2.0; chip ID: 0bda:c811).
our experience, Step by step..
01. We bought the first USB WiFi adapter for our old ASUS P5K desktop in May 2022. this was AC650 Long Range USB WiFi Adapter Mod. AC5L; FCC ID 2AZES-AC5L Brostrend - (4M4N0Z)
02. after to plug the dongle, it was not working at all but running few command at the terminal (suggested by forums members) the WiFi adapter started to work but randomly non so fine. Problems were solved running the commands suggested by the customer support service of the vendor.
03. the dkms status revealed that more than a driver was installed at that time, but there were no conflicts and everything was working fine.
04. most of the time we updated our system, following LM Update Manager suggestions (kernek and linux-firmware in particular), we suffered some problems recovered by a new installation of the driver.
05. the driver that worked fine was the 5.12.0-16b3 with the kernel 5.4.0-137
06. In late 2022 we made a second order of the same WiFi dongle from the very same vendor and the very same American multinational technology company focusing on e-commerce (4M4N0Z) and even the second WiFi adapter worked fine, also in our spare PC running LM as well (32 bit, LM 19 MATE).
07. in january 2023, we made some regular updates of our system suggested by LM Update manager and the WiFi adapter stopped to work after the update of the rtl8821cu.dkms package from version 5.12.0-16b3 to the version 5.12.0-16b4.
We were able to solve the problem rolling back the driver to the version 5.12.0-16b3, fortunally somewhere still present in our system folders.
We run the winning rollback following suggestions collected from the web:
first command: apt list -a rtl8821cu-dkms
the answer was:
rtl8821cu-dkms/unknown,unknown,now 5.12.0-16b4 all [installed]
rtl8821cu-dkms/unknown,unknown 5.12.0-16b3 all
then the downgrade to the 16b3 version:
sudo apt install rtl8821cu-dkms=5.12.0-16b3
08. in our Update Manager, we added the "rtl8821cu=5.12.0-16b4" in the blacklist (edit - preferencies - blacklist).
09. The 2023.03.29, after a couple of months of PC inactivity, we made a new cumulative update of dozens updates listed by the LM Update Manager (linux kernel, linux-firmware and many other updates without a deep check of each one update...IMHO NEVER-NEVER-NEVER DO THAT, NEVER) and the WiFi adapter stopped to work again.
10. We made a check and we realized that the version 5.12.0-16b5 of the rtl8821cu.dkms package was installed in place of the old (working) version 5.12.0-16b3.
11. This time there was no way to rollback to the "16b3" version because still not present in our system folders, maybe automatically deleted during the update session or maybe running "apt autoremove" from the terminal (not sure about this last one). Also the kernel was updated to the version 5.4.0-146.
12. after to contact the BrosTrend technical support team (very rapid and effective customer support team):
support@trend-tech.net.cn
we were able to make our dongle to works fine again installing the old dkms "rtl8821cu=5.12.0-16b3"
13. the old -16b3 driver version can be downloaded from the Brostrend github site:
https://github.com/brostrend/linux.bros ... /rtl8821cu
and can be installed with e.g. dpkg -i command, then reboot.
14. in our personal case, the WiFi adapter started to work (blinking the blue LED) immediately, without any reboot (
).
System: Kernel: 5.4.0-146-generic x86_64 bits: 64 compiler: gcc v: 9.4.0
Desktop: Cinnamon 5.0.7 wm: muffin dm: LightDM Distro: Linux Mint 20.2 Uma
base: Ubuntu 20.04 focal
Machine: Type: Desktop Mobo: ASUSTeK model: P5LD2-SE v: Rev 1.xx serial: <filter>
BIOS: American Megatrends v: 0316 date: 07/17/2006
CPU: Topology: Single Core model: Intel Pentium 4 bits: 64 type: MT
arch: Netburst Smithfield rev: 9 L2 cache: 1024 KiB
flags: lm nx pae sse sse2 sse3 bogomips: 12845
Speed: 3211 MHz min/max: N/A Core speeds (MHz): 1: 3211 2: 3211
Graphics: Device-1: NVIDIA GF119 [GeForce GT 610] driver: nvidia v: 340.108 bus ID: 04:00.0
chip ID: 10de:104a
Display: x11 server: X.Org 1.20.13 driver: nvidia
unloaded: fbdev,modesetting,nouveau,vesa resolution: 1280x960~60Hz
OpenGL: renderer: GeForce GT 610/PCIe/SSE2 v: 3.3.0 NVIDIA 340.108 direct render: Yes
Audio: Device-1: Intel NM10/ICH7 Family High Definition Audio
vendor: ASUSTeK P5LD2-VM Mainboard driver: snd_hda_intel v: kernel bus ID: 00:1b.0
chip ID: 8086:27d8
Sound Server: ALSA v: k5.4.0-146-generic
Network: Device-1: Realtek RTL8111/8168/8411 PCI Express Gigabit Ethernet vendor: ASUSTeK P5B
driver: r8169 v: kernel port: c800 bus ID: 02:00.0 chip ID: 10ec:8168
IF: enp2s0 state: down mac: <filter>
Device-2: Realtek 802.11ac NIC type: USB driver: rtl8821cu bus ID: 1-2:2
chip ID: 0bda:c811
IF: wlx60fb00644383 state: up mac: <filter>
USB: Hub: 1-0:1 info: Full speed (or root) Hub ports: 8 rev: 2.0 chip ID: 1d6b:0002
Device-1: 1-2:2 info: Realtek 802.11ac NIC type: Network driver: rtl8821cu rev: 2.0
chip ID: 0bda:c811
<snip>
Repos: No active apt repos in: /etc/apt/sources.list
Active apt repos in: /etc/apt/sources.list.d/additional-repositories.list
1: deb https: //dl.winehq.org/wine-builds/ubuntu/ focal main
Active apt repos in: /etc/apt/sources.list.d/brostrend.list
1: deb https: //linux.brostrend.com stable main previous
<snip>
