[Solved] Wifi no longer works after kernel update with Tp-Link AC600 adapter (rtl8821cu kernel module)

[mazda1]

hello,

Since my wifi doesn't work with kernel 1.0.26, I booted my laptop with kernel 1.0.25. I thought that maybe dkms-1.0.26 couldn't build install the driver for my Tp-Link AC600 adapter, I was therefore surprised to read the following

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~# uname -r
6.1.0-25-amd64

~# dkms status
rtl8821cu/5.12.0.4, 6.1.0-26-amd64, x86_64: installed

1- how, when I'm using kernel 1.0.25, can I get information about kernel 1.0.26?
2- is there anything I can do to fix this wifi problem, or do I have to wait for an update?

PS1: 2 weeks ago, I tried kernel 1.10: wifi was OK. ..... For some reason, it no longer works. (2024/10/18)
PS2: wifi is not blocked and there's nothing special about it in boot log

[Aki]

Hello,

Since my wifi doesn't work with kernel 1.0.26, I booted my laptop with kernel 1.0.25. I thought that maybe dkms-1.0.26 couldn't build install the driver for my Tp-Link AC600 adapter,

You are probably referring to:

• 6.1.0-25 debian kernel package (currently shipped with Debian stable repositories)

• 6.1.0-26 debian kernel package (currently shipped with Debian stable-security repositories)

I was therefore surprised to read the following

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~# uname -r
6.1.0-25-amd64

~# dkms status
rtl8821cu/5.12.0.4, 6.1.0-26-amd64, x86_64: installed

1- how, when I'm using kernel 1.0.25, can I get information about kernel 1.0.26?

According to your output, you are using the rtl8821cu/5.12.0.4 out-of-the-tree kernel module (which you probably downloaded from the Internet earlier), which was built using Dynamic Kernel Module Support (DKMS) for Debian kernel package version 6.1.0-26-amd64.

The DKMS attempts to rebuild its registered kernel modules each time you upgrade the kernel or manually request a rebuild. Each kernel has its own binary kernel modules in a separated directory, usually called /lib/modules/ .

So I have to assume that you previously installed the 6.1.0-26-amd64 Debian kernel package, and that the DKMS built the rtl8821cu/5.12.0.4 kernel module. Note that the version of the kernel module (/5.12.0.4) refers to a previous version of the kernel (it was probably originally built for the for 5.12.0.4 kernel version).

is there anything I can do to fix this wifi problem, or do I have to wait for an update?
PS2: wifi is not blocked and there's nothing special about it in boot log

You can try building the kernel module with your preferred running kernel version using the following commands (I assume you are using the amd64 architecture):

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sudo apt install linux-headers-amd64
sudo dkms status
sudo dkms build rtl8821cu/5.12.0.4

PS2: wifi is not blocked and there's nothing special about it in boot log

Check if the rkfill command can help you:

• RFKILL(8) - System Administration

Hope this helps.

note: please add the name of the kernel module in the subject of the first post; e.g.: "Wifi no longer works after kernel update with Tp-Link AC600 adapter (rtl8821cu kernel module)"

[mazda1]

Yesterday the wifi connection stopped working, even with kernel 6.1.0-25. So there must be another problem. Maybe the adapter is defective. I'll buy a new one, then install it for kernel-6.1.0-26.

I was wondering what happened when I updated a kernel, now I know a bit more thanks to your explanations.

PS:even though the README document for the 8821cu driver says :

Compatible kernels

Kernels: 5.4 - 5.11 (Realtek)

Kernels: 5.12 - 6.11 (community support)

https://github.com/morrownr/8821cu-20210916

It seems that there are exceptions and that even if it works for 2 or 3 weeks for a particular kernel, it can break at any time (my experience).

I changed this driver and replaced it with 8821au and it works; at least now.

PS: it's my fault, I should have stuck to what the lsusb command for the driver told me.