Garbled display with kernels 5.15 and 5.14

[schwarzwieweiss]

I am running Debian bullseye with a self-compiled kernel. The last working version I have available is kernel 5.12.12. 

I now decided to try a more recent kernel and compiled kernel 5.15. Booting this kernel results in a garbled display.

To check whether the display still worked fine with older versions, I tried kernel 5.14.14 (which I am using for another RK3399 device). The same result, garbled display.

Please see the attached image for what the garbled display looks like.

It's not damaged hardware: the display works perfectly with kernel 5.12.12. It's not something in the DT: kernel 5.15 still results in a garbled display with the DT from kernel 5.12.12 and kernel 5.12.12 with the DT from kernel 5.15 gives a working display.

I did not spot any difference in the kernel configurations that seemed likely to cause the issue. I am not applying any patches to the kernel.

I would appreciate any hint regarding the cause.

[schwarzwieweiss]

After a lot of kernel reconfiguration I now have a kernel 5.14.9 that works most of the time. Rebooting usually results in a garbled display, cold boot usually works fine (i.e. the display works perfectly).

I vaguely remember reading reports that rebooting result in display issues. However, I have never experienced this with kernel 5.12.12 (and also not the few times I rebooted with kernel 5.12.19, which I have built while trying to figure out the display issue).

I now also have a kernel 5.14.16, which I have build with the same config as the (mostly) working kernel 5.14.9. But this kernel 5.14.16 has so far always produced a garbled display. On reboots as well as on cold boots.

During testing I had the impression that the issue was in parts caused by USB and HID configuration of the kernel (switching three CONFIGs from module to builtin changed the display behaviour from working to garbled). I have, however not tested this properly.

I'll now try to find which kernel version from 5.14.9 to 5.14.16 is the one that "breaks" with my configuration.

[schwarzwieweiss]

I started with kernel 5.14.12 to find the version that breaks with my current configuration. The result was a kernel that worked (i.e. display was not garbled) roughly 50 % of the time. That was quite frustrating so I abandoned my plan to find the "breaking" version.

Instead I decided to try again with kernel 5.15 and start from its defconfig instead of a Debian kernel config (which I did previously). Some iterations later I have arrived at 11 CONFIGs that I need to change to get "everything" working with my Debian installation.

Display has so far not shown any issues, neither on cold boot nor on reboot.

I have no idea which part of the previous kernel configurations were responsible for the display issues. A bit disappointing but I probably will not investigate this further.

[kuleszdl]

I occasionally have experienced this with the stock kernel that ships with bullseye as well. Overall, the screen is sometimes a bit flaky which makes it much harder to track down issues in your kernel config.

Why have you tried using a more recent kernel than the 5.10 kernel shipped in bullseye btw?

[schwarzwieweiss]

Why have you tried using a more recent kernel than the 5.10 kernel shipped in bullseye btw?

Three reasons:
- Display does not turn on with the Bullseye kernel (I believe this is due to CONFIG_ROCKCHIP_CDN_DP in combination with the shipped DTB)
- Keyboard does not work until after I unlock the encrypted root (I believe this could be worked around by manually including a number of seemingly unrelated modules into the initrd, which in my kernel are configured as builtin)
- DisplayPort over USB-C does not work (but I haven't gotten it to work with the self-compiled kernel either). This is mainly "I'll just try the next kernel version, let's see what happens then".

Side note: On my RockPro64, the first two issues don't exist (HDMI monitor works perfectly fine and the standard USB keyboard does not require any unexpected modules) and I don't really care about the DisplayPort issue, so I just run Debian's stock kernel on the RockPro64.