[REGRESSION] No image on 4k display port displays connected through usb-c dock in kernel 6.10

[Linux regression tracking (Thorsten Leemhuis)]

[adding a few people and lists to the recipients]

Hi! Thx for your rpeort.

On 27.07.24 18:07, kevin at holm.dev wrote:
> Connecting two 4k displays with display port through a lenovo usb-c
> dock (type 40AS) to a Lenovo P14s Gen 2 (type 21A0) results in no
> image on the connected displays.
> 
> The CPU in the Lenovo P14s is a 'AMD Ryzen 7 PRO 5850U with Radeon
> Graphics' and it has no discrete GPU.
> 
> I first noticed the issue with kernel version '6.10.0-arch1-2'
> provided by arch linux. With the previous kernel version
> '6.9.10.arch1-1' both connected displays worked normally. I reported
> the issue in the arch forums at
> https://bbs.archlinux.org/viewtopic.php?id=297999 and was guided to
> do a bisection to find the commit that caused the problem. Through
> testing I identified that the issue is not present in the latest
> kernel directly compiled from the trovalds/linux git repository.
> 
> With git bisect I identified 4df96ba66760345471a85ef7bb29e1cd4e956057

That's 4df96ba6676034 ("drm/amd/display: Add timing pixel encoding for
mst mode validation") [v6.10-rc1] from Hersen Wu.

Did you try if reverting that commit is possible and might fix the problem?

> as the first bad commit and fa57924c76d995e87ca3533ec60d1d5e55769a27

That's fa57924c76d995 ("drm/amd/display: Refactor function
dm_dp_mst_is_port_support_mode()") [v6.10-post] from Wayne Lin.

> as the first commit that fixed the problem again.

Hmm, the latter commit does not have a fixes tag and might or might not
be to invasive to backport to 6.10. Let's see what the AMD developers say.

> The initial commit only still shows an image on one of the connected
> 4k screens. I have not investigated further to find out at what point
> both displays stopped showing an image.

Ciao, Thorsten (wearing his 'the Linux kernel's regression tracker' hat)
--
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