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

[kevin at holm.dev]

> [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?

Reverting is not easily possible:

$ git checkout v6.10
[...]
HEAD is now at 0c3836482481 Linux 6.10

$ git revert 4df96ba66760345471a85ef7bb29e1cd4e956057
Auto-merging drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm_mst_types.c
CONFLICT (content): Merge conflict in drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm_mst_types.c
error: could not revert 4df96ba66760... drm/amd/display: Add timing pixel encoding for mst mode validation

I do not know enough to try and solve the conflict myself without breaking more things.

> 
> > 
> > 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)
> 
> --
> 
> Everything you wanna know about Linux kernel regression tracking:
> 
> https://linux-regtracking.leemhuis.info/about/#tldr
> 
> If I did something stupid, please tell me, as explained on that page.
>