Re: 5.6 DP-MST regression: 1 of 2 monitors on TB3 (DP-MST) dock no longer light up

[Lyude Paul <lyude@redhat.com>]

On Wed, 2020-02-26 at 18:52 +0100, Hans de Goede wrote:
> Hi,
> 
> On 2/26/20 5:05 PM, Alex Deucher wrote:
> > On Wed, Feb 26, 2020 at 10:43 AM Hans de Goede <hdegoede@redhat.com>
> > wrote:
> > > Hi,
> > > 
> > > On 2/26/20 4:29 PM, Alex Deucher wrote:
> > > > On Wed, Feb 26, 2020 at 10:16 AM Hans de Goede <hdegoede@redhat.com>
> > > > wrote:
> > > > > Hi Lyude and everyone else,
> > > > > 
> > > > > Lyude I'm mailing you about this because you have done a lot of
> > > > > work on DP MST, but if this rings a bell to anyone else feel
> > > > > free to weigh in on this.
> > > > 
> > > > Might be a duplicate of:
> > > > https://gitlab.freedesktop.org/drm/amd/issues/1052
> > > 
> > > Looks like you are right, reverting the commit which the bisect
> > > from that issue points to:
> > > 
> > > cd82d82cbc04 ("drm/dp_mst: Add branch bandwidth validation to MST atomic
> > > check")
> > > 
> > > Fixes the issue for me. I will add a comment to the issue.
> > > 
> > > Note I'm using integrated Intel gfx, so that means that this issue
> > > definitely is not amdgpu specific.
> > > 
> > 
> > I'm not too familiar with the mst code, but I wonder if we were
> > exceeding the bandwidth limits in some setups and it just happened to
> > work, but now that we enforcing them, they don't which is correct, but
> > a regression from some users' perspective?
> 
> I seriously doubt that is the case according to:
> https://support.lenovo.com/nl/en/solutions/pd029622
> 
> The gen 2 tb3 dock can handle 2 external
> displays at 3840*2160@60Hz together with the internal
> panel being on and both my external displays run at
> 1920x1080@60 so I'm consuming less then half of the
> maximum bandwidth.

OK-so I wasn't actually able to reproduce this issue with my setup (I've got a
X1 Carbon 7th generation, but I don't have the 2nd generation dock - only the
first generation dock) but I'm certain I've actually fixed it now, since I
realized we did not have a very good understanding of how PBN limitations are
advertised with MST. I rewrote the bandwidth checks again, and in the process
also found a much more subtle regression that got introduced in 5.6, which
would sometimes cause MST probing to appear to just stop in it's tracks with
no messages.

I cc'd both patch series to you, so I'd recommend applying them both onto your
kernel and seeing if that fixes your issues. If it still doesn't, then get me
some kernel logs with:

drm.debug=0x116 log_buf_len=50M

And I'll take a closer look. I'm pretty confident this should fix everything
though :)

> 
> There definitely is a bug somewhere in the
> cd82d82cbc04 ("drm/dp_mst: Add branch bandwidth validation to MST atomic
> check")
> commit (or somewhere else and triggered by that commit).
> 
> Regards,
> 
> Hans
> 
> 
> 
> 
> 
> 
> 
> > Alex
> > 
> > 
> > > Regards,
> > > 
> > > Hans
> > > 
> > > 
> > > 
> > > 
> > > > > I'm currently using a Lenovo X1 7th gen + a Lenovo TB3 gen 2 dock
> > > > > as my daily rider for testing purposes. When 5.6-rc1 came out I
> > > > > noticed that only 1 of the 2 1920x1080@60 monitors on the dock
> > > > > lights up.
> > > > > 
> > > > > There are no kernel errors in the logs, but mutter/gnome-shell says:
> > > > > 
> > > > > gnome-shell[1316]: Failed to post KMS update: Page flip of 93 failed
> > > > > 
> > > > > With 93 being the crtc-id of the crtc used for the monitor which is
> > > > > displaying black. Since then I've waited for 5.6-rc3 hoping that a
> > > > > fix was already queued up, but 5.6-rc3 still has this problem.
> > > > > 
> > > > > gnome-shell does behave as if all monitors are connected, so the
> > > > > monitor is seen, but we are failing to actually send any frames
> > > > > to it.
> > > > > 
> > > > > I've put a log collected with drm.debug=0x104 here:
> > > > > https://fedorapeople.org/~jwrdegoede/drm-debug.log
> > > > > 
> > > > > This message stands out as pointing to the likely cause of this
> > > > > problem:
> > > > > 
> > > > > [    3.309061] [drm:intel_dump_pipe_config [i915]] MST master
> > > > > transcoder: <invalid>
> > > > > 
> > > > > Regards,
> > > > > 
> > > > > Hans
> > > > > 
> > > > > _______________________________________________
> > > > > dri-devel mailing list
> > > > > dri-devel@lists.freedesktop.org
> > > > > https://lists.freedesktop.org/mailman/listinfo/dri-devel
-- 
Cheers,
	Lyude Paul (she/her)
	Associate Software Engineer at Red Hat

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