Re: thunderbolt: DP tunnel activation not retried when display NAKs while settling after link reset (Apple Studio Display XDR 2026 on Panther Lake)

[Imre Deak <imre.deak@intel.com>]

On Tue, Aug 25, 2026 at 10:11:53PM +0800, Dennis Wang wrote:
> Hi Imre,
> 
> One clarification first: there is only one physical display - an Apple
> Studio Display XDR, a tiled 2x1 panel with two DP sinks, hence the two
> tunnels/connectors in the logs. The whole panel stays black from
> power-on until I type the LUKS passphrase blind; the compositor modeset
> after unlock is what first lights it. So it is not one display lighting
> up while another stays dark.
> 
> Tested xe.enable_fbc=0 on a cold boot with both sinks active: panel
> still black until the compositor modeset (~83s). In that boot I noticed
> Panel Replay enabled at 4.7s and disabled at 81.6s, bracketing the
> black window exactly, so I also tried xe.enable_panel_replay=0: still
> black (until ~123s that time).
> 
> What both logs show: the fbdev client's tiled two-pipe modeset
> completes without errors - links train at 5.4G x4, both tunnels get
> 17000 Mb/s allocated, transcoders enable (in the fbc=0 boot both pipes
> are up by 4.7s) - yet nothing ever appears on the panel. The first
> modeset that visibly lights it is the compositor's single-stream
> 5120x2880 DSC config after unlock.

Thanks. 

Not sure what could be wrong with the fbdev modeset, no obvious issues
in the logs around that.

The later compositor modeset config is a bit strange: one pipe is
enabled with a 5120x2880 mode and another pipe with a 2560x2880 mode, as
if the tile configuration would be ignored. Could you check what's the
compositor's idea of the monitors and modes and try to set a 2560x2880
mode (after booting with disabling both fbc and panel replay)?

I think it'd be easier to follow up on a ticket, could you please open
one at

https://gitlab.freedesktop.org/drm/xe/kernel/-/work_items/new?type=Issue

and attach the log after setting the above 2560x2880 mode?

Could you also attach the log there when booting with video=DP-2:d?

> Consistent with that: booting with video=DP-2:d (second sink masked,
> single-stream from the start) is fully clean on every boot, LUKS prompt
> visible at ~4s.
> 
> Both dmesgs attached (timestamps rebuilt from
> _SOURCE_MONOTONIC_TIMESTAMP as before). Happy to test patches or more
> knobs.
> 
> Thanks,
> Dennis
> 
> Imre Deak <imre.deak@intel.com> 于2026年8月25日周二 17:35写道：
> >
> >
> > Hi Dennis,
> >
> > thanks for the logs.
> >
> > Could you confirm that the problem you see is:
> >
> > After boot-up with both displays connected, one of the display is blank
> > by 4.839037, while the other display lights up okay.
> >
> > By 47.768154 both displays light up correctly.
> >
> > I can't see any obvious issues related to tunneling, both tunnels are
> > detected and the BW allocations for them succeed as expected.
> >
> > Could you try if the same sequence without the other issue you mentioned
> > happening (DP tunnel activation failed) the blank screen problem
> > reproduces booting with: xe.enable_fbc=0 ?
> >
> > Thanks,
> > Imre
> >
> > On Tue, Aug 25, 2026 at 09:42:23AM +0800, Dennis Wang wrote:
> > > Hi Mika, Imre,
> > >
> > > Attached: cold boot dmesg with thunderbolt.dyndbg=+p + drm.debug=0x10e
> > > (timestamps rebuilt from _SOURCE_MONOTONIC_TIMESTAMP; root fs is LUKS
> > > so journald re-stamps initramfs kmsg).
> > >
> > > This boot hit the graphics-side flavor - tunnels healthy, screen black
> > > until the LUKS passphrase was typed blind at ~40 s:
> > >
> > >   1.8s   XDR found, Gen3 x2, both DP tunnels allocated, no errors
> > >   3.9s   second sink (0:10 <-> 3:12): repeated "DPRX read timeout"
> > >          (tiled 2x1 panel, two DP sinks)
> > >   3.96s  fastset rejected: crtc_hblank_start expected 3840, found 1920
> > >   11.3s  AUX_TC2 power well disabled - nothing lights the panel again
> > >          until the compositor modeset after unlock (~60 s)
> > >
> > > A control cold boot with video=DP-2:d (masking the second sink) is
> > > fully clean, panel lit at ~4 s, single-stream 5K120 DSC. The original
> > > "DP tunnel activation failed" flavor is intermittent - will send it
> > > with full debug when I catch it.
> > >
> > > Happy to test patches on this hardware.
> > >
> > > Thanks,
> > > Dennis Wang
> >
> >
> 
> 
> -- 
> Best Regards
> Dennis.Wang
> Zoom