drm/msm/dp: Introduce link training per-segment for LTTPRs

[Dmitry Baryshkov]

On Tue, Apr 29, 2025 at 12:57:16PM +0200, Aleksandrs Vinarskis wrote:
> On Tue, 29 Apr 2025 at 10:03, Johan Hovold <johan at kernel.org> wrote:
> >
> > On Tue, Apr 29, 2025 at 10:50:55AM +0300, Abel Vesa wrote:
> > > On 25-04-29 09:23:46, Johan Hovold wrote:
> > > > On Mon, Apr 28, 2025 at 05:17:21PM +0300, Abel Vesa wrote:
> > > > > On 25-04-28 14:47:04, Johan Hovold wrote:
> > > > > > On Mon, Apr 28, 2025 at 11:06:39AM +0200, Aleksandrs Vinarskis wrote:
> > > > > > > On Mon, 28 Apr 2025 at 09:45, Johan Hovold <johan at kernel.org> wrote:
> >
> > > > > > > > Does this mean that the incomplete LTTPR support in 6.15-rc1 broke
> > > > > > > > adapters or docks with retimers in transparent mode?
> >
> > > > Ok, but my concern is if they may have worked in a default transparent
> > > > mode.
> > >
> > > But if they are by default in transparent mode, doing the setup to
> > > transparent mode will not break it in any way.
> >
> > It looks like it will, see below.
> >
> > > > > X13s doesn't have LTTPRs on-board so when reading the caps, LTTPRs count
> > > > > would return 0 and none of the of the transparent/non-transparent setup
> > > > > would happen.
> > > >
> > > > But this is the crux; does any off-board LTTPRs in transparent mode add
> > > > to the count or not? If they don't, how would you ever learn that there
> > > > are any LTTPRs? If they do, it seems we may have a problem here.
> > >
> > > Count gets increased either way. It doesn't matter if they are in
> > > transparent mode or not.
> >
> > Thanks for confirming. So then it seems we do have a problem as since
> > 6.15-rc1 drm_dp_lttpr_init() will switch all LTTPRs to non-transparent
> > mode.
> 
> In this case, let me add Fixes to the entire series. Do you think we
> could land it in 6.15-rcX then? The second option proposed to roll
> back current LTTPR support and wait until 6.16 will completely break
> DP output on all X1E, so it's very undesirable.

It should be possible, if you rebase onto 6.15-rc, add Fixes tags and
send it quickly enough.

> This series was tested quite a bit on at least the X1E/X1P devices,
> both with and without docking stations, as it is also (v2 iirc) part
> of Ubuntu's concept tree since little over a month ago. You have
> confirmed that x13s also works with this change but without a docking
> station. If someone could confirm that x13s with this change does work
> with a docking station as well, it would be safe to merge the entire
> series as fix to 6.15, correct? I could reach out on #aarch64-laptops,
> perhaps someone has both x13s (or another qcom-based non X1(E) device)
> and a docking station.
> 
> Thanks,
> Alex
> 
> >
> > Johan

-- 
With best wishes
Dmitry