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

[Johan Hovold]

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.

Johan