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

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Message-ID: <aBC4qS0fx61qJxkh@hovoldconsulting.com>
Date: Tue, 29 Apr 2025 13:31:53 +0200
From: Johan Hovold <johan@...nel.org>
To: Aleksandrs Vinarskis <alex.vinarskis@...il.com>
Cc: Abel Vesa <abel.vesa@...aro.org>, Dmitry Baryshkov <lumag@...nel.org>,
	linux-arm-msm@...r.kernel.org, dri-devel@...ts.freedesktop.org,
	freedreno@...ts.freedesktop.org, linux-kernel@...r.kernel.org,
	dmitry.baryshkov@....qualcomm.com, Rob Clark <robdclark@...il.com>,
	Abhinav Kumar <quic_abhinavk@...cinc.com>,
	Sean Paul <sean@...rly.run>,
	Marijn Suijten <marijn.suijten@...ainline.org>,
	David Airlie <airlied@...il.com>, Simona Vetter <simona@...ll.ch>,
	laurentiu.tudor1@...l.com
Subject: Re: drm/msm/dp: Introduce link training per-segment for LTTPRs

On Tue, Apr 29, 2025 at 12:57:16PM +0200, Aleksandrs Vinarskis wrote:
> On Tue, 29 Apr 2025 at 10:03, Johan Hovold <johan@...nel.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:

> > > > 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?

That's for the msm drm maintainers to answer. It should be possible, and
if we agree (or could confirm) that this breaks existing setups then
this needs to be addressed before 6.15 final is out in some way.

Note that you'd need to rebase on rc4 which doesn't have that return
value rework that's in msm-next.

> 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.

Right, but it wasn't enabled until 6.15-rc1 so that's technically not a
regression (i.e. since 6.14). Essentially, we'd only wait with enabling
DP for another cycle (and I believe DT changes to enable this never made
it into 6.15 either for most laptops anyway).

> 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.

If it fixes a regression then I would merge it for 6.15.

Johan