Message-ID: <CAD=FV=V4YGNUBRs2yTuEHKswsZs8TQdF8huRhCvtuvqc0R=P+Q@mail.gmail.com>
Date: Tue, 26 Mar 2024 09:03:13 -0700
From: Doug Anderson <dianders@...omium.org>
To: Dmitry Baryshkov <dmitry.baryshkov@...aro.org>
Cc: dri-devel@...ts.freedesktop.org, Abhinav Kumar <quic_abhinavk@...cinc.com>, 
	linux-tegra@...r.kernel.org, Mikko Perttunen <mperttunen@...dia.com>, 
	Thierry Reding <thierry.reding@...il.com>, Jonathan Hunter <jonathanh@...dia.com>, 
	Ankit Nautiyal <ankit.k.nautiyal@...el.com>, Daniel Vetter <daniel@...ll.ch>, 
	David Airlie <airlied@...il.com>, Imre Deak <imre.deak@...el.com>, 
	Jani Nikula <jani.nikula@...el.com>, Maxime Ripard <mripard@...nel.org>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] drm/dp: Clarify that wait_hpd_asserted() is not
 optional for panels

Hi,

On Tue, Mar 19, 2024 at 3:45 PM Dmitry Baryshkov
<dmitry.baryshkov@...aro.org> wrote:
>
> On Tue, 19 Mar 2024 at 22:58, Douglas Anderson <dianders@...omium.org> wrote:
> >
> > In response to my patch removing the "wait for HPD" logic at the
> > beginning of the MSM DP transfer() callback [1], we had some debate
> > about what the "This is an optional function" meant in the
> > documentation of the wait_hpd_asserted() callback. Let's clarify.
> >
> > As talked about in the MSM DP patch [1], before wait_hpd_asserted()
> > was introduced there was no great way for panel drivers to wait for
> > HPD in the case that the "built-in" HPD signal was used. Panel drivers
> > could only wait for HPD if a GPIO was used. At the time, we ended up
> > just saying that if we were using the "built-in" HPD signal that DP
> > AUX controllers needed to wait for HPD themselves at the beginning of
> > their transfer() callback. The fact that the wait for HPD at the
> > beginning of transfer() was awkward/problematic was the whole reason
> > wait_hpd_asserted() was added.
> >
> > Let's make it obvious that if a DP AUX controller implements
> > wait_hpd_asserted() that they don't need a loop waiting for HPD at the
> > start of their transfer() function. We'll still allow DP controllers
> > to work the old way but mark it as deprecated.
> >
> > [1] https://lore.kernel.org/r/20240315143621.v2.3.I535606f6d4f7e3e5588bb75c55996f61980183cd@changeid
> >
> > Reviewed-by: Abhinav Kumar <quic_abhinavk@...cinc.com>
> > Signed-off-by: Douglas Anderson <dianders@...omium.org>
> > ---
> > I would consider changing the docs to say that implementing
> > wait_hpd_asserted() is actually _required_ for any DP controllers that
> > want to support eDP panels parented on the DP AUX bus. The issue is
> > that one DP controller (tegra/dpaux.c, found by looking for those that
> > include display/drm_dp_aux_bus.h) does populate the DP AUX bus but
> > doesn't implement wait_hpd_asserted(). I'm actually not sure how/if
> > this work on tegra since I also don't see any delay loop for HPD in
> > tegra's transfer() callback. For now, I've left wait_hpd_asserted() as
> > optional and described the old/deprecated way things used to work
> > before wait_hpd_asserted().
> >
> > Changes in v2:
> > - Make it clear that panels don't need to call if HPD is a GPIO.
> >
> >  include/drm/display/drm_dp_helper.h | 13 ++++++++++++-
> >  1 file changed, 12 insertions(+), 1 deletion(-)
>
> Reviewed-by: Dmitry Baryshkov <dmitry.baryshkov@...aro.org>

I don't think this is controversial and I've been involved / written
most of the code for eDP panel interactions, so I think I can land
this myself with Dmitry and Abhinav's review (thanks!). If someone
later comes back with additional thoughts I'm happy to update things
more.

Pushed to drm-misc-next:

6376eb8b9115 drm/dp: Clarify that wait_hpd_asserted() is not optional for panels

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