linux-kernel - Re: [PATCH v2] drm/dp: Clarify that wait_hpd

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Date: Wed, 20 Mar 2024 00:44:57 +0200
From: Dmitry Baryshkov <dmitry.baryshkov@...aro.org>
To: Douglas Anderson <dianders@...omium.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

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>


-- 
With best wishes
Dmitry