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Message-ID: <b9e1a57d-0bdd-45f4-f589-792f8ea82835@quicinc.com>
Date: Tue, 19 Mar 2024 11:26:19 -0700
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Subject: Re: [PATCH] drm/dp: Clarify that wait_hpd_asserted() is not optional
 for panels
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To: Douglas Anderson <dianders@chromium.org>,
        <dri-devel@lists.freedesktop.org>,
        Dmitry Baryshkov
	<dmitry.baryshkov@linaro.org>
CC: <linux-tegra@vger.kernel.org>, Mikko Perttunen <mperttunen@nvidia.com>,
        Jonathan Hunter <jonathanh@nvidia.com>,
        Thierry Reding
	<thierry.reding@gmail.com>,
        Ankit Nautiyal <ankit.k.nautiyal@intel.com>,
        Daniel Vetter <daniel@ffwll.ch>, David Airlie <airlied@gmail.com>,
        Imre Deak
	<imre.deak@intel.com>, Jani Nikula <jani.nikula@intel.com>,
        Maxime Ripard
	<mripard@kernel.org>, <linux-kernel@vger.kernel.org>
References: <20240319111432.1.I521dad0693cc24fe4dd14cba0c7048d94f5b6b41@changeid>
From: Abhinav Kumar <quic_abhinavk@quicinc.com>
In-Reply-To: <20240319111432.1.I521dad0693cc24fe4dd14cba0c7048d94f5b6b41@changeid>
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On 3/19/2024 11:14 AM, Douglas Anderson 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
> 
> Signed-off-by: Douglas Anderson <dianders@chromium.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().
> 
>   include/drm/display/drm_dp_helper.h | 8 +++++++-
>   1 file changed, 7 insertions(+), 1 deletion(-)
> 

Thanks for the change,

Reviewed-by: Abhinav Kumar <quic_abhinavk@quicinc.com>