From: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
To: Abhinav Kumar <quic_abhinavk@quicinc.com>
Cc: Douglas Anderson <dianders@chromium.org>,
dri-devel@lists.freedesktop.org, 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
Subject: Re: [PATCH] drm/dp: Clarify that wait_hpd_asserted() is not optional for panels
Date: Tue, 19 Mar 2024 22:16:58 +0200 [thread overview]
Message-ID: <CAA8EJpqg+Di7PH2bmQ6uMidD3MhQ+N7w-1MWWEOBrH5DbsWSTA@mail.gmail.com> (raw)
In-Reply-To: <2fa9a262-de60-2c1e-9ac3-1e478ec65bb8@quicinc.com>
On Tue, 19 Mar 2024 at 21:02, Abhinav Kumar <quic_abhinavk@quicinc.com> wrote:
>
>
>
> On 3/19/2024 11:35 AM, Dmitry Baryshkov wrote:
> > On Tue, 19 Mar 2024 at 20:15, Douglas Anderson <dianders@chromium.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
> >>
> >> 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(-)
> >>
> >> diff --git a/include/drm/display/drm_dp_helper.h b/include/drm/display/drm_dp_helper.h
> >> index a62fcd051d4d..b170efa1f5d2 100644
> >> --- a/include/drm/display/drm_dp_helper.h
> >> +++ b/include/drm/display/drm_dp_helper.h
> >> @@ -422,7 +422,13 @@ struct drm_dp_aux {
> >> * @wait_hpd_asserted: wait for HPD to be asserted
> >> *
> >> * This is mainly useful for eDP panels drivers to wait for an eDP
> >> - * panel to finish powering on. This is an optional function.
> >> + * panel to finish powering on. It is optional for DP AUX controllers
> >> + * to implement this function but required for DP AUX endpoints (panel
> >> + * drivers) to call it after powering up but before doing AUX transfers.
> >> + * If a DP AUX controller does not implement this function then it
> >> + * may still support eDP panels that use the AUX controller's built-in
> >> + * HPD signal by implementing a long wait for HPD in the transfer()
> >> + * callback, though this is deprecated.
> >
> > It doesn't cover a valid case when the panel driver handles HPD signal
> > on its own.
> >
>
> This doc is only for wait_for_hpd_asserted(). If panel driver handles
> HPD signal on its own, this will not be called. Do we need a doc for that?
This comment declares that this callback must be called by the panel
driver: '...but required for DP AUX endpoints [...] to call it after
powering up but before doing AUX transfers.'
If we were to follow documentation changes from this patch, we'd have
to patch panel-edp to always call wait_for_hpd_asserted, even if HPD
GPIO is used. However this is not correct from my POV.
> >> *
> >> * This function will efficiently wait for the HPD signal to be
> >> * asserted. The `wait_us` parameter that is passed in says that we
> >> --
> >> 2.44.0.291.gc1ea87d7ee-goog
> >>
> >
> >
--
With best wishes
Dmitry
next prev parent reply other threads:[~2024-03-19 20:17 UTC|newest]
Thread overview: 8+ messages / expand[flat|nested] mbox.gz Atom feed top
2024-03-19 18:14 [PATCH] drm/dp: Clarify that wait_hpd_asserted() is not optional for panels Douglas Anderson
2024-03-19 18:26 ` Abhinav Kumar
2024-03-19 18:35 ` Dmitry Baryshkov
2024-03-19 19:02 ` Abhinav Kumar
2024-03-19 20:16 ` Dmitry Baryshkov [this message]
2024-03-19 20:38 ` Abhinav Kumar
2024-03-19 20:54 ` Dmitry Baryshkov
2024-03-19 21:00 ` Doug Anderson
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