Re: [PATCH] drm/dp: Clarify that wait_hpd_asserted() is not optional for panels

[Dmitry Baryshkov]

On Tue, 19 Mar 2024 at 21:02, Abhinav Kumar <quic_abhinavk@xxxxxxxxxxx> wrote:
>
>
>
> On 3/19/2024 11:35 AM, Dmitry Baryshkov wrote:
> > On Tue, 19 Mar 2024 at 20:15, Douglas Anderson <dianders@xxxxxxxxxxxx> 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@xxxxxxxxxxxx>
> >> ---
> >> 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