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 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
