On Tue, 19 Mar 2024 at 22:58, 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 > > Reviewed-by: Abhinav Kumar <quic_abhinavk@xxxxxxxxxxx> > 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(). > > 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@xxxxxxxxxx> -- With best wishes Dmitry
