Re: [PATCH v2] drm/dp: Clarify that wait_hpd_asserted() is not optional for panels

[Doug Anderson]

Hi,

On Tue, Mar 19, 2024 at 3:45 PM Dmitry Baryshkov
<dmitry.baryshkov@xxxxxxxxxx> wrote:
>
> 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>

I don't think this is controversial and I've been involved / written
most of the code for eDP panel interactions, so I think I can land
this myself with Dmitry and Abhinav's review (thanks!). If someone
later comes back with additional thoughts I'm happy to update things
more.

Pushed to drm-misc-next:

6376eb8b9115 drm/dp: Clarify that wait_hpd_asserted() is not optional for panels