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

[Dmitry Baryshkov]

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