Re: [PATCH v2 3/4] drm/msm/dp: Delete the old 500 ms wait for eDP HPD in aux transfer

[Abhinav Kumar]

On 3/18/2024 5:55 PM, Dmitry Baryshkov wrote:
> On Tue, 19 Mar 2024 at 02:19, Abhinav Kumar <quic_abhinavk@xxxxxxxxxxx> wrote:
> > +bjorn, johan as fyi for sc8280xp
> >
> > On 3/15/2024 2:36 PM, Douglas Anderson wrote:
> > > Before the introduction of the wait_hpd_asserted() callback in commit
> > > 841d742f094e ("drm/dp: Add wait_hpd_asserted() callback to struct
> > > drm_dp_aux") the API between panel drivers and DP AUX bus drivers was
> > > that it was up to the AUX bus driver to wait for HPD in the transfer()
> > > function.
> > >
> > > Now wait_hpd_asserted() has been added. The two panel drivers that are
> > > DP AUX endpoints use it. See commit 2327b13d6c47 ("drm/panel-edp: Take
> > > advantage of wait_hpd_asserted() in struct drm_dp_aux") and commit
> > > 3b5765df375c ("drm/panel: atna33xc20: Take advantage of
> > > wait_hpd_asserted() in struct drm_dp_aux"). We've implemented
> > > wait_hpd_asserted() in the MSM DP driver as of commit e2969ee30252
> > > ("drm/msm/dp: move of_dp_aux_populate_bus() to eDP probe()"). There is
> > > no longer any reason for long wait in the AUX transfer() function.
> > > Remove it.
> > >
> > > NOTE: the wait_hpd_asserted() is listed as "optional". That means it's
> > > optional for the DP AUX bus to implement. In the case of the MSM DP
> > > driver we implement it so we can assume it will be called.
> >
> > How do we enforce that for any new edp panels to be used with MSM, the
> > panels should atleast invoke wait_hpd_asserted()?
> >
> > I agree that since MSM implements it, even though its listed as
> > optional, we can drop this additional wait. So nothing wrong with this
> > patch for current users including sc8280xp, sc7280 and sc7180.
> >
> > But, does there need to be some documentation that the edp panels not
> > using the panel-edp framework should still invoke wait_hpd_asserted()?
> >
> > Since its marked as optional, what happens if the edp panel driver,
> > skips calling wait_hpd_asserted()?
>
> It is optional for the DP AUX implementations, not for the panel to be called.

Yes, I understood that part, but is there anything from the panel side 
which mandates calling wait_hpd_asserted()?

Is this documented somewhere for all edp panels to do:

if (aux->wait_hpd_asserted)
	aux->wait_hpd_asserted(aux, wait_us);

> > Now, since the wait from MSM is removed, it has a potential to fail.
> >
> > > ALSO NOTE: the wait wasn't actually _hurting_ anything and wasn't even
> > > causing long timeouts, but it's still nice to get rid of unneeded
> > > code. Specificaly it's not truly needed because to handle other DP
> > > drivers that can't power on as quickly (specifically parade-ps8640) we
> > > already avoid DP AUX transfers for eDP panels that aren't powered
> > > on. See commit 8df1ddb5bf11 ("drm/dp: Don't attempt AUX transfers when
> > > eDP panels are not powered").
> > >
> > > Signed-off-by: Douglas Anderson <dianders@xxxxxxxxxxxx>
> > > ---
> > >
> > > (no changes since v1)
> > >
> > >    drivers/gpu/drm/msm/dp/dp_aux.c | 17 -----------------
> > >    1 file changed, 17 deletions(-)
> > >
> > > diff --git a/drivers/gpu/drm/msm/dp/dp_aux.c b/drivers/gpu/drm/msm/dp/dp_aux.c
> > > index 75c51f3ee106..ecefd1922d6d 100644
> > > --- a/drivers/gpu/drm/msm/dp/dp_aux.c
> > > +++ b/drivers/gpu/drm/msm/dp/dp_aux.c
> > > @@ -313,23 +313,6 @@ static ssize_t dp_aux_transfer(struct drm_dp_aux *dp_aux,
> > >                goto exit;
> > >        }
> > >
> > > -     /*
> > > -      * For eDP it's important to give a reasonably long wait here for HPD
> > > -      * to be asserted. This is because the panel driver may have _just_
> > > -      * turned on the panel and then tried to do an AUX transfer. The panel
> > > -      * driver has no way of knowing when the panel is ready, so it's up
> > > -      * to us to wait. For DP we never get into this situation so let's
> > > -      * avoid ever doing the extra long wait for DP.
> > > -      */
> > > -     if (aux->is_edp) {
> > > -             ret = dp_catalog_aux_wait_for_hpd_connect_state(aux->catalog,
> > > -                                                             500000);
> > > -             if (ret) {
> > > -                     DRM_DEBUG_DP("Panel not ready for aux transactions\n");
> > > -                     goto exit;
> > > -             }
> > > -     }
> > > -
> > >        dp_aux_update_offset_and_segment(aux, msg);
> > >        dp_aux_transfer_helper(aux, msg, true);