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References: <20240315213717.1411017-1-dianders@chromium.org>
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From: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Date: Tue, 19 Mar 2024 02:55:11 +0200
Message-ID: <CAA8EJppau--vt3RLkH96K0SF2x-QGWz+5U8tErvLFDvz-GQN4Q@mail.gmail.com>
Subject: Re: [PATCH v2 3/4] drm/msm/dp: Delete the old 500 ms wait for eDP HPD
 in aux transfer
To: Abhinav Kumar <quic_abhinavk@quicinc.com>
Cc: Douglas Anderson <dianders@chromium.org>, Rob Clark <robdclark@gmail.com>, 
	Daniel Vetter <daniel@ffwll.ch>, David Airlie <airlied@gmail.com>, 
	Kuogee Hsieh <quic_khsieh@quicinc.com>, Marijn Suijten <marijn.suijten@somainline.org>, 
	Sean Paul <sean@poorly.run>, dri-devel@lists.freedesktop.org, 
	freedreno@lists.freedesktop.org, linux-arm-msm@vger.kernel.org, 
	linux-kernel@vger.kernel.org, Bjorn Andersson <quic_bjorande@quicinc.com>, 
	Johan Hovold <johan@kernel.org>
Content-Type: text/plain; charset="UTF-8"

On Tue, 19 Mar 2024 at 02:19, Abhinav Kumar <quic_abhinavk@quicinc.com> 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.

>
> 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@chromium.org>
> > ---
> >
> > (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);
> >


-- 
With best wishes
Dmitry