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

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Message-ID: <d04711c1-6df0-f988-9227-2161f4109dd1@quicinc.com>
Date: Tue, 19 Mar 2024 10:13:18 -0700
From: Abhinav Kumar <quic_abhinavk@...cinc.com>
To: Dmitry Baryshkov <dmitry.baryshkov@...aro.org>
CC: Douglas Anderson <dianders@...omium.org>, Rob Clark <robdclark@...il.com>,
        Daniel Vetter <daniel@...ll.ch>, David Airlie <airlied@...il.com>,
        "Kuogee
 Hsieh" <quic_khsieh@...cinc.com>,
        Marijn Suijten
	<marijn.suijten@...ainline.org>,
        Sean Paul <sean@...rly.run>, <dri-devel@...ts.freedesktop.org>,
        <freedreno@...ts.freedesktop.org>, <linux-arm-msm@...r.kernel.org>,
        <linux-kernel@...r.kernel.org>,
        "Bjorn
 Andersson" <quic_bjorande@...cinc.com>,
        Johan Hovold <johan@...nel.org>
Subject: Re: [PATCH v2 3/4] drm/msm/dp: Delete the old 500 ms wait for eDP HPD
 in aux transfer



On 3/18/2024 5:55 PM, Dmitry Baryshkov wrote:
> On Tue, 19 Mar 2024 at 02:19, Abhinav Kumar <quic_abhinavk@...cinc.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.
> 

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@...omium.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);
>>>
> 
> 
>