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

[Abhinav Kumar]

On 3/19/2024 11:35 AM, Dmitry Baryshkov wrote:
> On Tue, 19 Mar 2024 at 20:15, Douglas Anderson <dianders at chromium.org> 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 at chromium.org>
>> ---
>> 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 doc is only for wait_for_hpd_asserted(). If panel driver handles 
HPD signal on its own, this will not be called. Do we need a doc for that?

>>           *
>>           * 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
>>
> 
>