Re: [RFC] drm/msm/dp: Allow attaching a drm_panel

[Hans de Goede]

Hi all,

On 12/7/21 13:26, Heikki Krogerus wrote:
> +Hans and Imre
> 
> On Mon, Dec 06, 2021 at 02:31:40PM -0800, Bjorn Andersson wrote:
>> On Thu 07 Oct 03:17 PDT 2021, Heikki Krogerus wrote:
>>> On Wed, Oct 06, 2021 at 01:26:35PM -0700, Prashant Malani wrote:
>>>> (CC+ Heikki)
>> [..]
>>>> On Wed, Oct 6, 2021 at 8:19 AM Doug Anderson <dianders@xxxxxxxxxxxx> wrote:
>> [..]
>>>         void drm_connector_oob_hotplug_event(struct fwnode_handle *connector_fwnode);
>>>
>>> If your USB Type-C controller/port driver does not yet register the DP
>>> alt mode, the it's responsible of handling HPD separately by calling
>>> drm_connector_oob_hotplug_event() on its own.
>>>
>>
>> Finally found my way back to this topic and it doesn't look like I can
>> reuse the existing altmode code with the firmware interface provided by
>> Qualcomm, so  I just hacked something up that invokes
>> drm_connector_oob_hotplug_event().
>>
>> But I'm not able to make sense of what the expected usage is. Reading
>> altmode/displayport.c, it seems that I should only invoke
>> drm_connector_oob_hotplug_event() as HPD state toggles.
>>
>> I made a trial implementation of this, where my firmware interface
>> driver calls drm_connector_oob_hotplug_event() every time HPD state
>> changes and then in my oob_hotplug_event callback I flip the DP
>> controller between on and off.
>>
>> Unfortunately when I then connect my HDMI dongle, I get HPD state HIGH,
>> call the oob_hotplug_event, the DP driver powers up and concludes that
>> there's nothing connected to the dongle and goes to idle. I then connect
>> the HDMI cable to the dongle, the firmware sends me another message with
>> HPD irq and state HIGH, which I ignore because it's not a change in
>> state.
>>
>> In the end I hacked up drm_connector_oob_hotplug_event() to allow me to
>> pass the HPD state and this solves my problem. I can now distinguish
>> between connect, disconnect and attention.
>>
>> Can you please help shed some light on what I might be missing?

The plan always was to pass some extra information, like the number
of available DP lanes (which can make training faster) along as
parameter to the drm_connector_oob_hotplug_event().

The merged version ended up not doing this because there were no
consumers, but passing additional info like HPD state definitely
is ok.

Regards,

Hans


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