Re: [PATCH v3 0/4] drm/tidss: Add OLDI bridge support

[Aradhya Bhatia <aradhya.bhatia@linux.dev>]

On 09/09/24 15:20, Tomi Valkeinen wrote:
> On 09/09/2024 12:31, Aradhya Bhatia wrote:
>> Hi,
>>
>> Thank you, Francesco and Max, for testing and reporting this!
>>
>> On 09/09/24 13:45, Tomi Valkeinen wrote:
>>> Hi,
>>>
>>> On 06/09/2024 14:43, Francesco Dolcini wrote:
>>>> +Max
>>>>
>>>> Hello Aradhya,
>>>>
>>>> On Tue, Jul 16, 2024 at 02:12:44PM +0530, Aradhya Bhatia wrote:
>>>>> The addition of the 2nd OLDI TX (and a 2nd DSS in AM62Px) creates a
>>>>> need
>>>>> for some major changes for a full feature experience.
>>>>>
>>>>> 1. The OF graph needs to be updated to accurately show the data flow.
>>>>> 2. The tidss and OLDI drivers now need to support the dual-link and
>>>>> the
>>>>>      cloned single-link OLDI video signals.
>>>>> 3. The drivers also need to support the case where 2 OLDI TXes are
>>>>>      connected to 2 different VPs - thereby creating 2 independent
>>>>> streams
>>>>>      of single-link OLDI outputs.
>>>>
>>>> Have you considered/tested the use case in which only single link is
>>>> used?
>>>> You do not mention it here and to me this is a relevant use case.
>>>>
>>>> There is a workaround for this (use option 2, cloned, even if
>>>> nothing is
>>>> connected to the second link), but this seems not correct.
>>
>> I agree. The whole idea behind the series is to be able to accurately
>> describe the hardware. Putting the devicetree in clone mode in order to
>> get the single-link mode working is far from ideal.
> 
> Btw, with the fw_devlink=off hack, and removing the second link from k3-
> am625-sk-microtips-mf101hie-panel.dtso, is still not enough, as the k3-
> am62-main.dtsi contains the ti,companion-oldi property which makes the
> driver think it's a cloning case.

Yes!

> 
> So, I think, either the ti,companion-oldi and ti,secondary-oldi should
> only be set in the overlay when setting up cloning/dual-link, or the
> companion-oldi property shouldn't actually make a difference, and the
> selection between clone and single-link should be done via some other
> means.

Yep, those properties need to be set in the overlay file, and not in the
k3-am62-main.dtsi like it is the case in ti-6.6.

> 
>>>> We (Max in Cc here) noticed that this specific use case is broken on
>>>> your downstream v6.6 TI branch.
>>
>> Yes, it was been brought to my attention that the single-link usecase is
>> not working over the downstream ti-6.6 kernel. As I have since
>> discovered, it's not working on this series either.
>>
>> For some reason, the supplier-consumer dependency between the OLDI and
>> the panel devicetree nodes is not getting flagged as `FWLINK_FLAG_CYCLE`
>> in cases of single-link configuration.
>>
>> This flag allows the panel driver to continue to probe without waiting
>> for the OLDI driver (panel's supplier). Absence of the flag getting set
>> is causing these drivers to keep deferring probe in an endless cycle.
>>
>> Even with the flag, the OLDI (and tidss) cannot complete probe until the
>> panel driver is probed and ready. That is because the OLDI (and tidss)
>> need the panel to continue with the bridge-chain creation.
>>
>> However, over with the dual-lvds configuration (and as Francesco has
>> now mentioned the clone configuration as well), the flag gets set by
>> default, and everything works.
>>
>> This is what the debug has led to, so far.
> 
> Yes, I came to the same conclusion with my debug.
> 
>>>
>>> What if you set "fw_devlink=off" kernel boot parameter?
>>>
>>
>> Yes! I haven't tested it, but it seems that this will bypass the
>> supplier check and let the panel probe continue.
>>
>>
>> Tomi, any idea, why is this issue happening only for single-link in the
>> first place? It seems as if having 2 ports inside the panel devicetree
>> lets the probe mechanism recognize the circular dependency and ignore
>> the supplier OLDIs?
> 
> I have to say I have no idea...
> 
> I don't really understand the devlink code here, but I'm guessing that
> the "cycle" part comes from the fact that with a media graph we have
> links (remote-endpoint) both ways in the DT data. So it's not possible
> to say which side is the consumer, which one is the supplier. Thus, it's
> marked as a cycle and, I think, basically ignored for the probing purpose.

Okay! I am not too sure about the devlink code either, but this
reasoning makes sense.

> 
> But why not here? I can see the links being created both ways:
> 
> /display Linked as a fwnode consumer to /bus@f0000/dss@30200000/oldi-
> txes/oldi@0
> /bus@f0000/dss@30200000/oldi-txes/oldi@0 Linked as a fwnode consumer
> to /display
> 
> but it's never marked as a cycle.

Yes, this matches my observations.

> 
>> This is the function where the difference comes down to, by the way -
>> __fw_devlink_relax_cycles(), per my knowledge. I am still on my way to
>> understand what exactly it is doing and why is it not relaxing the
>> single-link case.
> 
> Yep. The answer is probably somewhere there =).
> 

Alright! We have an interesting problem in our hands now. =)


Regards
Aradhya