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

[Tomi Valkeinen <tomi.valkeinen@ideasonboard.com>]

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.

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.

>>> 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.

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.

> 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 =).

  Tomi