Re: [PATCH v4 0/3] drm/tidss: Add OLDI bridge support

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

Hi,

On 04/12/24 00:06, Tomi Valkeinen wrote:
> Hi,
> 
> On 03/12/2024 20:14, Aradhya Bhatia wrote:
>> Hi,
>>
>> On 03/12/24 17:42, Tomi Valkeinen wrote:
>>> Hi,
>>>
>>> On 24/11/2024 16:36, Aradhya Bhatia wrote:
>>>> Hello all,
>>>>
>>>> This patch series add support for the dual OLDI TXes supported in Texas
>>>> Instruments' AM62x and AM62Px family of SoCs. The OLDI TXes support
>>>> single-lvds
>>>> lvds-clone, and dual-lvds modes. These have now been represented
>>>> through DRM
>>>> bridges within TI-DSS.
>>>>
>>>>    - Some history and hardware description for this patch series.
>>>>
>>>> This patch series is a complete re-vamp from the previously posted
>>>> series[1] and
>>>> hence, the version index has been reset to v1. The OLDI support from
>>>> that series
>>>> was dropped and only the base support for AM62x DSS was kept (and
>>>> eventually
>>>> merged)[2].
>>>>
>>>> The OLDI display that the tidss driver today supports, could not be
>>>> extended for
>>>> the newer SoCs. The OLDI display in tidss is modelled after the DSS
>>>> and OLDI
>>>> hardware in the AM65x SoC. The DSS in AM65x SoC, has two video-ports.
>>>> Both these
>>>> video-ports (VP) output DPI video signals. One of the DPI output (from
>>>> VP1) from
>>>> the DSS connects to a singular OLDI TX present inside the SoC. There
>>>> is no other
>>>> way for the DPI from VP1 to be taken out of the SoC. The other DPI
>>>> output
>>>> however - the one from VP2 - is taken out of the SoC as is. Hence we
>>>> have an
>>>> OLDI bus output and a DPI bus output from the SoC. Since the VP1 and
>>>> OLDI are
>>>> tightly coupled, the tidss driver considers them as a single entity.
>>>> That is
>>>> why, any OLDI sink connects directly to the DSS ports in the OF graphs.
>>>>
>>>> The newer SoCs have varying DSS and OLDI integrations.
>>>>
>>>> The AM62x DSS also has 2 VPs. The 2nd VP, VP2, outputs DPI signals
>>>> which are
>>>> taken out of the SoC - similar to the AM65x above. For the VP1, there
>>>> are 2 OLDI
>>>> TXes. These OLDI TXes can only receive DPI signals from VP1, and don't
>>>> connect
>>>> to VP2 at all.
>>>>
>>>> The AM62Px SoC has 2 OLDI TXes like AM62x SoC. However, the AM62Px SoC
>>>> also has
>>>> 2 separate DSSes. The 2 OLDI TXes can now be shared between the 2 VPs
>>>> of the 2
>>>> DSSes.
>>>>
>>>> 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.
>>>>
>>>> Note that the OLDI does not have registers of its own. It is still
>>>> dependent on
>>>> the parent VP. The VP that provides the DPI video signals to the OLDI
>>>> TXes, also
>>>> gives the OLDI TXes all the config data. That is to say, the hardware
>>>> doesn't
>>>> sit on the bus directly - but does so through the DSS.
>>>>
>>>> In light of all of these hardware variations, it was decided to have a
>>>> separate
>>>> OLDI driver (unlike AM65x) but not entirely separate so as to be a
>>>> platform
>>>> device. The OLDI TXes are now being represented as DRM bridges under
>>>> the tidss.
>>>>
>>>> Also, since the DRM framework only really supports a linear encoder-
>>>> bridge
>>>> chain, the OLDI driver creates a DRM bridge ONLY for the primary OLDI
>>>> TX in
>>>> cases of dual-link or cloned single-link OLDI modes. That bridge then
>>>> attaches
>>>
>>> How does the clone case work, then? There are two panels, what does the
>>> second one connect to?
>>
>> For the clone case, the devicetree will show the true connections - as
>> they are in the hardware.
>>
>> 2 endpoints from a single DSS VP devicetree port will be connected to 2
>> OLDIs, OLDI0 and OLDI1. The outputs of these OLDIs will be connected to
>> 2 distinct single-link panels.
>>
>> The driver and DRM side of things do not show the same picture, however.
>> The tidss_oldi code creates and registers a drm_bridge only for the
>> primary OLDI. The driver is capable of detecting the expected OLDI mode,
>> and if a companion OLDI is present, then the primary OLDI drm_bridge
>> keeps a note of that.
>>
>> The clock and config resources are shared between the primary and
>> companion OLDI hardware. So configuring the primary OLDI takes care of
>> the companion too.
>> The only case where it is not shared is the OLDI IO bit in the Control
>> MMR (ctrl_mmr) region. But, since the primary OLDI drm_bridge remains
>> aware about the presence of companion OLDI, it makes sure to enable /
>> disable the comapnion OLDI IO when required.
> 
> But if there's just one bridge (for the first oldi), how is the second
> panel connected to the DRM pipeline? Who e.g. calls the
> drm_panel_funcs.enable() in the panel driver for the second panel?
> 
> Or, say, if we have two LVDS->HDMI bridges, with the cloning setup, how
> does all the plumbing work if "DRM framework only really supports a
> linear encoder-bridge chain"?
> 

Right... The driver does not account for such a case at present. The
simple panels don't require any additional programming, which is why a
clone mode with them just happens to work out.

Since there is still only 1 VP behind this, there could only be a single
crtc. Perhaps, we can have an additional tidss encoder (connected to the
same crtc) to start this another encoder-bridge chain. I am still murky
with the details here, but I will try to see what needs to be done.

Thank you! =)


-- 
Regards
Aradhya