Message-ID: <dcd04013-7e58-d36a-b2c4-21c6270747c1@ti.com>
Date:   Tue, 9 Aug 2022 19:04:14 +0530
From:   Aradhya Bhatia <a-bhatia1@...com>
To:     Tomi Valkeinen <tomi.valkeinen@...asonboard.com>
CC:     Darren Etheridge <detheridge@...com>, Nishanth Menon <nm@...com>,
        Vignesh Raghavendra <vigneshr@...com>,
        Rahul T R <r-ravikumar@...com>,
        Krunal Bhargav <k-bhargav@...com>,
        Devarsh Thakkar <devarsht@...com>,
        DRI Development List <dri-devel@...ts.freedesktop.org>,
        David Airlie <airlied@...ux.ie>,
        Daniel Vetter <daniel@...ll.ch>,
        Rob Herring <robh+dt@...nel.org>,
        Jyri Sarha <jyri.sarha@....fi>,
        Devicetree List <devicetree@...r.kernel.org>,
        Linux Kernel List <linux-kernel@...r.kernel.org>,
        Krzysztof Kozlowski <krzysztof.kozlowski+dt@...aro.org>
Subject: Re: [PATCH 4/8] drm/tidss: Add support for Dual Link LVDS Bus Format

Hi Tomi,

On 09-Aug-22 15:21, Tomi Valkeinen wrote:
> On 09/08/2022 12:06, Aradhya Bhatia wrote:
> 
>>>> Even in DT, the dss port (for OLDI) connects to the panel port's
>>>> endpoint directly. Even in cases of dual link or cloning, it's only a
>>>> singular remote-to-endpoint connection between the (OLDI) VP and the
>>>> panel port. Hence the requirement of the properties in the earlier
>>>> patches of the series.
>>>
>>> Sorry, I don't follow. If you use cloning, you have two TX outputs, 
>>> going to two panels, right? So you need two panel DT nodes, and those 
>>> would connect to two OLDI TX ports in the DSS.
>>>  > Afaics the existing dual link bridge/panel drivers also use two ports
>>> for the connection, so to use the dual link you need two ports in the 
>>> DSS.
>>>
>>> I admit I'm not familiar with LVDS dual link, but it's not clear to 
>>> me how you see the dual OLDI TX being used with other drivers if you 
>>> have only one port. What kind of setups have you tested?
>>>
>> In the DTs, the OLDIs are not modeled at all. Since the DSS only has a
>> single VP for OLDI, the DT dss port (for OLDI) is connected to a single
>> simple-panel node for dual link, bypassing the OLDI TX in DT. I have
>> this same OLDI setup and have been testing on this.
> 
> A DSS VP is a DSS internal port, whereas a port node in the DT is an 
> external port. There doesn't have to be a 1:1 match between those.
> 
> The port in the DT represents some kind of "connector" to the outside 
> world, which is usually a collection of pins that provide a video bus.
> 
Okay, I now understand what you are saying. Indeed, I was mapping the
DSS VP and DT DSS port as 1:1 in my mind, which should not be the case.

> Here, as far as I can see, the DSS clearly has three external ports, two 
> OLDI ports and one DPI port.
> 
>> I do not have a cloning display setup with me, but I have seen DT DSS
>> port connected to one of 2 panel nodes while the other panel (remains as
>> a companion panel to the first) without any endpoint connections. Since,
>> the OLDI TXes (0 and 1), receive the same clocks and inputs from DSS
>> OLDI VP, this 'method' has worked too.
> 
> This, and using simple-panel for dual link with single port connection, 
> sounds like a hack.
> 
> A practical example: TI's customer wants to use AM625 and THC63LVD1024 
> bridge. How does it work? THC63LVD1024 driver uses two LVDS ports for 
> input, both of which are used in dual-link mode.
> 
Right. There has to be 2 ports for OLDI in DSS, to be connected to 2
ports of a single panel (dual link) or 2 ports of 2 panels (cloning).

>>>> The use of lvds helper functions does not seem feasible in this case,
>>>> because even they read DT properties to determine the dual link
>>>> connection and those properties need to be a part of a lvds bridge
>>>> device.
>>>
>>> Can you elaborate a bit more why the DRM helpers couldn't be used here?
>>>
>> The drm_of.c helpers use DT properties to ascertain the presence of a
>> dual-link connection. While there wasn't a specific helper to determine
>> dual-link or not, the drivers use the odd/even pixel order helper which
>> is based on the properties "dual-lvds-odd-pixels" and "dual-lvds-odd-
>> pixels". If either of the properties are absent, the helper returns an
>> error making the driver to use single link.
>>
>> These properties are LVDS specific, but they could not be added in the
>> DT because there is no OLDI TX DT node for our case.
> 
> If I'm not mistaken, those properties are in the port node, not the 
> device node, and also, I believe those properties are on the sink side, 
> so they wouldn't even be in the AM625 data. See, for example:
> 
> arch/arm64/boot/dts/renesas/r8a774c0-ek874-idk-2121wr.dts
Yeah, they are indeed on the sink side. I was misunderstood about this.
And the onus for properties is not on DSS nodes anymore.

This probably is a different discussion, but since the sink is now
responsible for those properties, these should get introduced in the
panel-common bindings, right?

The above example has a separate binding, but many dumb dual-link panels
will require those properties in panel-common.

> 
>>>> I have also been considering the idea of implementing a new device
>>>> driver for the OLDI TXes, not unlike the renesas' one. That way the
>>>> driver could have the properties and the lvds helper functions at their
>>>> disposal. I am just slightly unsure if that would allow space for any
>>>> conflicts because of the shared register space.
>>>
>>> No, I don't think new devices are needed here.
>> Okay...
>>
>> I am not quite sure I understand completely what you are recommending
>> the OLDI to be. It seems to me that you want the OLDI TXes to be modeled
>> as nodes, right? Wouldn't that automatically require some sort of
>> standalone driver arrangement for them? Or am I missing something
>> important here?
> 
> No, I'm only talking about the DT port nodes. At the moment the AM65x DT 
> bindings doc says that there are two ports, port@0 for OLDI and port@1 
> for DPI. I'm saying AM625 needs three ports.
Agreed.

Moreover, I will update the binding to reflect 3 ports for am625-dss.


Regards
Aradhya

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