Re: [PATCH 4/8] drm/tidss: Add support for Dual Link LVDS Bus Format

[Tomi Valkeinen]

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.

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.

> > > 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

> > > 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.

 Tomi