[PATCH v15 04/10] dt-bindings: display: bridge: anx7625: Add mode-switch support

[Pin-yen Lin]

On Thu, Apr 20, 2023 at 5:10 PM Pin-yen Lin <treapking at chromium.org> wrote:
>
> On Thu, Apr 20, 2023 at 2:10 PM Stephen Boyd <swboyd at chromium.org> wrote:
> >
> > Quoting Stephen Boyd (2023-04-13 17:22:46)
> > > Quoting Pin-yen Lin (2023-04-13 02:50:44)
> > > >
> > > > Actually the `mode-switch` property here is mainly because
> > > > `fwnode_typec_mux_get`[1] and `typec_mux_match`[2] only return matches
> > > > when the property is present. I am not sure what side effects would be
> > > > if I remove the ID-matching condition in `typec_mux_match`, so I added
> > > > the property here.
> > > >
> > > > Is it feasible to remove the `mode-switch` property here given the
> > > > existing implementation of the Type-C framework?
> > >
> > > Omitting the mode-switch property would require changes to the type-c
> > > framework.
> > >
> > > I'm wondering if we can have this anx driver register mode switches for
> > > however many endpoints exist in the output port all the time when the
> > > aux-bus node doesn't exist. Then the type-c framework can walk from the
> > > usb-c-connector to each connected node looking for a device that is both
> > > a drm_bridge and a mode-switch. When it finds that combination, it knows
> > > that the mode-switch has been found. This hinges on the idea that a
> > > device that would have the mode-switch property is a drm_bridge and
> > > would register a mode-switch with the type-c framework.

I spent some time working on this approach on the Type-C side. The
issue I met is that the driver doesn't know whether a node is a
drm_bridge before the anx7625 driver probes. When there is a
"mode-switch" property in the node, the Type-C framework knows that
"here is a mode switch, but the corresponding driver hasn't registered
the typec_mux". So it returns -EPROBE_DEFER and retries later.
However, if we remove the property, the Type-C framework won't know
whether a node will be registered as a drm_bridge and register a
typec_mux.

Do you have other suggestions on this if we want to choose this approach?

Best regards,
Pin-yen
> > >
> > > It may be a little complicated though, because we would only register
> > > one drm_bridge for the input to this anx device. The type-c walking code
> > > would need to look at the graph endpoint, and find the parent device to
> > > see if it is a drm_bridge.
> >
> > I've been thinking more about this. I think we should only have the
> > 'mode-switch' property possible when the USB input pins (port at 2) are
> > connected and the DPI input pins are connected (port at 0). Probably you
> > don't have that case though?
>
> No we don't have the use case that uses the USB input pins on anx7625.
> >
> > In your case, this device should register either one or two drm_bridges
> > that connect to whatever downstream is actually muxing the 2 DP lanes
> > with the USB SS lanes onto the usb-c-connector.
>
> What do you mean by "muxing the 2 DP lanes with the USB SS lanes''? In
> our use case, the USB data lanes from both ports are connected to a
> USB hub, but the DP lanes are muxed by the crosspoint switch on
> anx7625. HPD and AUX for the external display are muxed by the EC. You
> can find the diagram at
> https://lore.kernel.org/linux-usb/YxGzk6DNAt0aCvIY@chromium.org/
>
> > If that is the EC for
> > ChromeOS, then the EC should have a binding that accepts some number of
> > input ports for DP. The EC would act as a drm_bridge, or in this case
> > probably two bridges, and also as two type-c switches for each
> > drm_bridge corresponding to the usb-c-connector nodes. When DP is on the
> > cable, the type-c switch/mux would signal to the drm_bridge that the
> > display is 'connected' via DRM_BRIDGE_OP_DETECT and struct
> > drm_bridge_funcs::detect(). Then the drm_bridge in this anx part would
> > implement struct drm_bridge_funcs::atomic_enable() and configure the
> > crosspoint switch the right way depending on the reg property of the
> > output node in port at 1.
>
> So there will be two drm bridges that act as the downstreams for
> anx7625, and we find the downstream with connector_status_connected to
> configure the crosspoint switch? How do we support that kind of
> topology given that the drm bridge chain is currently a list? Are you
> suggesting making the bridge topology to a tree, or maintaining the
> two downstreams inside the anx7625 driver and not attaching them to
> the bridge chain?
>
> Also, if we still register mode switches on the two downstream
> bridges, why do you prefer that over the original approach that
> register switches in the anx7625 driver?
>
> >
> > Because you don't have the part that implements the orientation-switch,
> > you don't need to implement the code for it. I think simply adding
> > support in the binding for mode-switch and orientation-switch if this is
> > directly wired to a usb-c-connector should be sufficient. Those
> > properties would be at the top-level and not part of the graph binding.