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From:   Pin-yen Lin <treapking@chromium.org>
Date:   Wed, 10 May 2023 11:41:54 +0800
Message-ID: <CAEXTbpcWfYV_58pw_VupjhAFZsUU3pkLRN_8JoASyLLBmgTYqQ@mail.gmail.com>
Subject: Re: [PATCH v15 04/10] dt-bindings: display: bridge: anx7625: Add
 mode-switch support
To:     Stephen Boyd <swboyd@chromium.org>
Cc:     Andrzej Hajda <andrzej.hajda@intel.com>,
        Andy Shevchenko <andriy.shevchenko@linux.intel.com>,
        Benson Leung <bleung@chromium.org>,
        Daniel Scally <djrscally@gmail.com>,
        Daniel Vetter <daniel@ffwll.ch>,
        David Airlie <airlied@gmail.com>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Guenter Roeck <groeck@chromium.org>,
        Heikki Krogerus <heikki.krogerus@linux.intel.com>,
        Jernej Skrabec <jernej.skrabec@gmail.com>,
        Jonas Karlman <jonas@kwiboo.se>,
        Krzysztof Kozlowski <krzysztof.kozlowski+dt@linaro.org>,
        Laurent Pinchart <Laurent.pinchart@ideasonboard.com>,
        Neil Armstrong <neil.armstrong@linaro.org>,
        Prashant Malani <pmalani@chromium.org>,
        "Rafael J . Wysocki" <rafael@kernel.org>,
        Rob Herring <robh+dt@kernel.org>,
        Robert Foss <rfoss@kernel.org>,
        Sakari Ailus <sakari.ailus@linux.intel.com>,
        Xin Ji <xji@analogixsemi.com>, Marek Vasut <marex@denx.de>,
        Hsin-Yi Wang <hsinyi@chromium.org>,
        Thomas Zimmermann <tzimmermann@suse.de>,
        AngeloGioacchino Del Regno 
        <angelogioacchino.delregno@collabora.com>,
        Lyude Paul <lyude@redhat.com>, devicetree@vger.kernel.org,
        dri-devel@lists.freedesktop.org, linux-acpi@vger.kernel.org,
        chrome-platform@lists.linux.dev,
        =?UTF-8?B?TsOtY29sYXMgRiAuIFIgLiBBIC4gUHJhZG8=?= 
        <nfraprado@collabora.com>,
        Javier Martinez Canillas <javierm@redhat.com>,
        linux-kernel@vger.kernel.org,
        Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>,
        Chen-Yu Tsai <wenst@chromium.org>, jagan@amarulasolutions.com
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+Jagan who worked on a similar design and initiated the thread.

Hi Stephen,

On Sat, Apr 29, 2023 at 12:47=E2=80=AFPM Stephen Boyd <swboyd@chromium.org>=
 wrote:
>
> Quoting Pin-yen Lin (2023-04-20 02:10:46)
> > On Thu, Apr 20, 2023 at 2:10=E2=80=AFPM Stephen Boyd <swboyd@chromium.o=
rg> 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 ma=
tches
> > > > > when the property is present. I am not sure what side effects wou=
ld 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 th=
e
> > > > > 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 t=
he
> > > > 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.
> > > >
> > > > It may be a little complicated though, because we would only regist=
er
> > > > 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 devic=
e 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@2) are
> > > connected and the DPI input pins are connected (port@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_bridg=
es
> > > 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/
>
> I mean that you must have some sort of orientation switch hardware that
> takes the 2 DP lanes and the 2 USB SuperSpeed "lanes" or "pairs" and
> puts them all onto a usb-c-connector. The usb-c-connector node can't be
> connected directly to the anx7625 in your diagram because there must be
> some sort of "flipper" that does the orientation control. Otherwise the
> usb-c-connector wouldn't work if the user flipped the cable. Probably
> this is some TCPC or redriver controlled by the EC.
>
> >
> > > 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 woul=
d
> > > 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@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?
>
> Good point. I'm suggesting to make the drm bridge chain into a tree. We
> need to teach drm_bridge core about a mux, and have some logic to
> navigate atomically switching from one output to another. I was talking
> with dianders@ and he was suggesting to use bridge attach/detach for
> this. I'm not sure that will work though because that hook is only
> called when the encoder is attached to the bridge.
>
> It may also turn out that this helps with DP multi-stream transport
> (MST). As far as I can recall DP MST doesn't mesh well with drm_bridge
> designs because it wants to operate on a drm_connector and
> drm_bridge_connector_init() wants to make only one drm_connector for a
> chain of bridges. If you squint, the anx7625 could be an MST "branch"
> that only supports one drm_connector being enabled at a time. Maybe that
> is what we should do here, make drm_bridge support creating more than
> one drm_connector and when there is a mux in the tree it walks both
> sides and runs a callback similar to struct
> drm_dp_mst_topology_cbs::add_connector() to tell the encoder that
> there's another possible drm_connector here.

I have been surveying the approaches to change the bridge chain in
runtime, and I found this thread[1]. Quoting from Daniel:
"... exchanging the bridge chain isn't supported, there's no locking
for that since it's assumed to be invariant over the lifetime of the
drm_device instance. The simplest way to make that happen right now is to
have 2 drm_encoder instances, one with the lvds bridge chain, the other
with the hdmi bridge chain, and select the right encoder/bridge chain
depending upon which output userspace picks.
...
I wouldn't try to make bridge chains exchangeable instead, that's
headaches - e.g. with dp mst we've also opted for a bunch of fake
drm_encoders to model that kind of switching."

I'm not sure how we register two encoders properly, though. Do we make
the encoder driver check all the downstream bridges and register two
drm_encoder when a bridge is acting as a mux?

[1]: https://www.spinics.net/lists/dri-devel/msg340511.html

>
> >
> > 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?
>
> I prefer to not have a mode-switch property here for a couple reasons:
>
>  1. The binding is usb type-c specific, and in the case of the IT6505
>  part there is nothing that indicates this is a usb type-c piece of
>  hardware. The IT6505 is simply a display bridge. The anx7625 part
>  actually does accept usb signals though, but that isn't being used or
>  described here. That's where my disclaimer about mode-switch making
>  sense applies when the usb input is used.
>
>  2. Putting mode-switch into the graph endpoint nodes is awkward. It is
>  a device property, and graph nodes are not devices. Some patches in
>  this series have to work around this fact and special case the graph
>  walking logic to treat the graph itself as a place to look for the
>  property.
>
>  3. The mode-switch property probably isn't necessary at all. The DT
>  reviewers have been asking why it is needed. The EC driver that
>  registers the usb-c-connectors can be the mode-switch and the
>  orientation-switch. And in reality, it _is_ both. The DP signals and
>  the USB signals go to the TCPC/redriver that is controlled by the EC
>  and the EC is the device that's doing the mode switching to push DP and
>  USB through the TCPC/redriver out on the right pins of the
>  usb-c-connector.
>
> I guess another way to think about it is that the DP signal coming out
> of the anx7625 part is not "usb type-c" at all, unless the USB signal is
> coming out on the other side of the crosspoint switch and all four lanes
> are wired to some usb-c-connector or redriver. Similarly, the situation
> could look like trogdor, where DP is produced by the DP PHY in the SoC
> and goes through an analog mux to steer DP to one or the other TCPC
> that's wired to the usb-c-connector. There isn't any driver to control
> that mux, but if there was it would be a gpio controlled mux that would
> be a drm_bridge, because there isn't anything type-c about this
> hardware.
>
> And finally, I can see a possibility where the IT6505 is actually wired
> to two different dp-connector ports. In that situation, there is no
> type-c involvement, but we would still want to expose that to userspace
> as two drm_connectors where only one encoder can be attached to them. If
> we did that with drm_bridge, then anyone could make these sorts of
> chains with muxes and it would present a sane userspace interface.

Thanks for the detailed explanation. Yes, our use case in this series
is not related to the USB-C port from the bridge's perspective. The
bridge only cares about the output changes, and it doesn't need to
know whether the downstream is a USB-C port or a DP connector.

Best regards,
Pin-yen